Voyager 2 at Neptune: Imaging Science Results B. A. SMITH, L. A. SODERBLOM, D. BANFIELD, C. BARNET, A. T. BASILEVKSY, R. F. BEEBE, K. BOLLINGER, J. M. BOYCE, A. BRAHIc, G. A. BRIGGS, R. H. BROWN, C. CHYBA, S. A. COLLINS, T. COLVIN, A. F. COOK II, D. CRisp, S. K. CROFT, D. CRUIKSHANK, J. N. Cuzzi, G. E. DANIELSON, M. E. DAVIES, E. DE JONG, L. DONES, D. GODFREY, J. GOGUEN, I. GRENIER, V. R. HAEMMERLE, H. HAMMEL, C. J. HANSEN, C. P. HELFENSTEIN, C. HOWELL, G. E. HUNT, A. P. INGERSOLL, T. V. JOHNSON, J. KARGEL, R. KIRK, D. I. KUEHN, S. LIMAYE, H. MASURSKY, A. MCEWEN, D. MORIUSON, T. OWEN, W. OWEN, J. B. POLLACK, C. C. PoRco, K. RAGES, P. ROGERS, D. RUDY, C. SAGAN, J. SCHWARTZ, E. M. SHOEMAKER, M. SHOWALTER, B. SICARDY, D. SIMONELLI, J. SPENCER, L. A. SROMOVSKY, C. STOKER, R. G. STROM, V. E. SuoMI, S. P. SYNOTr, R. J. TERRILE, P. THOMAS, W. R. THOMPSON, A. VERBISCER, J. VEVERKA Voyager 2 images of Neptune reveal a windy planet characterized by bright clouds of methane ice suspended in an exceptionally clear atmosphere above a lower deck of hydrogen sulfide or ammonia ices. Neptune's atmosphere is dominated by a large anticyclonic storm system that has been named the Great Dark Spot (GDS). About the same size as Earth in extent, the GDS bears both many similarities and some differences to the Great Red Spot of Jupiter. Neptune's zonal wind profile is remarkably similar to that of Uranus. Neptune has three major rings at radii of 42,000, 53,000, and 63,000 kilometers. The outer ring contains three higher density arc-like segments that were apparently responsible for most of the ground-based occultation events observed during the current decade. Like the rings of Uranus, the Neptune rings are composed of very dark material; unlike that of Uranus, the Neptune system is very dusty. Six new regular satellites were found, with dark surfaces and radii ranging from 200 to 25 kilometers. All lie inside the orbit of Triton and the inner four are located within the ring system. Triton is seen to be a differentiated body, with a radius of 1350 kilometers and a density of 2.1 grams per cubic centimeter; it exhibits clear evidence of early episodes of surface melting. A now rigid crust of what is probably water ice is overlain with a brilliant coating of nitrogen frost, slightly darkened and reddened with organic polymer material. Streaks of organic polymer suggest seasonal winds strong enough to move particles of micrometer size or larger, once they become airborne. At least two active plumes were seen, carrying dark material 8 kilometers above the surface before being transported downstream by high level winds. The plumes may be driven by solar heating and the subsequent violent vaporization of subsurface nitrogen. OYAGER 2 ACQUIRED MORE THAN The combination of the very low light levels experienced at Neptune [30 astronomical units (AU) from the sun] and the low albedo of its rings and inner satellites, however, made it necessary to employ uncom- B. A. Smith, S. K. Croft, V. R. Haemmerle, J. Kargel, C. C. Porco, R. G. Strom, University of Arizona, Tucson, AZ 85721. L. A. Soderblom, R. Kirk, H. Masursky, A. McEwen, E. M. Shoemaker, U.S. G.S., Flagstaff, AZ 86001. D. Banfield, G. E. Danielson, E. De Jong, C. Howell, A. P. Ingersoll, J. Schwartz, California Institute of Technol- Cornell University, Ithaca, NY 14853. A. F. Cook II, Center for Astrophysics, Cambridge, MA 02138. T. Colvin and M. E. Davies, P. Rogers, Rand Corporation, Santa Monica, CA 90406. D. Cruikshank, J. N. Cuzzi, D. Morrison, J. B. Pollack, C. Stoker, NASA Ames Research Center, Moffett Field, CA 94035. L. Dones, University of Toronto, Toronto, Ontario M5S lAl, Canada. D. Godfrey, National Optical Astronomy Observatories, Tucson, AZ 85726. G. E. Hunt, Logica International, Ltd., 64 Neisman Street, London, England WIA 4SE. S. Limaye, L. A. Sromovsky, V. E. Suomi, University of Wisconsin, Madison, WI 53706. T. Owen, State University of New York, Stony Brook, NY 11794. K. Rages, Mycol, Inc., Sunnyvale, CA 94087. M. Showalter, Stanford University, Stanford, CA 94305. J. Spencer, Institute for Astronomy, University of Hawaii, Honolulu, HI 95822. VO 9000 images of Neptune, its rings, and its satellites during a 6-month interval surrounding the spacecraft's closest approach to the planet on 25 August 1989. ogy, Pasadena, CA 91125. C. Barnet, R. F. Beebe, D. I. Kuehn, New Mexico State University, Las Cruces, NM 88003. A. T. Basilevsky, Vernadsky Institute for Cosmochemistry, USSR Academy of Science, Moscow. K. Bollinger, R. H. Brown, S. A. Collins, D. Crisp, J. Goguen, H. Hammel, C. J. Hansen, T. V. Johnson, W. Owen, D. Rudy, S. P. Synnott, R. J. Terrile, Jet Propulsion Laboratory, Pasadena, CA 91109. J. M. Boyce and G. A. Briggs, NASA Headquarters, Washington, DC 20546. A. Brahic, I. Grenier, B. Sicardy, Observatoire de Paris, Meudon, Paris, France. C. Chyba, C. P. Helfenstein, C. Sagan, D. Simonelli, P. Thomas, W. R. Thompson, A. Verbiscer, J. Veverka, 1422 fortably long exposure times for many of the sequences. This, in turn, required the use of a special sequence design to compensate for image smear caused by spacecraft motion. In addition, spacecraft engineering teams modified the attitude control software to provide further reduction of random spacecraft motion over that achieved earlier at the Uranus encounter (1). The success of these techniques is aptly demonstrated in the striking photographs that Voyager 2 has sent back to Earth from the very edge of our planetary system. The Atmosphere of Neptune Neptune's atmosphere provided many surprises during the Voyager encounter. The high wind speeds, the persistence of large oval storm systems, and the hour-tohour variability of small-scale features were unexpected in an atmosphere that receives 1/20 as much energy (power per unit area) from internal heat and absorbed sunlight as Jupiter and only 1/350 as much energy as Earth. Large features near the equator move westward relative to the interior at speeds up to 325 m s-l, making Neptune one of the windiest planets (with Saturn) in the solar system. Small-scale features appear to move at twice this speed. The Great Dark Spot (GDS), a weather system comparable to Earth in size, stretches and contracts as it rolls in a counterclockwise direction with a 16-day period. Bright clouds cast shadows on the main cloud deck 50 to 100 km below. At some latitudes, the bright clouds resemble mountain lee waves, where largescale patterns remain fixed while small-scale elements move through them. Such patterns were not seen on the other gas giant planets. Although the atmospheres of the giant planets do exhibit some common phenomenacolored clouds and hazes, latitudinal banding, strong winds, for example-there remain many differences and they do not follow obvious rules. Discrete cloud features and banded structure. The largest discrete feature in Neptune's atmosphere is the GDS, which resembles Jupiter's Great Red Spot (GRS) in several respects. Neptune's GDS has an average extent of 38° and 150 in longitude and latitude, respectively (Figs. LA, 2, and 3A), compared with 30° and 200 for the GRS (2). The GDS is located at about the same latitude as the GRS (20°S) and seems to have a similar anticyclonic circulation sense (counterclockwise in the southern hemisphere). Evidence for the anticyclonic rotation of the GDS is based primarily on visual impressions gained from a time-lapse sequence, rather than on cirect measurement SCIENCE, VOL. 246 of the displacement of small features. How- smallest features appeared to move relative ever, there are significant differences be- to the structure as a whole. The role of tween the GDS and the GRS. The circula- topography, necessary for the formation of tion of the GRS modifies its surrounding terrestrial orographic clouds, may be played environment, forming a turbulent-wake re- by temperature and pressure anomalies assogion to the west ofthe feature (2). Although ciated with the GDS. the GDS on Neptune also appears to influA bright feature near the southern pole ence its immediate environment, the regions (71°S) was first seen in April 1989 (Fig. surrounding it appear morphologically 1B). This south polar feature (SPF) appears more uniform compared to those on Jupiter. to be not one discrete feature but rather an Furthermore, the GDS drifts rapidly west- active arc extending over some 900 of longiward at speeds over 300 m s- I relative to the tude and constrained to a latitudinal band planetary radio rate (3, 4), while the GRS spanning less than 5°. The intensity and drifts westward at an average rate of only 3 distribution of brightness along this arc was ms-' (5). seen to vary significantly during a single The bright companion along the southern rotation of the planet. edge of the GDS (Figs. 1 and 3A) was The third bright feature discovered by detected in January 1989; it was the first Voyager (after the bright companion to the discrete feature observed on Neptune by GDS and the SPF) was the "Scooter," a Voyager 2, and was also seen in ground- bright compact feature centered near 42°S based images (4, 6). This feature persistently (Fig. 1A); despite its name, however, it is appeared on the southern edge of the GDS, not the fastest moving feature (4). The although its brightness and precise shape "Scooter" is composed of many small streaks varied with time. In Fig. 3A, the bright stacked in latitude (Fig. 3C) rather than companion is resolved into east-west linear being a single round or oval storm system. structures. The number and length of these streaks Time-lapse images of the GDS and its varies (causing the feature to change shape vicinity (Fig. 4) suggest that the bright from round to square to triangular), but the companion may be similar to orographic composite structure identified as the "Scootclouds observed on Earth, that is, clouds er" persisted as a unit throughout the 80created by air being forced upward by the day encounter period. presence of a mountain. Specifically, the Soon after the discovery of the "Scooter," Fig. 1. Color images of Neptune, taken on 21.7 August 1989. These color images were reconstructed by combining images obtained through green and clear filters. North is up in all figures of Neptune except where explicitly noted otherwise. (A) The Great Dark Spot and its associated bright companion are visible, along with various bright streamers at the same latitude. The "Scooter" is the bright triangular feature south ofthe GDS. D2 (the second dark feature) is visible even further south. [Image processing by 15 DECEMBER I989 a second dark feature (D2) was identified in the southern hemisphere at a latitude of 550S (Figs. 1, A and B). Several weeks after its discovery, a bright core developed at the center of the dark feature. The core remained visible for the remainder of the encounter, although its brightness varied. Small-scale cloud features were clearly visible within the bright core (Fig. 3B). Figure 5 shows three high-resolution images of the cloud structure in the core of D2. The size and shape of the local details varied on time scales of hours. On Jupiter, features similar to D2 rotate anticyclonically, but the sense of circulation has not yet been detected in D2. Voyager images of Neptune's atmosphere (Fig. 1, A and B) revealed a banded zonal structure with the brightest region located near 20°S. Bands of lower reflectivity in the northern and southern hemispheres were located at 6°N to 25°N and 45°S to 70°S, that is, the brightness variation was not symmetrical about the equator. This asymmetry is also seen in Fig. 2, where the effect of viewing geometry was removed to show the brightness at normal incidence (7). Bright ephemeral streamers were seen both at the latitude of the GDS (Fig. 6) and at about 27N. These streamers were highly variable in both time and brightness. Methane-band images of the southern hemi- C. J. Levine] (B) This view focuses on the south polar region, showing the south polar feature and another view of D2. The south polar feature (SPF) is not a single discrete feature, but rather an arc of activity, extending about 900 in longitude and constrained to a latitudinal band less than 50 wide. Only the ends of the arc are bright in this image. This picture was obtained 14 hours after the first image. REPORTS 1423 sphere revealed a bright band within 150 of the south pole. To within an uncertainty of 10, a small feature lies at the south rotational pole (Figs. 7 and 8). This structure suggests a well-organized polar circulation that has no analogue in the polar regions ofthe other giant planets. Vertical structure from feature contrast. Analyses of ground-based observations of Neptune (8-11) and analogous studies of Uranus (11, 12) indicated the presence of three major particulate layers in the observable atmosphere of Neptune (Fig. 9). A photochemical smog layer was predicted at pressures starting at about 5 mbar in the lower stratosphere and extending to lower altitudes in the stratosphere and upper troposphere (8, 10, 13). This layer, composed of lower order hydrocarbons such as ethane, acetylene, and diacetylene, results from photochemistry of methane driven by solar ultraviolet radiation within the lower and upper stratosphere. 3000 1 Two condensation cloud layers are thought to occur in the upper troposphere. Methane should begin condensing at about the 1.5-bar level (8-10, 14) while a more optically thick cloud appears to exist near the 3-bar level (8-10). This deeper cloud may be made of hydrogen sulfide ice partides (8, 10), but ammonia also is present (14). The Voyager images provide both vertical and horizontal structural information (Fig. 1, A and B). We used several complementary approaches for estimating the absolute and relative heights of these features. In the first approach, we studied the wavelength dependence of the contrast of features. As shown in Figs. 8 and 10, the contrast of bright and dark features varies markedly with wavelength. For example, the "Scooter" displays the greatest contrast in the orange-filtered image and much lower contrast in both the UV and longer wavelength methane filter (designated MeJ, with center 1O 2400 I . wavelength 619 nm). On the other hand, most other bright features, such as the bright companion to the Great Dark Spot, show an enhanced contrast in the MeJ band relative to orange. The Great Dark Spot has its maximum negative contrast in the blue filter. We interpret the contrast variations in the following way: molecular Rayleigh scattering in the upper layers tends to mask deeplying features at the shortest wavelengths (the Rayleigh scattering optical depth is about 5 above the 3-bar cloud in the UV filter), while absorption by gaseous methane tends to mask deep features in the methane filters (especially the MeJ filter). Thus, the doud tops of the major bright features are not all located at the same altitude. In particular, the "Scooter" is situated at a much lower altitude than the bright companion of the GDS, as evidenced by the "Scooters" decreasing contrast at methaneband wavelengths relative to blue. Further- 6O0° 1200 O0 3000 30- -30 0- -0 -30- --30 -60- --60 I I I I I I I I I I I . a I . . . I I I I I I I I I 30- -30 0- -0 -30- --30 -60- --60 I 300 I I I I 2400 I I I I I* I 1800 Fig. 2. Simple cylindrical projection mosaics of narrow-angle images taken through the blue (top) and green (bottom) filters on 17-18 August 1989. Latitude is planetocentric and longitude is based on a rotation period of 17 hours 52 min, the "predict" period used to plan Voyager observation sequences. This is longer than the 16.1 hour period derived from the Planetary Radio Astronomy investigation (3). For each mosaic, approximately ten images were shuttered, during which time the planet rotated through slightly more than one rotation. Thus features visible near the right 1424 I I 120° I Il * 600 I * ..I 0° I I 3000 edge of the mosaic appear one rotation later near the left edge. Differential rotation is evident near longitudes of 3000 and latitudes of -41° (410S) and -460 (460S). Feature evolution is evident at +260 (26°N) and -68° (680S). An inverse photometric function was applied to each image to reduce limb darkening. A digital filter was applied to the completed mosaics to reduce the contrast of the large-scale bands and bring out the small-scale features. [Image processing by J. R. Yoshimizu] SCIENCE, VOL. 246 more, the "Scooter" must be located below the base of the methane cloud, because it is masked in the MeJ filter (assuming there is little methane above the base of the methane cloud). Preliminary radiative transfer calculations that approximately reproduce measured contrasts substantiate these conclusions (8, 15). The generally negative contrast of the GDS and the fact that its blue and MeJ contrasts are comparable might indicate that the atmosphere is relatively clear above the GDS; that is, the amount of scattering above the top of the 3-bar cloud is low (corresponding to a lower optical thickness of the methane cloud). If so, there is a gradual change from the outer to inner regions of the GDS, with the clearest atmosphere occurring toward its center. Altematively, the negative contrast might indicate that the "3-bar" cloud is actually at a deeper level in the center of the GDS. With the exception of the "Scooter," the discrete bright features are probably methane condensation clouds. These features are prominent, localized, and dynamically active. In contrast, a photochemically produced haze is likely to be spatially uniform and inactive. Such a haze layer is visible at the limb in the images taken through the MeJ and MeU filters (the latter with center wavelength 541 nm) (Fig. 6) and was inferred from ground-based observations of center-to-limb brightness profiles (8). In the troposphere, methane gas is probably abundant enough to produce a prominent localized cloud. The deep (labeled H2S?) condensation cloud (Fig. 9) is not a candidate for features that are bright in the MeJ filter, because that cloud has too much absorbing methane gas above it to appear bright at this wavelength. Figure 7 shows D2 and the SPF visible on Neptune's crescent in all filters from UV to orange. Such visibility of features on the crescents was not apparent at Jupiter, Satumr, or Uranus, and this is consistent with the idea that the bright white features on Neptune extend to considerable heights above most of the haze and atmospheric gases. path optical depth exceeds unity (or the vertical optical depth exceeds several hundredths). On the basis of experience with similar images obtained at Uranus (11), we suspect that the transition to constant brightness (400 km relative alitude in Fig. 1LA) occurs within the bottom scale height of the stratosphere (80 to 100 km altitude in Fig. 9). Light scattered by a combination of aerosols and gas molecules is detectable to altitudes above 550 km in Fig. 11A at least 150 km above the constant brightness portion. The sharp increase in brightness with decreasing altitude just above the constant brightness region indicates the existence of particles in this region of the stratosphere, since the Vertical structure from limb scans and cloud brightness increases more rapidly than shadows. Images obtained at high phase an- would be expected from molecular Rayleigh gles provide evidence for the existence of a scattering alone. This point is further demstratospheric haze and also place constraints onstrated in Fig. 1 1B, which shows an on its properties. Such images are particular- extinction profile derived by inverting one ly effective for detecting the presence of of the limb scans of Fig. 11A (11, 13, 16). optically thin stratospheric hazes, since the The region with a rapid increase in the submicronmeter-sized haze particles prefer- extinction coefficient may mark the altitude entially scatter sunlight at small scattering where temperatures become cold enough angles (large phase angles). Particle visibility for gaseous ethane to condense into ice is also enhanced by the long slant paths particles (13). through the atmosphere that occur under Cloud shadows provided additional data these viewing conditions. Figure 1 1A shows about vertical structure. As shown in Fig. several radial line scans across the planet's 12, some discrete clouds that were observed limb. The scans reach a nearly constant brightness at lower altitudes where the slant- Fig. 3. High-resolution images of Neptune's largest discrete features. (A) In this high-resolution image of the GDS and its bright companion taken 45 hours before closest approach, the companion is resolved into many bright E-W linear streaks. [Image processing by C. L. Stanley] (B) Our highest resolution image of D2 was obtained during an IRIS observation on 24.4 I5 DECEMBER I989 August 1989. (C) This image taken on 23.3 August 1989 reveals the detailed structure of the "Scooter." It is composed of many bright streaks which vary on time scales of days, causing the "Scooter" to sometimes appear square or triangular (for example, see Fig. LA). REPORTS 1425 'v t~~ ~2 sis were consistent for observations made at both high and low phase angles (about 140 and 20 degrees, respectively), where the viewing geometry is quite diffcerent. Large-scale variability offeatures. All of the larger-scale features tracked during observatory phase exhibited latitudinal drifting associated with changes of wind speed (4). D2 exhibited the largest change of wind speed. When first measured, it was located at 550S and had a rotation period of 16.0 hours (Figs. 1 and 5). Then D2 drifted northward to 51°S, and its period increased to 16.3 hours. When the feature returned to 550S, the rotation period decreased to 15.8 hours. Twenty-five days after the start of the cycle, the feature again drifted north with an associated increase of rotation period (4). The range of rotation periods spanned by D2 indudes the 16.05 hour period of planetary radio emissions (3). The latitudinal drift of the "Scooter" was small in amplitude (less than 2 degrees) and was neither monotonic nor periodic. The feature was initially detected at 420S, where its rotation period was 16.74 hours; it remained at this latitude for more than three weeks. Then over the course of a week, the "Scooter" drifted north to 40°S, with a new rotation period of 16.76 hours. The "Scooter" then remained at this latitude for the remainder of the observations (Fig. 1A). The Great Dark Spot drifted steadily northward at a rate of about 0.110 per day during most of the encounter period. During this time, its rotation period increased monotonically from 18.28 to 18.38 hours. For comparison, Jupiter's Great Red Spot displayed sinusoidal motion in longitude with a 90-day period and peak speeds between -1 and 5 m s-1 (17). The Great Dark Spot exhibited a "rolling" motion around its circumference; the motion is anticyclonic and is best seen in timelapse sequences (Fig. 4A). The boundary between the darker blue of the oval and the lighter blue outside the oval (Fig. 1A) changed shape as if a two-lobed structure were rotating inside it. Every 10 or 11 rotations of Neptune the long axis of the structure pointed east-west, and the GDS reached its maximum longitudinal extent. This configuration appeared twice during each rotation of the structure, whose fundamental period is therefore about 21 Neptune rotations (21 x 18.3 hours at the latitude of the GDS, or approximately 16 Earth days). A rough estimate of vorticity within the GDS is 4'r divided by the fundamental period, or 0.9 x 10-5 s-1. In contrast, the vorticity of the ambient shear flow at the latitude of the GDS (assuniing a difference in velocity of 100 m s- over 10* of latitude) is 2.3 x 10-5 s-1. The vorticity of the GDS appears to be less than that of the ambient shear flow-the opposite situation from that of the Great Red Spot of Jupiter. The interpretation of the rolling motion as wind (that is, fluid motion) is ambiguous. For example, it may represent the propagation of a wave around the edge of the Great Dark Spot. Associated with the rolling motion, the Great Dark Spot exhibited other morphological changes. For example, dur- 4. Time-lapse sequences of the Great Dark Spot (GDS). (A) The "roling motion of the GDS can Flg. be seen in this 32-rotation ofclear-filter The to left o sequence images (top bottom, right). longitudinal extent of the GDS varies from 20 to 400 during a roll, while the latitudinal extent. can vary by 5°. The last three images are the first three images of Fig. 4B. Black regions on the right and in the upper righthand corners of some images are regions where no data were available. (B) This seveni-rotation sequence of violet images shows the dissipation of a large western extension into a "string of beads" which moves westward relative to the GDS. The first image was taken on 13.3 August 1989, and the rotation period of the GDS is about 18.3 hours. Each imagc is centered near 22°S and extends ± 15° north and south of this latitude. Each frame includes 60° of longitude. (C) These six images represent our best highresolution tine sequence ofthe GDS (top to bottom, left to right). The first image was acquired at 20.3 August 1989. The time interval between images is one rotation. In the last two frames, a subde dark band appears extending horizontally across the center of the GDS. Each image extends in latitude from 7°S to 370S and covers 600 of longitude. IS DECEMBER 1989 REPORTS I427 Fig. 6. False-color view of Neptune. This color-enhanced image was created with images taken on 22.7 August 1989 through the orange, MeU, and MeJ filters. The images were assigned to the blue, green, and red channels of the false-color image, respectively. High clouds appear red, while low clouds appear blue. The companion to the GDS is bright in all filters and is a high cloud, although its appearance here is distorted by saturation during image processing (as is the appearance of the northern cloud band near the western limb). The high-altitude haze layer shows up as a red region on the limb; the haze is transparent when viewed at normal incidence. Subtle shades ofblue and green may represent differences in altitude, optical thickness, or composition of the cloud particles. [Image processing by L. K. Wynn] it may have moved significantly over the last 3 years. If this is indeed the case, observers may have been detecting the Great Dark Spot companion as it drifted north and south, giving the Great Dark Spot a lifetime of more than 5 years. All ground-based images obtained prior to 1985 showed multiple bright features in both the northern and southem hemispheres (18). The Great Dark Spot was not detected in Fig. 5. Short-term variability in the core of D2. ground-based images obtained at 550 nm, These three images of D2 are taken at intervals of one rotation (about 16 hours), beginning at 23.0 but a future search at 400 to 500 nm may August 1989. They show the remarkably rapid have more success (see Fig. 10A). The Hubgrowth and dissipation ofdetail in the core ofD2. ble Space Telescope should have sufficient resolution and wavelength coverage to detect the Great Dark Spot or any similar features. Other cloud systems that have been ing one particular rotation, a series of dark correlated with features in current groundcloud features (a "string of beads") devel- based images (6) are the south polar feature, oped after the maximum western extension the core of D2, some ephemeral bright of the Great Dark Spot occurred (Fig. 4, B streamers at the latitude of the Great Dark and C). Spot and in the northern hemisphere, and Ground-based images have shown bright the bright band surrounding the south pole cloud features on Neptune for more than a (seen in MeJ images). decade (18). By comparing spacecraft imZonal velocity profile. Hammel et al. (4) ages with near-simultaneous ground-based discussed the rotation period and zonal veimages, we identified the discrete bright locities of the four largest features visible in feature seen this year in 890-nm ground- the Voyager images during the 80 days based images as the bright companion at prior to encounter. Here we discuss the 330S associated with the southern edge of motions of smaller features observed during the Great Dark Spot (Fig. 8B). The GDS the last few days before closest approach. If itself is not visible in the ground-based the small-scale measurements and large-scale images. The bright features seen in previous measurements were to disagree, we would years in ground-based images were located suspect either that different altitudes were at 30S in 1988 and 38°S in 1986 and 1987 involved or that the patterns move relative (18, 19). Since the Great Dark Spot appears to the wind, as with a propagating wave. At to drift in latitude by about 0.10 per day (4), Jupiter and Saturn, the winds derived from 1428 tracking small features over short time intervals generally agree with those derived from the larger features when allowance is made for the tendency of larger features to move with the average flow in a latitude band. The measurement of velocity seemed to converge with smaller spatial and time scales. This Fig. 7. Neptune's bright crescent taken in six filters (from bottom to top: UV, violet, blue, clear, green, orange) on 31.3 August 1989 (7). The images were shuttered in temporal order: violet, blue, UV, clear, green, orange. These images show the bright core of D2, the south polar feature, and the symmetric structure immediately surrounding the south pole. The relatively high contrast of the features in these images indicates that they extend above most of the scattering haze and absorbing methane gas in Neptune's atmosphere. [Image processing by D. A. Alexander] SCIENCE, VOL. 246 agreement suggests that cloud displacements are indicators of the wind at roughly the same altitude for both of these planets. On Neptune, the small-scale features evolve rapidly and disappear quickly, so the interpretation of cloud displacements is less straightforward (Fig. 4C). Because of the rapid evolution of small-scale features, the best measurement strategy must be a compromise. On the one hand, reducing the time interval over which displacements are measured reduces the effects of evolution of the features, and this ensures that they are recognized on successive time steps. On the other hand, the error in velocity goes up as the time interval is reduced. This error is proportional to the resolution in kilometers per line pair divided by the time interval (in other words, the speed of a feature that moves 2 pixels in one time step). By using sequences of three or more images, we can shift the focus from one rapidly evolving feature to another while broadening the time base. Although the error is reduced by this technique, there is still no guarantee that one is measuring actual fluid motion. Not all regions on Neptune have smallscale features suitable for tracking. At scales of 50 to 200 km per line pair, there are eight general ypes of disernible cloud structures; (i) structure in the polar cloud feature at71°S, (ii) variable brightening within the central region of D2 near 55°S, (iii) stnations in the "Scooter" at 420S, (iv) smal bright features similar to the "Scooter" that appear in the latitudinal range from 400 to 500S, (v) individual bright structures around the perimeter of the GDS, (vi) structure within the large-scale banding to the east of the GDS, (vii) details within the hazy bright equatorial patch located north of the GDS, and (viii) structure within the cloud bands at about 27N. Figure 13, A and B, gives the rotation periods and zonal velocities derived from both small-scale and large-scale features. Figure 13 reveals that measurements over different time intervals do not agree. The disagreement is particularly evident near 200S and 700S. The +'s and x's, for which the resolution per time step is less than 50 km per line pair, show less dispersion among themselves and better agreement with the solid curve. The diamonds, which use three or more images in a sequence, generally show less dispersion among themselves than the squares, which use image pairs only. The measurements of the south polar feature (70°S in Fig. 13) provide an example of how the dispersion arises. The measurements fall into three groups. Those with rotational periods near 16.0 hours are derived from data with 49-hour time separaIS DECEMBER 1989 tion. Those with periods near 17.5 hours have 16-hour separation, and those with periods from 12 to 14 hours are from images separated by less than 2 hours. This grouping suggests that the observations in the first set track the large-scale structure measured during observatory phase (4), while the observations in the short-interval group track small-scale features that move through the larger structures. Simple measurement error attributable to limited resolution is not the main source for the dispersion, particularly when three or more images are used in sequence. The dispersion is of order 300 m s-1, whereas the error is only 50 m s- at 70°S (Fig. 13B). Wind shear with respect to altitude is one possible source. However, the infrared interferometer spectrometer (IRIS) observations of temperature as a function of latitude (20) suggest that the vertical wind shear is small, on the order of 30 m s per scale height. For wind shear to be important, the structures must be distributed over a range of altitudes extending ten scale heights. The other possible source for the dispersion is propagation, either of the large-scale features or the small-scale features, relative to the fluid. One interpretation is that the diamonds in Fig. 8. Neptunes app e as a funion of waveilngth (7). (A) Six images of Neptume taken in six Voyager filters (left to righ top to bottom: UV, dear, green, orange, MeU, and MeJ) on 21.3 August 1989. (B) These three images, obtained on 24.1 August 1989, show violet- and blue-filter images along with a near-simultaneous ground-based image taken in the strong methane band at 890 nm (the ground-based image was shuttered approximately 4 hours later than the spacecraft images to account for light travel time). Contrast of the Voyager images was increased by a constant amount (a factor of -2.5). The ground-based image was stretched with a nonlinear function to enhance the doud feature relative to the disk. Quantitative measurements ofthe relative brightnesses of specific regions are shown in Fig. 1OA. REPORTS I4.29 Fig. 13 represent the true motion of the fluid, making Neptune the windiest planet in the solar system. Wind speeds would then be close to the speed of sound, which is about 560 m s-' at T-60K (Fig. 9). The solid curve might represent the slower motions at deeper levels or perhaps a large-scale wave. The other interpretation is that the diamonds and squares represent short-lived features that propagate relative to the flow. The solid curve, perhaps including the points at 5°N with periods near 19.5 hours and those at 27°N with periods near 17.5 hours, would then be our best estimate of the zonal wind. This is the most conservative interpretation, since the velocities of the large-scale features are the most certain. Regardless of the interpretation of the points in Fig. 13, the periods of rotation at the equator are longer than both the radio period (3) and the periods at high latitudes. This fact puts Neptune in a class, with Uranus and Earth, of equatorial subrotators. Venus, Jupiter, Saturn, and the sun are all equatorial superrotators, since the periods of the equatorial atmospheres are shorter than 160 those of the interiors. Neptune is by far the most extreme subrotator, as measured both by the speed of the atmosphere relative to the interior and by the fractional difference o Bright companion ^ GDS (outer region) GDS (central region) 4120 00 0.0- a 66 , x LAT= 42°S 0.20 - -0.4 i- 0.30 UV VIO 0.40 MeU ORA MeJ BLU 0.50 0.60 VIO 0.00 -UV v. MeU ORA MeJ BLU 3v 0.30 0.40 0.50 0.60 Wavelength (gm) Wavelength (gm) Fig. 10. (A) Contrast as a function of wavelength between discrete features and the nearby atmosphere. Contrast is defined as (IJlIb - 1), where If and Ib are the brightnesses of a feature and its surrounding background, respectively. For the Great Dark Spot, two regions were measured, the central region and the outer edges. The contrast shown here for the bright companion was measured in low-resolution images. In higher resolution images, the feature was resolved into thin streaks (Fig. 3A), each of which has even higher contrast than that measured for the overall feature. The Voyager filter passbands are indicated along the bottom axis. (B) Reflectivity as a function of wavelength for the main cloud deck at different latitudes. The three curves show the wavelength-dependent absolute reflectivity (I/F) at latitudes 22°S, 33°S, and 42°S. The regions are at the same latitudes but different longitudes as the GDS, its bright companion, and the "Scooter," respectively. The feature contrasts shown in Fig. lOA were measured relative to these values. There are no significant differences between the measured reflectivities at these latitudes. Small differences in the reflectivity ofother latitude regions give Neptune a banded appearance. The Voyager filter passbands are indicated along the bottom axis. 10-2 10 l ALAT = 220S ;;0.40 D- A=3° oLAT = 335S 0 //yC HH /X'/ C2H2 . 0.60 X Scooter 0.4 B v.Ov A 0.8 between the angular velocity of the interior and that of the equatorial atmosphere. In general, subrotation at the equator is easier to understand than superrotation. B e 0 2 .0 10 a. +>H 40 \ I 1000 s O...............40 60 [ . . . . I... 80 -X<(~~~~~~~~~?).i -... 100 Temperature (K) 120 ... 140 Fig. 9. Vertical aerosol structure of Neptune's atmosphere. The zero of altitude corresponds to the 4-bar pressure level. The existence of the main cloud deck near the 3-bar level was first inferred from ground-based spectroscopic observations of hydrogen quadrupole lines (9). The large tropospheric concentrations of methane gas derived from ground-based visible and infrared observations suggest the presence of a methane condensation cloud near the 1.2-bar level, but there is little direct observational evidence of a global methane cloud at this level. The bright cloud features seen in Voyager images may provide evidence for smaller-scale methane condensation clouds there. The existence of the high-altitude hydrocarbon aerosol layers was first predicted by theoretical models of Neptune's atmospheric chemistry (13). Brightness variations seen in limb scans of highphase-angle Voyager images also suggest the presence of these aerosols. 1430 195 10-4 500 400 Relative altitude (km) 600 550 500 450 400 350 Relative altitude (km) Fig. 11. Limb profiles of Neptune. (A) Radial scans across crescent images of Neptune's limb provide evidence for high-altitude stratospheric aerosol layers. These limb scans were extracted from a narrowangle clear-filter image taken at a phase angle of 158°. The altitude resolution is approximately 1.9 km per line-pair. Image motion compensation prevented the spacecraft's translational motion from significantly smearing this image. The constant-brightness region extends from the main cloud top to a point near the tropopause (400 km on this plot; the zero of altitude is arbitrary). Brightness variations above this level indicate the presence of aerosols. (B) The steep slope of the vertical aerosol extinction profiles derived from the limb brightness scans in Fig. lA indicates the presence of a discrete aerosol layer located almost 150 km above the tropopause (the stippled area encloses the uncertainty). This high-altitude aerosol layer may be related to the hydrocarbon condensation clouds predicted by theoretical models of Neptune's atmospheric chemistry (13). SCIENCE, VOL. 246 Without dissipation, rings of fluid circling the planet at a given latitude tend to conserve their angular momentum as they move from one latitude to another. Maintaining equatorial subrotation merely requires taking rings of fluid from higher latitudes and moving them to the equator. Maintaining equatorial superrotation requires some pumping of angular momentum into the equator by organized waves or eddies. The stability of a zonal flow depends not only on the velocity profile, but also on the density structure of the atmosphere. At the moment, we do not know the density field of Neptune's atmosphere over a sufficient range of depths and latitudes to do a comprehensive stability analysis. One necessary condition for stability, however, is that the angular momentum per unit mass, Qr2 cos2p, should decrease monotonically from equator to pole. Here Ql is the atmospheric angular velocity, r is the planetary radius, and up is the latitude. On Neptune, as on Earth and Uranus, the decrease of cos2wp outweighs the increase of Ql with latitude, and the condition is satisfied. This conclusion holds for all interpretations of Fig. 13 for which the equatorial period is less than 20 hours (21). One of the most remarkable aspects of Neptune's zonal wind profile is its similarity to that found on Uranus (1). That these two planets, which have such different internal energy sources and such different obliquities, should have the same pattern of zonal winds requires an explanation that will certainly add to our understanding of atmospheric dynamics. 1$ DECEMBER I989 Fig. 13. Rotation periods (A) and zonal velocities (B) as a function of latitude. Velocity is measured with respect to the 16. 1-hour period derived from the planetary radio emissions (3). The solid line represents the motion of the largest features over the longest time intervals (4). It is composed of nine individual measurements, two from groundbased observations and seven from the four largest features seen in the early Voyager images (3 of these were measured at two different latitudes). The symbols are measurements of individual small-scale features, often at time steps less than 2 hours and resolutions better than 100 km per line pair. The diamonds and crosses are values calculated with software developed at the University of Wisconsin in which displacements are measured in a sequence of three or more images. The squares and +'s use software developed at JPL in which displacements are measured in pairs of images only. The resolution (in km per line pair) divided by the time step (in hours) is less than 50 for the crosses and +'s and is greater than 50 for the diamonds and squares. The error estimates are in the same units as the figures and are computed from the statistics of the observations. 40 20 in G = o + vI -20 A 000 .p R + + c'o 0 - x 0 0 -40 -60 --VU 22 0 C)o 0 + 20 18 16 14 Period (hours) 40 122 )r1. Error .... I oo 20 ° B- ... ... ++°c )8-3 00 la S -20 o 0 c~~~~~~ -i 1 -700 -500 -300 -100 Zonal wind 100 (m/s) 300 0 140 Error tion, observed by more than one telescope, of either a small satellite or an optically Prior to the Voyager reconnaissance of thick, azimuthally incomplete ring of about Neptune, very little was known of its ring 80 km radial width. Several other detecsystem. By August 1989, about 50 stellar tions, two ofwhich were confirmed, indicatoccultations had been observed from ed narrower (approximately 15 to 25 km) ground-based observatories, representing but also discontinuous rings. Minimum in100 separate scans through the system. Al- ferred lengths were 100 km. Thus, the existhough more than 90% of these observa- tence of relatively narrow "ring arcs" orbittions yielded no detection, at least five oc- ing between 41,000 and 67,000 km from cultations observed between 1981 and 1985 Neptune was commonly accepted. demonstrated with high confidence the On the basis ofthese ground-based obserpresence of material in orbit around Nepvations alone, however, it was impossible to tune (22). distinguish between a family of permanent One of these observations was a detec- or transient arcs around Neptune or continuous rings of highly variable optical depth. Neptune's ring features became members of a sparsely populated class of ring structures including the narrow, azimuthally incomplete rings in Saturn's ring system [in the Encke gap, the Cassini Division, and around the F ring; see, for example, (23)] and possibly in the Uranian ring system as well (24). The presence of short evolutionary timeFig. 12. Cloud shadows in scales in ring systems is a well known and asthe northern hemisphere. Bright cloud bands near the yet unsolved puzzle (25). The time required terminator at latitude 27°N for longitudinally localized material 20 km appear to cast shadows on in radial width to spread 3600 as a result of the main cloud deck. Mea- differential rotation is only about 5 years. surements of these shadows indicate that the tops of Stable, non-transient ring arcs would obvithese cloud bands are 100 ously require a longitudinal confinement 50 km above the back- mechanism (26, 27); transient arcs require ground cloud deck. Similar the continual creation, dispersal, and replenshadows were seen for clouds in the south polar ishment of local concentrations of ring material. We present here the initial results on region. This image was created by combining violet-, the nature and dynamics of Neptune's rings green-, and orange-filter imfrom analyses of Voyager imaging observaages acquired near closest approach at 25.1 August tions, discuss the various theories suggested 1989. [Image processing by for the existence of arcs in light of our G. W. Gameau] findings, and compare Neptune's system The Neptune Ring System REPORTS 1431 with the other ring systems we see in the outer solar system. Radial distribution of ring material. The Neptune ring system, as seen in Voyager images, contains two narrow rings, 1989N1R and 1989N2R, at radial distances of 62,900 km and 53,200 km, respectively; a broad ring, 1989N3R, at a radial distance of 41,900 kin; a second broad ring, 1989N4R, extending outwards from 1989N2R to a distance of nearly 59,000 km; and an extended sheet of material that may fill the inner Neptunian system. (We will alternatively refer to 1989N1R, 1989N2R, and 1989N3R as the N63, N53, and N42 rings.) The N63 ring is outermost and includes three arcs of substantially greater optical depth than the ring average. The three arcs are clustered together within a total range of 33 degrees in longitude. 1989N1R and 1989N2R lie about 1000 km outside the newly discovered satellites 1989N3 and 1989N4, respectively. Neptune's rings were most easily visible at the moderately high phase angles obtained after closest approach to the planet, and three images at these phase angles best characterize the overall distribution of material. Figure 14, a 111-second exposure, and Fig. 15, a composite of two 591-second exposures taken 1.5 hours apart, were all imaged through the clear filter of the wide angle camera and show the ring system at forward scattering phase angles of about 1350. Figure 14 clearly shows the arcs which are 12 data number units (DN, out of a total range of 255 DN) above background. In comparison, Fig. 16 is also a 111-second exposure taken through the clear filter of the Voyager wide angle camera, but at a phase angle of only 15.50. It shows the same three arcs within the optically thin N63 ring; the second, even fainter N53 ring is also visible. Though Fig. 16 is one of the best Voyager images taken of Neptune's ring system at low phase, the average brightness in the arcs measured only about 2.5 DN above background. The arcs were not captured in Fig. 15 because of the 50 degrees of orbital motion between the two frames. However, one can easily see the N63 and N53 rings as well as Fig. 14. This Voyager 2 image (FDS 11412.51), a 111-s exposure obtained through the clear filter of the wide angle camera, shows the ring system in forward scattering geometry (phase angle of 1340). Clearly seen are the three ring arcs and the N53 and N63 rings. The direction of motion is clockwise; the longest arc is trailing. The resolution in this image is about 160 km per line pair; the trailing arc was imaged at this same time at a higher resolution of about 20 km per line pair (Fig. 18). Fig. 15. This pair of Voyager 2 images (FDS 11446.21 and 11448.10), two 591-s exposures obtained through the clear filter of the wide angle camera, shows the fill ring system with the highest sensitivity. The ring arcs seen in Fig. 14, however, were at an unfortunate orbit longitude and were not captured in either of these frames taken 1.5 hours apart. Visible in this figure are the bright, narrow N53 and N63 rings, the diffuse N42 ring, and (faintly) the plateau outside ofthe N53 ring (with its slight brightening near 57,500 km). [Image processing by L. A. Wainio]. 1432 Fig. 16. This Voyager 2 image (FDS 11350.23), aI11-s exposure obtained through the clear filter of the wide angle camera, shows the N53 and N63 rings faintly in backscattered light (phase angle of 15.50). Relatively prominent in the N63 ring are the three Neptune ring arcs. The satellite 1989N2 can be seen in the upper right corner, streaked by its orbital motion; the other bright object is a star. The arcs are unresolved in this image (resolution of 37 km per line pair); their apparent width is due to image smear. SCIENCE, VOL. 246 N42 (1989N3R). This latter ring, which is seen at low phase angles only with great difficulty, is clearly resolved and has a full width at half maximum of about 1700 km. Also faintly visible in Fig. 15 is a sheet of material beginning midway between the two outer rings at approximately 59,000 km and possibly extending down to the planet. (We discuss this further below.) There are identifiable features within this sheet. The most prominent among them is the plateau, 1989N4R. A distinct feature or ring at 57,500 km, 1989N5R, on the outer edge of the plateau, can be seen above and below the ansa in Fig. 15. In addition, there are hints of other radial structure in the plateau. Although one gets the impression from Fig. 15 that material extends continuously inward from 59,000 km, this is difficult to confirm because of the uncertainty in the distribution of scattered light. Figure 17 is a radial profile of the intensity seen in the left hand frame of Fig. 15. In this scan, a smooth function has been subtracted to remove the scattered light from the planet; this has the effect of tapering off the brightness distribution to zero at small radii. Though this removal of scattered light is uncertain, the N42 ring does appear to be embedded in material which extends out to the N53 ring, with brightness only slightly less than that of the plateau region but having a local minimum at 52,000 km, approximately the orbit of 1989N3. A similar configuration was observed in the Uranian system between Cordelia and the A ring 10 FDS 11446.21 z 95 30 ,,,,1,,,1,,,,1,, 40 ,, 60 50 70 Orbital radius (103 km) 80 Fig. 17. This radial profile of the brightness of the rings as seen in Fig. 15, in units of DN (data number) above background as a fimction of orbital radius, was obtained by averaging all points lying within narrow radial bins and excluding the stars. A smooth background has been subtracted out to account for glare from the planet; since the true background is uncertain, it is not possible to say whether the rings extend all the way in to the planet. High-resolution imaging coverage of the rings, excluding the arc retargeting that is discussed below, was made mostly at sky plane resolutions of 15 and 40 km per line pair. Due to the smear resulting from the long exposures, we have been unable to resolve in the radial direction either the N53 ring or the non-arc part of the N63 ring. Our highest resolution images, obtained within 13 hours ofclosest approach, were of the arcs and were retargeted approximately 4 days before closest approach, based on earlier imaging observations of arc locations and orbital motion. The single highest resolution frame, FDS (1986U1R) (1). In other high-phase frames, a narrow, 11386.17, has a sky plane resolution of only clumpy ring (as yet unnamed) is also visible 3.0 km per line pair. At the geometry of this just interior to 1989N1R. This feature is observation, radial foreshortening results in not seen in any low phase angle images, and a ring-plane resolution of 15 km per line appears to lie at about the same radius as pair. The trailing arc has a full radial width satellite 1989N4. Imaging observations of at half maximum that is close to the resoluthe arcs and the satellites within the ring tion of the camera. However, in our narrow region indicate beyond doubt that the direc- angle outbound retargetable image (Fig. tion of orbital motion is prograde. Because 18), measurements of variations in the ring of remaining uncertainties in the Laplacian width indicate that the ring may be just plane pole orientation, we cannot at the barely resolved, implying a radial width of present time completely rule out very small about 15 km. This value is in good agreering eccentricities and inclinations relative to ment with the groundbased and Voyager this plane, which presumably is identical to photopolarimetry stellar occultation meathat of the inner, regular satellites. surements (22, 29). Voyager observations have put strong Longitudinal distribution of material. On lonconstraints on the existence of a potential gitudinal scales of a radian, all three rings polar ring system (28); several high and low and the plateau region are continuous phase angle images of Neptune's north and around the planet. This is very clearly seen in south polar regions, covering a radial range forward-scattering geometry (Fig. 15), but out to several hundred thousand kilometers it is also evident, though with greater diffifrom the planet, reveal nothing. Although culty, in backscattered light for the N53 and the upper limiting optical depth for broad N63 rings. On intermediate scales, the only sheets of material is roughly 10-5, it is still prominent structures are the three bright impossible to rule out the existence of nar- arcs seen in the N63 ring (Figs. 14 and 16). row clumps in polar orbits with somewhat The azimuthal lengths of these arcs, meahigher optical depths. sured to be the distance between the half15 DECEMBER I989 Fig. 18. This narrow angle, clear filter image (FDS 11412.46) was part of the outbound (high phase angle) retargetable sequence along with Fig. 14 and shows a portion of the trailing arc. The sky plane resolution is about 14 km per line pair and the phase angle is about 1350. The combination of uncompensated spacecraft motion and ring orbital motion causes points to streak in the image. However, the width and separation of the streaks reveal the scale size and longitudinal distribution of clumps in the rings. In addition to the fine scale information revealed in the streaked clumps, this image seems to show width variations which would imply that the arc may be resolved (see text). intensity points in azimuthal brightness scans in Fig. 14, are approximately 40, 40, and 100 for the leading, middle, and trailing arcs, respectively. The distances between the midpoints of these features are about 140 (leading-middle) and 120 (middle-trailing). To within the measurement uncertainties, the same values are obtained from low phase images. Given the signal-to-noise ratio of the data, we find no convincing evidence at this time for longitudinal structure on scales greater than about 50 in either of the two remaining rings or in the remainder of the outer ring in either forward or backscattering geometry. Small-scale azimuthal structure is most easily seen in the retargeted image of the trailing arc taken at high phase (Fig. 18). Several long linear features are apparent, which we believe are formed by discrete clumps in the ring, trailed out by a combination of orbital motion along the ring and the motion of the spacecraft across the ring. REPORTS I433 These features are unresolved and appear to and 57,500 km from Neptune. In order to be associated with microscopic particles be- suppress the effects of possibly variable cause of their enhanced brightness in for- smear in the long exposures that were ward scattered light (phase angle of 135 used, the radial integral of the brightness degrees). One of the retargeted images taken profile, or "equivalent width," was emof the trailing arc at low phase shows struc- ployed (33). ture similar to that seen in Fig. 18 when the Figure 19 shows the observations for the image contrast is strongly enhanced. different regions, converted to the quantity Though it is not yet certain if these are the (,PfTdr, as functions of phase angle (33). same clumps seen in Fig. 18, it is notewor- The fact that all regions are more reflective thy that the typical separation between these at high phase angles is direct evidence for a clumps, approximately 0.10 to 0.20, is the substantial dust population, since the same in both frames. A similar object within brightness of macroscopic objects depends the leading arc is seen in another low phase primarily on the fraction of the visible illuimage not reproduced here. These features minated area and decreases by an order of may be large embedded ring particles or magnitude as phase angle increases over this associated clumps of debris similar in mor- range. We have modeled the particle properphologv to the discrete features found in the ties in these regions to obtain the optical F-Ring or Encke gap ringlet of Saturn (23). depth of macroscopic and microscopic maMoon/let search. A search for additional terial, using assumptions as to the individual small satellites orbiting near the rings of particle properties based on prior experiNeptune is still being conducted (30), but ence. We have assumed that the large macrothe lack of confirmed sightings implies that scopic particles in the Neptune ring system few or no additional satellites larger than 12 have the Uranus ring particle phase function km in diameter with assumed geometric and a Bond albedo between 0.01 and 0.02, albedo of 0.05 are orbiting in the ring which brackets the Uranus ring particles, region. To discriminate actual sightings Phobos, Diemos, and Amalthea. This comfrom noise, we required each candidate object to be in a circular, equatorial orbit. Because of this requirement, our limiting radii are roughly twice as large for satellites arc/3 with orbits inclined by more than 100 or eccentric by more than 0.1. 0.10 Ritng photometry atnd particle properties. The ability of spacecraft to observe a system over a range of viewing angles is of great importance to our understanding of planetary ring 0.08 systems, since the scattering behavior of particles of microscopic and macroscopic sizes differs dramatically with phase angle. N53 Observations at high phase angles (>1500) 0.06 have been important in establishing that the microscopic "dust" particle fractional area in the main rings of Saturn and Uranus is quite < 4 N63 small-between 0.01 and 0.001 by area 0.04 PL (31). In other rings, the dust fraction can be considerably larger; for example, Saturn's F ring, a narrow, clumpy ring, and Saturn's E N42 ring, a broad, diffuse ring, are both visible 0.02 primarily because of microscopic particlesdust fractions greater than 80% (32). In the case of Neptune, the spacecraft trajectory I I II did not allow any extremely high phaseon 50 100 0 150 angle observations; however, the rings Phase angle (degrees) themselves compensated for this lack of observational sensitivity by being extremely Fig. 19. In this figure, observations of the radially a~- "dusty." Our preliminary photometric analysis relies on only twvo phase angles (approximately 140 and 1350) for the following regions: the middle arc of 1989N1R, wide azimuthal averages in the N42, N53, and N63 rings (excluding the arc material) and the constant brightness region ranging between 54,500 '434 integrated brightness of different regions of the Neptunian rings are shown as functions of phase angle. The brightness has been converted to the "equivalent width" product .a0PfTdr, in units of kilometers, where fTdr is the "equivalent depth" and is the normal optical depth. The regions are the N42, N53, and N63 rings (not including the ring arcs), the plateau region averaged between 55,000 and 58,000 km, and the middle ring arc (plotted at one-third of its actual values). T bination results in a geometric albedo of about p = 0.05, similar to that seen for the newly discovered Neptune satellites. To bound the properties of the microscopic dust component, we chose coal dust (which is used to model the Uranus rings) and a typical silicate (which is used to model the Jupiter ring). Mie scattering, as modified by a simple irregular particle algorithm, was used to obtain the range of dust particle albedo and phase functions for material with this range of composition. We assumed a power law size distribution with index of 2.5, such as has been observed to characterize the dust in the Uranus and Jupiter rings; microscopic dust in ring systems tends to have a somewhat flatter size distribution than typical comminution products due to the size dependence of removal processes. The results of this preliminary photometric modeling are shown in Fig. 20. The N42 ring and the plateau are clearly low optical depth structures, although two orders of magnitude more substantial than the Jupiter ring or the E and G rings of Saturn and about one order of magnitude more substantial than the Uranus dust bands. The optical depth of the middle arc, about 0.04 to 0.09, is in excellent agreement with ground-based values when appropriate diffraction corrections are made. Within the uncertainties, the N42, N53, and arc regions have the same dust fraction (0.5 to 0.7), which is about twice as large as the fraction found in the N63 ring and the plateau regions, and significantly larger than found in the main rings of Saturn or Uranus (10-3 to 10-2). Comparison ofground-based and Voyager observations. Re-analysis of the geometries for the most reliable ground-based stellar occultation tracks by means of the improved Voyager Neptune pole position has indicated that the observations of 22 July 1984, 7 June 1985, and 20 August 1985 are all consistent with occultation by material near the radius of 1989N1R (34). These are also the only three observations which yield equivalent widths of up to 2.0 km, consistent with the abundance of material that we find in the three Voyager arcs (Figs. 19, 20) when diffraction effects are appropriately accounted for. Although this suggests that the Voyager arcs are the same as those observed from Earth in 1984 and 1985, a much more stringent test of this hypothesis, and one that could rule out the notion of transient and evolving arcs, would be the successful prediction of longitudinal location at the time of the Voyager encounter by use of the observed arc locations in both ground-based and Voyager observations and the mean motion derived from Voyager images. SCIENCE, VOL. 246 A precise determination of the arcs' mean motions must necessarily await careful arc position measurements in images spanning as large a temporal range as possible, and the fitting of such measurements with a general model describing an eccentric and inclined orbit, as well as an evaluation of the pole of Neptune's Laplacian plane, which is independent of the assumed ring arc model. A preliminary value was obtained, however, by assuming the arcs' orbits to be identical, circular, and equatorial [Neptune rotational pole orientation, (a, 8)1950 = (298.904, +42.841)], and by measuring the beginning and ending longitudes of each arc in smoothed, radially averaged, azimuthal scans taken from at most five images spanning a total of about 6.5 days. For each of the three arcs, a weighted least-squares fit to the measured locations versus time was used to determine the mean motions; the final value and its uncertainty, 820.12 ± 0.06, are the mean and the standard error of the mean, respectively, of these three numbers. Using this value, we projected forward from the longitudes of arc detection in the three reliable ground-based stellar occultations in the N63 region mentioned above (35), correcting for light travel time, to the epoch of our best single image (FDS 11412.51, Fig. 14): for 22 July 1984, the precessed longitude is 227°; for 7 June 1985, 238°; and for 20 August 1985, 2240. The large uncertainty in the rate, 0.06° per day, maps into a longitude uncertainty at the time of Fig. 14 of approximately ± 1000. Nonetheless, the agreement found with the use of the nominal rate is astonishingly good: the groundbased observations fall within about 150 of each other and comfortably intercept the longitude range, 2060 to 2400, subtended by the three arcs in Fig. 14. (Although the ground-based longitudes are measured in the Neptune-Triton invariable plane, and the Voyager longitudes in the Neptune equator plane, the difference amounts to at most a few degrees.) We consider these results convincing evidence that the Voyager arcs themselves were the occulting material for all ground-based occultations at this radius and, therefore, that the arcs are stable over intervals of at least 5 years. Future work in this area, taking into account the geometrical considerations mentioned above, may in fact allow us to determine with high probability which of the three arcs was observed in each of these three ground-based occultations and to predict the locations of these features at the times of upcoming Neptune stellar occultations observable from the ground or from the Hubble Space Telescope. It is not surprising that ground-based observations, in general, do not detect material in the noniS DECEMBER i989 Fig. 20. In this figure, we have estimated the total optical depth and fraction in microscopic dust particles from the data of Fig. 19, making certain assumptions as to the phase function and albedo of the ring particles. The total normal optical depth, plotted vertically, has been obtained from the equivalent depth by dividing by a physical width of 15 km for N53, N63 and the arc, by the 1700-km full-width at half maximum for N42, and by the 3000-km width of the plateau. The fraction of this total represented by "dust" is plotted horizontally. In all cases, the patches are bounded on the lower right and upper left by large particle single scattering albedos of 0.02 and 0.01 respectively, and on the upper right and lower left by microscopic material composed of coal and of silicates, respectively. The large particle phase function and the dust particle size distribution are those ofthe Uranus ring particles. Although these specific values are clearly modeldependent, the relative differences between the regions (that is, corresponding corners of the patches) are real. arc regions of the N53 and N63 rings-the estimated optical depth of 0.01 to 0.02 in these regions is below the ground-based threshold. However, it is worth noting that (unconfirmed) ground-based stellar occultation observations, revised by using Voyager improvements for the Neptune pole orientation (35), indicate events at around 42,000 kilometers and 55,000 kilometers-opening the possibility that clumpy material may now, or did then, exist in regions where we now observe broad, diffuse belts of material (see Fig. 21). One other important result concerns the identity of the object that was responsible for the first occultation observation of material orbiting Neptune (22). Using the mean motions for the small satellites determined from imaging observations, we obtained their positions at the time ofthe 1981 event. Since this event did not occult the planet, some astrometric uncertainty remains in addition to the uncertainty in the mean motion of the candidate satellites. However, we find that 1989N2 falls within the total uncertainty (about 8° of orbital longitude or less than 1 arc second) of the location of the event, and that the only other possible candidate (1989N4) is on the other side of the planet. This excellent positional agreement, combined with the fact that the 1981N1 occultation was completely opaque and 180 km across and 1989N2 has a diameter of about 200 km, makes us confident that 1981N1 and 1989N2 are one and the same object (Fig. 21). Discussion of ring observations. Despite the singular nature of Neptune's system of relatively large satellites (in particular, retrograde Triton and highly eccentric Nereid), and despite the dramatically different visual impression that Neptune's rings give in comparison with those of Jupiter, Saturn, II Ill 0.1 E. 0.01 0 I0.001 : 0.0001= (1- o3km) I I I 0.00001 0.1 '5 1 Fraction "dust" Groundbased Voyager 2 7 __-41989 N2 71 Radius (10Okm) * 61;0 50O __ __ __ __ _ 1989 N4 01989 N3 *1989 N5 01989 N6 40 Fig. 21. This figure shows the radial locations of the most reliable ground-based occultation detections of material around Neptune compared against the locations of ring material as seen in Voyager imaging data. Asterisks denote groundbased observations confirmed by more than one telescope. and Uranus, the combined system of rings and inner satellites has surprised us by sharing many characteristics with the other giant planet ring-satellite systems. To wit, Neptune's rings comprise an extensive prograde system, essentially confined to the planet's equatorial plane and filling the planet's Roche zone, a region lying between the classical stability limit for a liquid satellite [r/ Rp = 2.44 (Ps/Pp)033, where Ps and pp are the densities of the satellite and the planet, respectively, Rp is the planet's radius and r is the distance from the planet's center] and the "accretion limit" at which equal-sized particles are destabilized by differential Keplerian motion [1.44 (Ps/Pp033)] (36). It is a system containing narrow dusty REPORTS I435 Table 1. Small satellites of Neptune, where a is the semimajor axis. a Satellite (10+3 km) Nereid 1989N1 1989N2 1989N3 1989N4 1989N5 1989N6 551 117.6 73.6 52.5 62.0 50.0 48.0 Mean radius (km) 170 ± 200 ± 95 ± 75 ± 90 ± 40 ± 27 ± 25 10 10 15 10* 8* 8* Geometric albedo Best resolution (km per pixel) 0.14 ± 0.035 0.060 ± 0.006 0.056 ± 0.012 0.054 ± 0.024 43.3 1.3 4.1 16.9 18.4 17.4 23.6 *Assumes albedo equivalent to 1989N1 and 1989N2. rings, like the Uranus X ring (1986U1R) and the Saturn F ring; diffuse dusty rings, perhaps similar to the Jupiter ring and Saturn G ring; azimuthally confined arcs embedded with a ring, reminiscent of Saturn's F and Encke rings; and possibly a broad sheet of dust like that which is seen around Uranus at high phase angles. An examination of Neptune's system of satellites and its distribution with orbital radius (Table 1) supports the generality that, as the distance from a giant planet decreases, there is a gradual transition from large, isolated satellites to families of more numerous, smaller satellites and ring material. The presence of relatively massive objects (for instance, satellites at Neptune, rings at Saturn) well within the outer planets' Roche limits, where structural stability depends on internal strength but where accretion to radii of tens of kilometers is difficult or impossible, supports the idea that the parent objects of these outer planet ring-satellite systems migrated into their respective Roche zones from elsewhere long ago (37). In comparing only Uranus with Neptune, we find that the reflectivities of the surfaces of their inner satellites are similar and very low. Also, the agreement between our derived Neptune ring arc optical depths and those obtained from ground-based stellar occultation measurements supports equivalently low albedos for the ring particles, comparable to those found for Uranus. Compositionally, therefore, the Neptune and Uranus ring-satellite systems appear to be quite similar, suggesting chemical origins and/or evolutionary histories that proceeded in tandem, histories that may well characterize the outer solar system beyond Saturn. However, the dramatic differences also call for our attention. The amount of mass in Neptune's rings is approximately 10,000 times less than that at Uranus and many orders of magnitude less than that at Saturn, yet the inner Neptune satellites are significantly larger, and presumably more massive, than the bodies in similar locations in the ring-satellite systems of the other giant planets. At Neptune, the five satellites 1989N2 I4-36 through 1989N6, with diameters ranging from approximately 55 to 190 kin, all fall within the Roche "liquid" limit at roughly 77,000 km; at Uranus, the nine satellites falling within its Roche limit, 1986U1 through 1986U9, range from about 25 to 110 km in diameter (38). If we assume the overall satellite-size distributions to be similar among the outer planets, the relatively large number of big bodies close to Neptune might lead one to expect a proportionately large abundance of smaller moonlets in the ring region. Yet, our search for satellites to date does not support the existence of more than two objects (1989N5 and 1989N6) with diameters less than 100 km. Evidently, the distribution of mass among the inner satellites of each planet, and between each planet's satellites and rings, is very different. The absolute amount of mass within these zones is also notably different: All the mass in Saturn's rings, which fill Saturn's Roche zone, can be contained within an icy body approximately the size of Mimas, 195 km in radius; in Neptune's Roche zone, the rings' and satellites' masses can be contained within an icy body 130 km in radius; and for Uranus, within an icy body 75 km in radius. It is interesting to compare these differences with the variation in present-day cratering rates on the inner satellites of these three planets: The ratio for Saturn/Neptune/Uranus is roughly 3/50/100 (39). It would appear that where bombardment is greatest, there is less overall mass. However, the present-day differences in the distribution of mass around the giant planets may reflect, in part, the varying degrees to which bombardment and collisional processes have combined to shape their ringsatellite systems. The presence and distribution of dust in these systems may provide a direct indication of the relative importance of these processes today. The relatively large number of microscopic particles spread throughout the Neptune rings (Fig. 20) is not unique; the Jupiter ring and the Saturn E ring contain a fractional optical depth of 50% to 80% in dust. However, the absolute abundance of dust in the Neptune system, which is about two orders of magnitude larger than the Jupiter and Saturn counterparts, presents a serious problem. Because microscopic particles are very short-lived (40), they must be continually replenished. When material is in a state of dynamic balance, equilibrium abundances are maintained by equal rates of creation and destruction. Different removal processes dominate in different environments: In the Uranus rings, microscopic material is removed primarily by gas drag (41) and, in the Jupiter rings, by plasma drag (40). In the Neptune rings, in which dust has a relatively large optical depth (approximately 10-4) and which are relatively free of plasma or neutral gas, simple sweep-up on the surfaces of macroscopic particles dominates dust removal. If the source of the dust is meteoroid bombardment, as believed for the Jupiter ring, the creation and removal processes are both proportional to parent body optical depth. Consequently, the dust optical depth resulting from meteoroid bombardment is independent of the large particle optical depth and depends instead on the meteoroid flux and impact yield parameters (40). For heliocentric distances less than 15 to 20 AU observed by the Pioneer 10 and 11 dust detectors, a value of interplanetary meteoroid flux of about 10-16 g cm-2 s-' is generally accepted. Given this value and ejecta yields of about 104 [see, for example (42)], "Jupiter ring" dust optical depths on the order of 10-6 are easily obtained. However, dust optical depths in N42 and the plateau (and the Uranus dust bands) are around 10-4 (Fig. 20), requiring a bombarding flux roughly two orders of magnitude larger than found at Jupiter and Saturn. This larger dust abundance is qualitatively consistent with the previously mentioned larger estimated projectile population at Uranus and Neptune. Although the estimate falls short by a factor of 3 to 10, this may be within the uncertainty in the Neptune dust optical depths and in the estimated projectile populations. However, the dust within the N53 and N63 rings is orders of magnitude larger than can be explained by the meteoroid bombardment mechanism. Therefore, we suspect that it is most likely generated locally through vigorous collisions between larger, unseen particles. In this mode, the equilibrium dust optical depth depends in a more complicated and model-dependent way on the optical depth of the parent bodies that create it. The dust optical depth does tend to increase with that of the colliding parents as long as the latter is much less than unity and the mass injected per collision is a constant. This creation of dust through interparticle collisions, assuming relative velocities conSCIENCE, VOL. 246 sistent with radial excursions as large as the observed ring widths, may explain the generally large dust abundance in the Neptune rings, given reasonable assumptions about the yield per impact (43). The brightening at 57,500 km (1989N5R) near the outer edge of the plateau is one Neptune ring feature which, by analogy with Uranus, may be the manifestation of a moonlet belt (1). The lack of other noticeable fine structure, in contrast to the nearly 100 belts revealed in the Uranus rings or the internal structure of the Saturn D ring, may be due to long exposure times of the images and the corresponding smear rather than to actual absence. Of course, greatly increased optical depth will diminish the dust population because the attendant large collision rate tends to damp the relative velocities. For instance, in the large optical depth rings of both Saturn and Uranus, the dust fraction is extremely small. For this reason, it is not clear whether the amount of dust in the Neptune arcs is consistent with a relatively simple moonletbelt model or whether additional stirring of the ring arc material by unseen perturbing bodies is implied. Nonetheless, it appears overall that the moonlet-belt hypothesis that has been proposed for the Uranus rings (1) and recently modeled in more detail (44) may go a long way toward explaining some of the global characteristics seen in the Neptune ring system. While it seems clear that the 1989N1R arcs are stable over an interval of at least 5 years, a search for dynamical relationships between the Neptune rings, ring arcs, and the new satellites found within the ring region has demonstrated that none of the current hypotheses based on the combined action of satellite corotation and Lindblad resonances can explain the persistence of the arcs. Confinement of the ring arcs through a combination of corotational and Lindblad resonances with a single satellite (27) can be easily discounted on several grounds: (i) 1989N4 has zero inclination and eccentricity to within measurement uncertainties, thereby rendering it incapable ofcorotationally shepherding 1989N1R; (ii) the 3:2 corotation resonances of 1989N6 fall some 250 km outside 1989N1R, and the expected scale for this resonance (about 60°) is too large; (iii) the 50 inclination of this satellite is insufficient to allow it to confine a ring arc 15 km wide, given its radius of 27 km and a reasonable assumed density. The Lagrange point satellite shepherding model (26) can be discounted because no Lagrange point satellite of sufficient mass has been found. The only relationships that hold some promise for being significant are the standard outer Lindblad-type resonances of 1989N4 on the N63 ring and 1989N3 on the N53 15 DECEMBER I989 Pluto v 2.0 Ail Umbriel 4 t Dione Enceladus o Rhea T E 0, cm Miranda- 1.0 Mimaso_ Titania Tethys vTriton Ganyrmede Callisto' Ttan a n _lpt 0 0 0.51 100 Radius (km) 1000 ring, which might account for the locations ofthe inner edges of these rings in much the same way as Cordelia shepherds the inner edge of the Uranus e and probably X (1986U1R) rings (45). Though both inertial and acoustic waves in Neptune were proposed as a possible mechanism for the azimuthal confinement of arc material, these mechanisms can now be discounted because the required planetary wave amplitudes would have to be impossibly large to produce arcs with the observed longitudinal scale of 120 (46). Moreover, acoustic mode corotation resonances at Neptune do not fall outside 28,000 km (47). At the present time, therefore, there are no theories explaining ring arcs that are verified in Voyager imaging data. The effort to date that has focused on the details of a longitudinal confinement mechanism for the arcs, while now clearly justified, does not address the larger question of the origin of rings and ring arcs themselves. This question will require studies of catastrophic disruptions and the subsequent dispersal and distribution of collisional fragments, as well as the study of the coupled behavior of ensembles of moonlets and ring material, both under the influence of a variety of ongoing processes, like tidal evolution and meteoroid bombardment, which continually sap orbital energy and angular momentum from the system. However, knowing as we do now the basic properties of the four ring-satellite systems of the outer solar system, we can begin to explore comprehensive models of their undoubtedly complex evolution. The Satellites of Neptune Prior to the Voyager 2 encounter, Neptune's known satellite system consisted of one large retrograde satellite, Triton, a smaller satellite, Nereid, in a direct but highly eccentric orbit, and the tentatively identified satellite (22) in the vicinity of the ring arcs. Triton was discovered in 1846 by Lassel, immediately following the discovery Fig. 22. Comparison of radius and observed mean density for outer solar system objects. Filled circles are satellites of Jupiter, open circles satellites of Saturn, triangles satellites of Uranus, and inverted triangles Pluto and Triton. The upper curve represents a simple compression model for a satellite with 60% water ice and 40% silicate (53); the lower curve is a pure waterice model for comparison. of Neptune; Nereid was not discovered until the 20th century by Kuiper in 1949. Unsuccessful satellite searches using modem CCD cameras from ground-based telescopes placed upper limits on the diameters of additional satellites. These limits ranged from about 40 km at 15 Neptune radii (RN) up to about 2000 km at 3 RN (48). Of the six newly discovered satellites, two (1989N1 and 1989N2) were imaged with sufficient resolution to study their surfaces. Its proximity to Neptune and low relative brightness make ground-based observations of Triton difficult; many of its fundamental properties (diameter, albedo, mass, atmospheric pressure) remained obscure prior to Voyager's encounter (49). Spectroscopic studies established the presence of methane (CH4) ice or gas (or both) but showed no water-ice absorption features which are so prominent in the spectra of most outerplanet satellites. These studies also tentatively identified molecular nitrogen (N2) in the gaseous and condensed states on the basis of a weak absorption band at 2.15 ,m (50). Triton's physical properties. Triton's radius (1350 + 5 km), determined from limb measurements on the Voyager images, and mass, obtained from analysis of radio tracking data (14), yield a density of about 2.075 ± 0.019. With the exceptions of the rocky satellites Io and Europa, Triton's density is the highest observed for an outerplanet satellite and is very similar to that of the Pluto/Charon system; see Fig. 22 (51, 52). If these bodies are assumed to be composed chiefly of silicates and water ice, the rock/ice mass fractions can be derived from the bulk density by using interior structure models (53). Two extreme cases are examined here: differentiated (silicate core and ice mantle) and homogeneous (uniform mixture ofsilicate and ice or clathrate). A silicate density of 3.361 g cm-3 was used (54); a "chondritic" value of 3.6, used in some other studies, would result in slightly lower silicate mass fractions than those quoted here. In the case that Triton was melted and REPORTS 1437 Table 2. Triton interior model parameters, where P, is the model central pressure in bars, Xjj is the silicate mass fraction (assuming a model uncompressed silicate density of 3.361 g cm-3), and Pu is the model uncompressed density of Triton in grams per cubic centimeter. would be reached at a depth of only 200 to 300 km; the melting point of water ice would be near the core-mantle boundary. Convective heat transport in sub-solidus Silicate core Silicate core Undifferentiated Ice-I mantle Ice-I/Ice-II mantle Silicate + Ice-I + Ice-II convection would cool the interior more rapidly than conduction alone, and more 20360 20150 11290 PC complicated models need to be considered. 0.75 0.69 0.65 Xr,ii Local topographic relief on cliffs, ridges, 2.01 1.83 1.74 Pu knobs, pits, and craters commonly exceeds -1 km over most of Triton's surface. This completely differentiated, it would have Triton's extensive resurfacing is also incon- implies that a rigid material, which would formed a silicate core approximately 1000 sistent with an inactive, homogeneous inte- not flow at the 40 to 50 K near-surface km in radius overlain by an Ice-I mantle rior. temperatures over billions of years, is reapproximately 350 km thick (Table 2, colTriton's estimated silicate fraction is high- quired to support them. The rheologies of umn 1). As the interior cooled, a layer of er than those of the large icy satellites of solid N2 and CH4 are not well known at Ice-II may have developed due to the very Jupiter, Saturn, and Uranus, but similar to these temperatures but it seems unlikely that low surface temperature. The existence and that of the Pluto/Charon system; see Fig. 23 a 1-km-high cliff could be supported over thickness of such a layer strongly depend on (53). This is consistent with the hypothesis geologic time in a material held at half to the details of the Ice-I/Ice-II phase bound- that Triton was formed in the solar nebula two-thirds of its melting temperature. Such ary, poorly known for these low tempera- and subsequently captured by Neptune (55). relief could easily be supported in water ice tures. Ice-II also will not be present if These high densities have important impli- or water-dominated ammonia-water ice, the ammonia hydrate/water eutectic is cations for the carbon chemistry of the outer however. We suspect that water ice is in fact reached and some of the interior is partially solar nebula. If the nebular carbon were the primary component of the near-surface molten. Column 2 of Table 2 gives the mostly in CO gas, the H20 abundance crustal materials overlain by thin veneers of characteristics of models for a mixture of would be depressed relative to that of the nitrogen and methane ices and their derivaIce-I and Ice-II by means of the phase silicates (53). Modifications to the simple tives. relation given by Lupo (54). A homoge- concept of CO-rich versus CH4-rich nebulae Triton spectrophotometry. Clear-filter, narneous undifferentiated model (Table 2, col- may be required, however, in response to row-angle images acquired during the last 2 umn 3) would result in a layered structure of recently suggested revisions of the cosmic weeks of approach were analyzed to detersilicate mixed with Ice-I, Ice-II, and Ice-VI carbon abundance (57) and to the lack of mine Triton's light curve (Fig. 24). Integralin the deep interior. detected CO on Triton or Pluto. disk brightness measurements were scaled to The differentiated models are clearly the Another important aspect of these models constant distance from Triton and corrected most plausible. Triton is large enough that is the relatively high interior temperatures for the disk-averaged phase curve described radiogenic and accretional heating alone expected, even at the current epoch. If con- below. Confirmed by preliminary geodetic likely resulted in differentiation. In addition, ductive heat transport alone is considered, control measurements, these data indicate if Triton is a captured satellite, tidal evolu- with the radiogenic heat produced by a synchronous rotation. A Cassini state 2 rotation of its orbit also would have produced 2000-km-diameter chondritic core, the eu- tion state (rotation axis not perpendicular to significant heating and melting (55, 56). tectic melting point of ammonia hydrate the orbital plane) was suggested as a theoretical possibility based on dynamical arguments (58) but is ruled out by the data. The 0.8 light curve agrees well in magnitude and Triton Pluto/Charon CO-Rich sense (leading side brighter-90° longitude, ,1"",,, Uste,........................ Figs. 24 and 25) with broadband V filter 0.7 Ft * ~~Satellite system averages measurements, but telescopic measurements Titania at 890 nm show no light curve larger than I Oberon about 2% (59). This suggests that the surAriel/Umbriel c 0.6 V Uranus differentiated face contrast at 890 nm is less than at visible ________\\\\\\>\\\\X co Jupiter wavelength and implies that the chromoU) 0 phores (possibly silicates or hydrocarbons or LII ~~~~~~~~~~~~~Saturn con 0.5 V both) responsible for albedo and color variaUranus Homogeneous Rhea E 0 tions on Triton are more absorbing at shorter wavelengths. Miranda * 05 0.4 Triton was imaged at phase angles beMd ~~~~Saturn CH4-Rich (object average) tween 12 degrees and 156 degrees through j Mimas the violet, green, and clear filters. These data 0.3 p are combined with ground-based V bandI L Leaend- __ niffrnfntei *Bysullu. IVR;Homogene Io U mril uu 11PI III pass data (similar to the green filter in effective wavelength) in Fig. 26. Also shown Homogeneous model 2 are fits to the green and violet data using Fig. 23. Comparison of model silicate mass fractions for several outer planet satellites and the Hapke's photometric model; see (61) and Pluto/Charon system (53, 54). Two types of satellite interior structure are given, a fully differentiated Table 3. The phase curves display remarkbody with a silicate core and an ice mantle, and a homogeneous undifferentiated mix of water ice and silicates. Radiogenic heating is taken into account in the current thermal structure of the interiors. Also able wavelength-dependent differences. At shown are approximate values for silicate mass fraction for bodies formed in CO-rich and CH4-rich small phase angles Triton is brighter in the nebular conditions and the system averages from several satellites as compiled in (51). green filter than in the violet; at large phase I 0 I jt as ane(mas average) U *1 1438 I U dl SCIENCE, VOL. 246 Table 3. Triton photometric parameters. The V band value is from a ground-based measurement (60). 0 Violet 0.61 1.5 0.90 0.999 +0.16 30 Filter p q A w g influenced by atmospheric scattering than are the violet data and because both groundbased and Voyager-imaging observations can be used to constrain global photometric parameters. redder areas may be contaminated with darker, redder material. The reddest areas measured are in the plains north of the bright collar that extend all the way to the terminator in the north. Even Triton's "darkest" regions are in actuality quite bright in comparison to most outer-planet satellites. Even the "dark" streaks show normal reflectances in the range of 0.40 to 0.75. Only a few small spots in the northern plains have normal albedos with values as small as -0.20. One explanation for Triton's reddish color is that the surface is dominated by a physical mixture or solid solution of methane in nitrogen and that organic polymers produced by photolysis and charged-particle bombardment of methane are responsible for the red coloration (69). Although alternative explanations do exist (for instance, that the coloring agent is derived from primordial organic material or from material that continues to accrete onto Triton's surface), it is certain (i) that methane is present on the surface and in the atmosphere and (ii) that methane will be polymerized by interaction with cosmic rays, UV photons, or charged particles, or all of these. Figure 27A compares Triton color data derived from Voyager imaging observations with contemporaneous ground-based observations (64) and with ground-based observations acquired about a decade earlier (65, II 66). The Voyager data were reduced to Triton clear filter light curve -0.05 normal albedos by using the global-average phase function discussed above. Both the Voyager imaging and contemporaneous * ground-based measurements of Tholen (64) show Triton to be substantially less red than * indicated by the earlier ground-based obser_ 111111 _ ' i vations. Because both the 1979 and the 0.05 1989 measurements have each been confirmed by two sets of independent observa300 0 200 100 tions, it is clear that Triton's global color Subspacecraft longitude Fig. 24. Triton light curve. The disk-integrated changed in the intervening decade. This is brightness of Triton in arbitrary units normalized not surprising. The subsolar latitude ranges to a constant distance from Triton is shown as a between about 55°N and 55°S on time function of subspacecraft longitude (in degrees). scales of several 100 years (67). GroundThe data were derived from clear-filter narrowangle camera. The period of revolution was as- based measurements indicate solid nitrogen and methane on Triton's surface and Voyagsumed to be synchronous. er UVS observations show these comTriton's geologic processes and evolution. Voypounds are dominant in its atmosphere (50, angles the reverse is true. Clear-filter data are 68). These ices are quite mobile, even at the ager 2's highest resolution view of Triton intermediate. 37 to 39 K surface temperature of Triton. It was of the hemisphere that faces Neptune in At large phase angles (>150°) the violet- is quite plausible that volatile transport, synchronous rotation. Figure 28, the first and green-filter data both deviate from the combined with the 100 change in the subsofitted Hapke functions, with the deviation lar latitude between 1979 and 1989, could Table 4. Voyager green-filter normal albedos of being larger in the violet filter. This may account for the large change in the disk- Triton terrains. result from the effects of atmospheric scat- averaged color. Terrain Normal albedo tering at large phase angles, which are not Comparison of the colors of several large accounted for in our models. Atmospheric albedo units and streaks (Fig. 27, B and C) Average dark polar cap 0.82 0.03 scattering is expected to be greater at large reveals that the central polar unit is slightly Bright ice (polar cap edge) 0.88 0.02 0.89 0.02 phase angles and at shorter wavelengths. reddish, similar in color to the dark streaks. Bright polar frost 0.62 0.04 The more positive value of g (indicative of The bright equatorial collar along the outer Dark streaks 0.76 0.02 Dark pond (centers) forward scattering particles) for the violet fringe of the cap is neutral and very bright, Dark pond (edges) 0.95 0.04 filter Hapke fit is consistent with the pres- suggestive of freshly condensed frost. The ence of atmospheric scattering. Triton's global-averaged, single-scattering albedo is among the highest of the outerplanet satellites studied to date. Only values for Enceladus (w 0.998) and Europa (w 0.97) are comparable (62). The Hapke parameters can be used to estimate violetand green-filter geometric albedos (p), phase integrals (q), and spherical albedos (A = pq). These values are given, along with the ground-based V filter value of p in Table 3. The greater-than-unity phase integrals are unusual for icy, airless satellites, but such values are expected for an object covered by transparent grains of frost or terrestrial snow (63). The green-filter Hapke parameters were used to derive crude normal albedos for various regions imaged at high resolution Fig. 25. Global map of Triton. The airbrush drawing shown here covers the 300 to 45°N to 90°S (+ 300 through the green filter (Table 4). Green- to -90°) region in cylindrical projection. It was drawn by J. L. Inge of the United States Geological filter data were chosen as they are less Survey. Green 0.71 V 0.78 1.2 0.88 0.996 -0.22 5° 0 @0 o,0.0 0 0 0 . ± ± ± - 15 DECEMBER I989 REPORTS I439 I I 0 l~~~ 0) ~ N * Earth-based V filter 2 II 0 m - 4 liolet fit --~~~ - Jreen fit ' o Voyager green filter A A Voyager clear filter IQo o Voyager violet filter 6 0 50 100 Phase angle (a) o \ 150 Fig. 26. Voyager camera violet-, grreen-, and clear-filter phase curve for Triton. Maignitude is normalized at zero phase angle to -2.5loglo(p), where p is the geometr icqalbedo The solid line represents the best-fit IHapke parameters for the violet filter; the dashedl line is for the green filter (see Table 3). Both cunves assume Bo = 0 (no opposition surge). image to clearly show surface feat ures, and Fig. 29, a mosaic of higher resoluttion mapping frames, both show this heimisphere. Less than 40% of Triton was imagced at high resolution; it should be kept in rnind that our understanding of Triton's geollogic processes and history is limited to thi Lssample. By comparison to most planetar)y surfaces, Triton's appears geologically younig; like Io and Europa, heavily cratered terrrains are absent. All other outer-planet sate-llites display regions of heavily cratered ter-rains that evidently date back to the early post-accretional bombardment. Triton's surfaace is consistent with an object that was geologically active and resurfaced well after the period of heavy bombardment. Albedo patterns. Triton's color atnd albedo patterns can be broadly divided into (i) units of the brighter polar cap that occuipy nearly the entire southern hemisphere and (ii) somewhat darker and redder pllains that extend roughly from the equator niorthward to the terminator (Fig. 25). Data c)f Table 4 show that all ofthese surfaces have very high albedos (0.6 to 0.9); all are likely ccomposed substantially of volatiles. In most cases the color/albedo units do not correlate with geologic terrain s or topographic features, suggesting they represent thin veneers draped over the tenrain units (Fig. 30). For example, clusters )f strange albedo patterns with irregular, rounded, dark centers surrounded by brigh't aureoles occur near the eastern limb (Fig. 30, lower right). A few impact craters can be seen near and on them but they themselves clisplay no obvious topography. The interic:r of the polar cap displays discrete boundlaries and irregular, patchy areas that appcear to be "windows" in the bright reddish iccy deposit (Fig. 30, lower left). There are no convincing cases of relief along these discriete edges, which is again suggestive of thin deposits 1440 below the limits of detection. craters. Our descriptions of Triton's surface The slightly darker streaks scattered over morphology are confined to the terrains to the interior of the ice cap resemble the the north (Fig. 31). ubiquitous martian "wind streaks" attributAn extensive unit termed the "cantaed to eolian erosion and deposition. The loupe" terrain (ct) dominates the western Triton streaks range in length from a few part of the equatorial region. It consists of a 10's up to about 100 km. Often small, dense concentration of pits or dimples that darker, irregular patches a few kilometers are crisscrossed by ridges ofviscous material across occur at the heads of the streaks; less commonly a bright region also appears near the head. Although between roughly 100 .. A. ......,. and 30°S, the streaks are preferentially ori0 1.00 -A A ented toward the northeast; farther to the south the directions are highly irregular m 0.80 Cruikshank, with streaks crossing one another. In a few 20.60 et al., 19797 0 cases streaks of opposite direction originate * 0.40 A Tholen, 1989 from the same point. - RBll La'al et al.. 1,7 u al#., 1979 Whether the streaks are modern or an- C 0.20 0 VGR - ISS .... I ... . ..... cient features is difficult to say from their u.uu I . 0.40 0.60 0.50 0.70 0.30 surface patterns alone. It seems unlikely that Wavelength (jgm) they are older than very many Triton years 1.2 (that is, a few thousand years), because the B dark particles of which they are probably formed would migrate deep into the ice c 1.0 _ Bright equatorial collar Bright central deposits after multiple cycles of pole-to-pole a) areas migration of the volatiles. 0.8 Dark streak Transport of material by the winds in -a E Triton's tenuous atmosphere seems required z 0.6Equatorial dark zone as part of the explanation for the streaks. As mentioned earlier, some methane will inev0.4 0.4 0.6 0.7 0.5 0.3 itably be converted to particles of dark comWavelength (gm) plex hydrocarbons that will be found in Triton's surficial deposits. Those particles c that are fine enough (a few micrometers or Brighter background 0.9 less) can be suspended in the atmosphere and carried substantial distances downwind. 0.8 Streak 3For Triton dust settling occurs in the Epstein or kinetic regime as opposed to the a1) 0.7 _ Streak 2 _ Streak 1 Stokes or viscous regime as the mean free 0.6 _ path in the tenuous nitrogen atmosphere is large compared to the particle size. From -a _ simple momentum transfer arguments, we c 0.5 estimate, for instance, that a 1-,um particle 0.4 _ would settle through the bottom scale 0.3 _ height (about 14 km) in about 5 days. The Darkest spots basic question is how the dark particles 0 VTMAP 0.2become airborne in the first place. The lI estimated threshold velocity for the surface 0.4 0.5 0.6 0.3 wind to pick up particles a few micrometers Wavelength (uim) in diameter could be as low as roughly 1 ms- . Such surface winds are quite conceiv- Fig. 27. Triton colors and albedos. (A) GroundTriton in the visual and near able. As discussed below, however, at least based spectra of infrared taken in 1979 as well as contemporanetwo dark geyser-like plumes have been iden- ously with the Voyager encounter of Neptune are tified. In one, dark material is clearly erupt- shown (64, 65). Plotted with the ground-based ing nearly vertically to an altitude of about 8 data are the Triton disk-averaged colors as obkm. Purely eolian explanations are not re- tained from Voyager narrow-angle camera imNote the substantial color change from quired unless the dark streaks are polygenet- ages. 1979 to 1989 shown by these data. (B) Normal ic. reflectance of selected areas on the encounter Morphological units. The topography of the hemisphere of Triton are shown (see text). (C) central part of polar cap (br, bs, bst in Fig. Normal reflectances of selected dark areas on are shown. The brighter background spec31) is difficult to interpret owing to the Triton trum is an average of bright areas adjacent to the albedo a patterns. Only exceedingly complex dark streaks whose normal albedos are also plotfew inarguable topographic landforms can ted. The darkest spot is located north of the be distinguished in those units, all impact equator in the darker, redder plains. V' . C7& , 0 0 0 z SCIENCE, VOL. 246 erupted into grabens. The grabens are global in scale and organization, continuing into other terrains to the east and south. Most of the dimples fall into two roughly uniform size classes of about 5-km and 25-km diameter; the smaller ones are found dominantly in the western part of the terrain. Both sizes are often organized into linear, equally spaced sets. It is possible that none of these features is of impact origin. In fact, recognition of any impact craters in the cantaloupe terrain is extremely uncertain. Seen at the highest resolution (Fig. 30, upper left) the cantaloupe terrain displays an extremely rugged and mottled texture. Some combination of viscous flow and collapse and deterioration of the landforms by extensive sublimation of surface materials may have been responsible for the complex landscape observed here. A morphological sequence in the degree of eruption into the grabens can be discerned. The sequence begins with the Ushaped floor at the Y-shaped branch in the top of Fig. 30, lower left. Farther northwest along the valley floor, a narrow ridge is visible that erupted along a section of the valley floor and along an intersecting fault, crossing out into the plains to the south. Even farther north in the cantaloupe terrain and to the east limb along the equator, viscous material welled up in the valley floors forming ridges that stand above the adjacent plains (Fig. 29). In contrast to the cantaloupe terrain, the eastern plains are dominated by a series of much smoother units. The first of these is the floor material (sv) in two "lake-like" features a few hundred kilometers across located near the terminator (Figs. 29 and Fig. 30, upper right). The floors are extremely flat compared to other plains units, embaying the scarp-rimmed margins and surrounding numerous hills and knobs that protrude through the deposit. The floors are terraced, occurring at several levels separated by scarps a few hundred meters high, suggesting multiple episodes of emplacement. A cluster of small irregular pits surrounding a large central pit, likely associated with an eruptive vent, is found in each of the floors. A single impact crater about 15 km in diameter shows that material that is rigid on geologic time scales makes up the bulk of the floor material. Another smooth plains unit dominates the region just south of the two flat-floored lake-like depressions. It occurs as highstanding smooth plains (sh) that appear to have erupted from large quasi-circular depressions; one can see that strings of irregular rimless pits occur commonly in this unit (Fig. 31). Like the flat-floored deposits, the high-standing plains are quite smooth and IS DECEMBER I989 have a very low density of superimposed impact craters. In contrast, however, these deposits stand as thick masses extruded onto and standing above preexisting terrains. The deposits terminate with rounded flow margins partially burying subjacent landforms. In places these deposits appear to be a few kilometers thick. Eruptions of comparable style, recognized on Ariel, are thought to have been formed by highly viscous material, such as partially crystalline ammoniawater mixtures (70). The third plains unit in the eastern region is a hummocky plain (th). It appears to have been formed by extensive eruption of material along sections of the grabens that flowed out onto adjacent plains forming hummocky, rolling deposits and obliterating sections of the graben entirely. Figure 29 shows this unit to have the greatest abundance of impact craters. In general, Triton's global fault system evidences a tensional regime. Several features in the transition zone between the lake-like features and cantaloupe terrain, however, suggest lateral displacement along strike-slip faults. Some show an offset of a few kilometers, others up to 30 km. The best example is an irregular depression 100 to 120 km (Fig. 32, left). It resembles many other depressions so common in this region; by contrast its margins appear to have been offset along ridge-and-groove lineaments. We speculate that the feature was deformed by two subsequent strike-slip motions, each with an amplitude of about 30 km. When the image is sheared along the hypothesized fault lines, the outline of the feature is restored to one typical of other cantaloupe depressions (Fig. 32, right). Other truncated features also appear to line up after restoration. Impact crater abundances and relative ages. At the resolution of the global mapping images of Fig. 29 (1.5 to 3 km per line pair) impact craters are generally rare. Craters with diameters from the limit of resolution up to about 12 km display sharp rims and bowlshaped interiors. Larger craters, ranging up to the largest (27-km diameter) are complex, having flat floors and central peaks. The transition diameter from simple to complex and depth/diameter ratios for Triton's craters are similar to those observed on other satellites whose crustal materials are thought to be dominantly water ice. Ejecta blankets are not visible, probably due to inadequate resolution. The craters also do not have rays, probably because veneers of mobile surface volatiles mask them. Impact crater statistics were collected for four areas (Fig. 33, top). Area 1, the most heavily cratered region, occurs in the leading hemisphere ofTriton's rotation-locked, synchronous orbit. Area 2, the least cratered, coincides with units sv and sh. Area 3 is in the cantaloupe terrain. Area 4 is a part of the ice cap interior. Owing to difficulty of recognizing impact craters with any confidence, crater statistics for the cantaloupe terrain have been excluded. Figure 33, bottom, compares the cratersize frequency distribution for areas 1, 2, and 4 with those of the lunar highlands, typical lunar maria, and the fresh crater population on Miranda's rolling cratered plains (1, 71). Triton's most heavily cratered terrain displays a population similar to that Fig. 28. Color image of Tri- ton acquired at a range of 530,000 km with a resolution of 10 km per line pair. Narrow angle images obtained with green, violet, and ultraviolet filters were used as the red, green, and blue components. High frequency information from a clear image, containing the greatest detail, was merged with the color data. REPORTS I441 V, n",~~~~~~~~~~~~~~~~~4 4 fact seen in the Triton images. These include clouds above the limb and extending into the terminator and an extensive optically thin haze that appears to be uniformly distributed around the disk. The haze is difficult to detect in limb images due to scattered light from Triton's bright surface, but it is easily seen in crescent images where it shows an extension of the cusp beyond the terminator. It apparently extends to an altitude of about 30 km. more than two scale heights, and has an optical depth of about 2 x 10-4. We interpret it to be composed of photochemically generated smog-like particles described above. Several clouds are seen in backscatter above both the east and west limbs (Fig. 34). In places these clouds are clearly detached; in others they appear to extend to the surface. The limb clouds appear above diffuse bright regions seen on the limb, suggesting that they are composed of bright particles that can be seen both in backscatter as well as in projection against the disk. All of the limb clouds so far detected are located over the subliming south polar ice cap. Brightness scans perpendicular to the limb EXPLANATION - -I- Contact-Dasd wheapproximae Grabens Faultorscap-Balondownhownskn -4e - Rides gula der Smal irregular depressions o Lar o Fig. 31. Sketch map of Triton's terrains and south polar units. Units mapped are as follows: cantaloupe rolling plains-th, linear ridge materials-ri, bright spotted polar unit-bs, bright streaked polar unit-bst, bright rugged polar terrain-ct, smooth floor material-sv, high-standing smooth matenials-sh, hum unit-br. I,o I0 10 100 Crater diameter (km) Fig. 32. Possible strike-slip faulting on Triton. (Left) The irregular depression near the center of the view appears to have been distorted by lateral faulting. (Right) One possible reconstruction of the feature shown in this view requires two episodes of strike-slip faulting, each with roughly 30 km of displacement. 14441 Fig. 33. Impact crater statistics for various regions of Triton. (Top) The locations of regions for which data were collected indude: area 1, the most heavily cratered; area 2, lightly cratered; area 3, cantaloupe terrain, and area 4, a plains region just outside the inner cap zone. The dash-endosed areas (6, 7, and 8) were analyzed to detect a possible gradient in the crater flux (see text). (Bottom) Triton crater size/frequency distribution for area 1 (Triton HC), area 2 (Triton LC), and area 4 (Triton SH) compared with the fresh crater population on Miranda, the lunar highlands, and the lunar post-mare. The curves were normalized to a standard -2 cumulative distribution power law as in (1). SCIENCE, VOL. 246 on one of these images (Fig. 35) show a cloud that extends from close to the surface to an altitude of about 4 kmi, above which its brightness sharply decreases. We interpret this feature to be a nitrogen condensation cloud. The ubiquitous diffuse haze can barely be discerned in the scans extending from the surface to an altitude of almost 30 km. Because they are thin, the optical thickness of the limb clouds can be estimated from their observed brightness. Assuming isotropic scattering (both of the direct and surface-reflected sunlight), we estimate optical depths of about 10-3. If more realistic particle phase functions are used, the estimated optical depths are several times larger (73). Also, in some cases the brightness of the discrete clouds is as much as three times larger than shown in Fig. 35. Together these factors imply optical depths ranging up to projecting these images to a common perspective onto a sphere, it was possible to study topographic landforms and search for material aloft. In this way two active geyserlike plumes have been confidently identified; several others are suspected. All are located well inside the complex central zone of Triton's south polar cap. The first of the two well-observed plumes (termed here the west plume) is situated at 3340E, 50°S (Fig. 38, top). West plume is visible in at least four views, which have emission angles of 370, 620, 670, and 750 at its location. The feature appears as an 8-km- 10-2. Bright clouds extending beyond the terminator are visible in a long sequence of outbound images of the opposite hemisphere to that seen in the mapping coverage (Fig. 36). The terminator is at roughly 45°S; again the clouds are seen above the south polar cap. From the distance they extend beyond the terminator, their altitudes are estimated to be in excess of 13 kilometers. These clouds remained stationary relative to the surface for the 2-day period as they rotated through the terminator region. They appear to correlate with dark surface markings seen in the low resolution approach images 3 days prior to closest approach. Several east-west, elongated clouds can also be seen over the illuminated part of the crescent in a color image set acquired soon after Triton closest approach at a phase angle of approximately 1400 (Fig. 37). Both clouds and surface detail can be discerned. The clouds apparently cast shadows, giving a very rough estimate of their altitudes of a few kilometers. Like the clouds associated with the geyser-like plumes described below, these elongated clouds are roughly 100 kmn long and 10 km wide. They may also be associated with erupting plumes, but no direct evidence exists for plumes connecting them to surface sources. Triton's geyser-like plumes. The Voyager 2 images of Triton were acquired over a sufficiently wide range of viewing angle that many regions can be studied stereoscopically. The last global view of Triton, acquired just prior to closest approach to Neptune, was imaged from a sub-spacecraft latitude of about 15°S. Subsequent higher resolution frames of the mapping coverage were acquired from latitudes ranging from about 100 to 250N at about the same longitude. Through stereoscopic examination and by 15 DECEMBER I989 Fig. 34. Limb clouds above Triton's south polar cap. Each of the images is shown with two different stretches to enhance the limb clouds and surface features separately. Dark pixels mark the best estimate of Triton's limb. (Top) Cloud on the west limb that extends about 100 km along the limb and appears detached over much of its length. (Bottom) Cloud imaged above the east limb that extends about 200 km along the limb and is asymmetrical. 2000 Latitude 1500 _ 27.79 28.28 o----C --- 31.15 \o --- 13.66 i. ! \ Longitude 296.13 296.33 297.59 291.55 1000 - Fig. 35. Brightness scans across sections of the cloud shown in Fig. 34, top. The latitudes and longitudes of the scans are indicated. 1350 1360 1370 Radius (km) 1380 REPORTS 1390 1445 Fig. 36. Clouds extending over the terminator in an overexposed image of Triton's crescent acquired during the outbound sequence. The hemisphere shown is that facing away from Neptune at a resolution of 20 km per line pair. tall narrow, dark stem rising vertically from a dark spot on the surface; the upper end terminates abruptly in a small dense dark cloud. A more diffuse cloud, appearing as a narrow dark band, can be seen extending westward for at least 150 km in the highest emission angle view, at which point it diffusely disappears. The cloud band maintains a very narrow projected width (approximately 5 km) along its length. In the later, higher resolution and higher emission angle images, the shadow of the band of cloud is visible; the sun was nearly directly above the west plume (incidence angle approximately 100). The east plume (Fig. 38, bottom) is located at 12°E, 57°S, and is also visible in at least four views with emission angles near the plume of 530, 720, 76°, and 77°. Like the west plume, it rises to an altitude of about 8 km where a small, dark, dense cloud is formed. In contrast to the west plume, however, its dark cloud of material is generally denser and diverges as it extends westward. The cloud is also quite dense in the vicinity of the source. Hence, although the Fig. 37. High phase-angle image of Triton showing surface features and elongated clouds (to the left) evidently casting shadows near the terninator. In this view the resolution is about 9 km per line pair, the phase angle about 1400, and the subspacecraft position was centered at 208°E near the equator. Green-, clear-, and violet-filter narrowangle images were used as the red, green, and blue components. [Image processing by T. L. Becker] 1446 rising column appears to be tilted slightly westward, its geometry is less obvious than in the case of the west plume. In both cases the plume material rises roughly vertically to the 8-km altitude before being carried downwind. The geometry suggests stratification of the atmosphere may be controlling the form. Perhaps the 8km altitude represents an inversion at the tropopause; as the plumes rise under some combination of momentum and buoyant force, they cease to rise above this altitude. The cloud geometry also suggests vertical structure in the wind speeds, in which the winds aloft increase abruptly at this altitude, although this dearly depends on the timescales for rising, suspension, and downwind transport. It seems likely that the active dark plumes are related to the ubiquitous dark streaks scattered over the south polar region described earlier. The observed variation in the two plumes in terms of cloud density and degree of divergence is consistent with the variance seen in the collection of dark streaks. Most plausible mechanisms to drive the plumes involve the venting of some gas from the surface, entraining fine dark particles. The particles are carried by some combination of ballistic and buoyant forces to altitudes where they are left suspended in the thin atmosphere to be carried downstream by the complex, sublimation-driven winds. The most likely driving gas is nitrogen, although models involving concentrated methane gas rising buoyantly are conceivable. Secondly, different types of energy sources can be considered, including insolation and geothermal energy sources either localized, for instance by intrusion of cryogenic lavas into areas ofvolatile material, or more broadly distributed through Triton's overall radiogenic heat flow. The fact that the active plumes are near the current subsolar latitude argues for a solar-driving mechanism. We discuss here one model out of many conceivable, simply to demonstrate Fig. 38. Profile view of Triton's active geyser-like plumes. These two high-emission-angle views are of the regions near the southern limb; south is up, west to the right. In both views the spacecraft was about 15° above the horizon as seen from the base of each plume near its source. The plumes are about 8 km tall; the east-west dimension of the two views is about 150 km. The west plume is shown in top image; the east plume in the bottom image. SCIENCE, VOL. 246 that simple, plausible explanations do exist. The model we describe is for solar-driven nitrogen gas geysers. The insolation-driven mechanism involves a "greenhouse" effect. A way to construct an extremely efficient greenhouse is to cover a dark absorbing layer with a relatively transparent layer, which we propose to be a layer of nitrogen ice, that is both volatile and has a low thermal conductivity. Groundbased spectroscopic observations of Triton show an absorption feature at 2.15 pm that was attributed to nitrogen, requiring a path length in the nitrogen equivalent to a meter or more of solid or liquid (50). In this model the radiation is absorbed in a dark substrate beneath the nitrogen layer. The temperature will rise until the thermal gradient reaches a point where the excess heat is conducted and reradiated back to the surface. The vapor pressure of nitrogen ice increases rapidly with increasing temperature. A temperature rise of 10 degrees above Triton's 37 K surface temperature results in roughly a one hundred-fold increase in pressure. If the layer of nitrogen is thick enough (>1 or 2 meters), sufficiently transparent, and locally seals off the subsurface, the subsurface vapor pressure will increase, filling permeable subsurface reservoirs. If the seal is ruptured or if the pressurized gas migrates laterally to an open vent it will rapidly decompress, launching a plume of nitrogen gas and ice, entraining dark particles encountered in the exit nozzle, and carrying them to altitude. Small satellites. Of the newly discovered satellites (Table 1), only 1989N1 and 1989N2 were well enough resolved to see surface features (Fig. 39). As resolution was sufficient to measure radii directly for 1989N1, 1989N2, and 1989N3, their albedos are also reasonably well determined. One color sequence acquired of 1989N1 shows it to have a flat spectrum; violet-, green-, and clear-filter albedos all fall within 10% of each other, similar to Voyager photopolarimetry observations for 1989N2 (29). The low resolution view of Nereid provided only an approximate albedo; no surface or limb features were detectable (Fig. 41). Nereid's spectral reflectivity (geometric albedo in the violet, green, and clear filters) is flat within 10%. Clear-filter images used to derive Nereid's phase curve between 250 and 550 (Fig. 40) yield a linear phase coefficient of 0.021 magnitudes per degree. No rotational effects were detected in the Nereid images; the amplitude of its light curve is less than 10%. Inasmuch as the orientation of Nereid's pole is unknown, we cannot derive limitations of its shape from these data. The Voyager viewing direction differed only by approximately 250 from that of recent ground-based observations (74) that suggested a large amplitude light curve. We see no evidence for an amplitude of more than 10%. The rotation period remains unknown. 1989N1 and 1989N2 are irregularly shaped; 1989N1 displays craters, one near the terminator about 150 kilometers across. 1989N1 was resolved at several longitudes and is slightly elongated; topography of about 20 km can be seen on its limb. The closest view weakly shows linear features; no substantial albedo features are seen. The irregular shape of 1989N1, which had a diameter of 400 kilometers, might seem surprising. However, the internal strength required to support such topography falls within the expected range even for an icy object (75). Collisional histories of satellites. Bombardment by comets over the last -3.5 billion years probably accounts for most of the craters observed on Triton and for the origin of the small satellites 1989N5 and 1989N6 by collisional fragmentation. The comet flux close to Neptune is dominated by comets captured into relatively short-period Neptune-crossing orbits by encounters with that planet. A rough estimate of the number of these Neptune family comets can be made by extrapolation from the observed Jupiter family of short-period comets (76, 77). On the basis of theoretical studies of the capture process (78), we infer that there is a fairly steady population of the order of a million Neptune family comet nuclei with absolute B-magnitude greater than 18 (this corresponds approximately to diameters greater than about 2.5 kilometers). This estimate is conservative; if most short-period comets have been derived from a region lying a moderate distance beyond Neptune or between Uranus and Neptune (79), the population of Neptune family comets might be 10 to 20 times higher. Long-period comets also strike the satellites of Neptune, but their impact contributes, at most, only a few percent to the estimated production of craters on Triton and on the small regular satellites. Sun 2's best views of 1989N1 and 39. Voyager Fig. 1989N2. (A) 1989N1 is shown in this image at a resolution of 7.9 km per pixel; (B) 1989N1 is at 1.3 km per pixel; and (C) 1989N2 at 4.2 km per pixel. (D) Schematic of the viewing geometry. Arrows labeled S/C indicate spacecraft view; letters correspond to images A, B, and C. IS DECEMBER I989 REPORTS 1447 Q~~~~~~ 0.0 l l llll Nereid disk integrated brightness 01 c -1.0 -2.0 40 60 80 Solar phase angle (degrees) Fig. 40. Voyager 2's best view of Nereid. (A) Acquired from a range of 4.7 million km at a phase angle of about 900, this image has a resolution of 43 km per pixel. (B) Schematic of viewing geometry. Fig. 41. Disk-integrated phase curve for Nereid. The photometric data were extracted from 15-s exposures taken through the narrow angle clear filter during the last 12 days of approach. The present cratering rates on the satellites, estimated from methods described earlier (76) and from the conservative estimate ofthe Neptune family comet population, are given in Table 5. At Triton, the conservatively estimated present production of craters >10 km in diameter is about half the present rate on Earth (80) and similar to the average rate on the Moon over the last 3.3 billion years. At this rate, the craters observed on most of the sparsely cratered terrains (marked sv and sh in Fig. 31) of Triton could all have been generated in the last billion years or so. Even at the conservatively estimated cratering rates, the small, innermost satellites 1989N5 and 1989N6 are not likely to have survived intact over the last 3.5 billion years. These satellites are probably fragments pro- bardment period or somewhat later. Distant Nereid appears to have been almost immune to collisional disruption by comet impact in the period since heavy bombardment. duced by catastrophic disruption of a larger body in the last 2 or so billion years, possibly even in the last half-billion years. 1989N2, 1989N3, and 1989N4, on the other hand, might have escaped destruction by comet impact at the present rate, but would almost certainly have been destroyed by collisions during an early period of heavy bombardment. The rate of cratering on these satellites by comet impact is about three to five times higher than on 1989N1; this result is independent of the estimate of the comet population. The large craters observed on 1989N1 suggest a fluence of impacting bodies that would have destroyed 1989N2, 1989N3, and 1989N4. Most likely these satellites are the product of a disruption of a body comparable in size to 1989N1 near the end of the heavy bom- Table 5. Estimated present cratering rates and past production of large craters on the satellites of Neptune. The entries are as follows: P > 10 km is the estimated present rate of production of craters >10 km diameter in units of 10 to 14. Dmax is the diameter of the largest crater likely to have been formed in 3.5 billion years at the current rate of cratering. Frsat is the frequency of production of craters with diameters larger than the radius of the satellite over a time period of 3.5 billion years. The column headed D > Rs.t gives the number of craters with diameters greater than the radius of the satellite that would have been formed while the observed craters on Ni were formed. Ni equivalent Present rate Satellite P > 10 km Nereid 0.06 Triton 0.84 NI 5.7 (kma) Frsat D> Implications for impact history 0.0015 0.01 Little modification by comet impacts in last 3.5 billion years. 193 0.014 0.1 Oldest cratered surfaces probably about 3.5 billion years old. Youngest crater surfaces probably less than 0.5 billion years old. 81 0.14 1.0 Observed surface probably dates to period of heavy bombardment. 3.2 4.4 6.2 These satellites probably were derived from fragmentation of a larger body near the end of heavy bombardment. 8.8 N2 N4 N3 16 21 29 66 71 70 0.44 0.60 0.85 N5 41 46 1.4 10 N6 50 35 1.8 13 I448 Rsat Probably formed by fragmentation of larger parent body about 2 to 2.5 billion years ago. REFERENCES AND NOTES 1. B. A. Smith et al., Science 233, 43 (1986). 2. B. A. Smith et al., ibid. 204, 951 (1979); ibid. 206, 927 (1979). 3. J. W. Warwick et al., ibid. 246, 1498 (1989). 4. H. B. Hammel et al., ibid. 245, 1367 (1989). 5. B. A. Smith and G. E. Hunt, in Jupiter, T. Gehrels, Ed. (Univ. of Arizona Press, Tucson, 1976), pp. 564-585. 6. H. B. Hammel, Bull. Amer. Astron. Soc., in press. 7. The Voyager images were processed with software that greatly reduces limb darkening or limb brightening. Specifically, a Minnaert function was used with k = 0.7. 8. H. B. Hammel, K. H. Baines, J. T. Bergstralh, Icarus 80, 416 (1989). 9. W. H. Smith, W. V. Schempp, K. H. Baines, Astrophys. J. 343, 450 (1989). 10. K. H. Baines and W. H. Smith, Icarus, in press. 11. J. B. Pollack et al., ibid. 65, 442 (1986); J. B. Pollack et al., J. Geophys. Res. 92, 15037 (1987). 12. K. H. Baines and J. T. Bergstralh, Icanrs 65, 406 (1986). 13. P. N. Romani and S. K. Atreya, Geophys. Res. Lett. 16, 941 (1989). 14. G. L. Tyler et al., Science 246, 1466 (1989). 15. The radiative transfer model was a multilayer model based on the delta-Eddington approximation. The background atmosphere contained stratospheric aerosols, a methane cloud, and an optically thick cloud whose top was at 3 bars. Parameters were adjusted so the background model matched groundbased spectrophotometry in the visible and near infrared. A cloud layer was introduced to represent a feature; its optical depth and altitude were adjustable parameters. 16. We assumed that the product ofthe single-scattering albedo and phase function was independent of altitude. This product, determined from the value ofthe nearly constant brightness at low altitudes, was significantly greater (by a factor of 1.7) than that expected just from Rayleigh scattering, implying that scattering by stratospheric aerosols makes an important contribution to the observed brightness. 17. H. G. Solberg, Planet. Space Sci. 17, 1573 (1969); L. A. Youngblood and R. F. Beebe, Nature 280, 771 (1979). 18. B. A. Smith et al., Bull. Am. Astron. Soc. 11, 570 (1979); R. J. Terrile and B. A. Smith, ibid. 16, 657 (1984); H. B. Hammel, Icanrs 80, 14 (1989). 19. H. B. Hammel, Science 244, 1165 (1989). 20. B. Conrath et al., ibid. 246, 1454 (1989). 21. Another stability condition involves the potential vorticity q which is given by (C + J)l/h, where ; is the relative vorticity (approximately -duldy), u is the zonal velocity, y is the northward coordinate, f is 2 SCIENCE, VOL. 246 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. Msingp, and h is the thickness (mass per unit area) of the atmospheric layer. A zonal flow is stable when the latitudinal gradient of q is positive everywhere. Although we do not know ; accurately, (; + j) is likely to vary as fi which is proportional to sin(p. Thus, 1; +fl is large at the poles and zero at the equator. Because the interior of Neptune rotates faster than the atmosphere at most latitudes, the pressure surfaces become more oblate with increasing depth below the cloud tops. This means that the thickness h of the layer just below the cloud tops is also large at the poles and small at the equator. The potential vorticity q = (C + J)lh might then be nearly constant in each hemisphere, merely changing sign at the equator. A constant-potential-vorticity atmosphere is marginally stable and is diagnostic of horizontal mixing [P. S. Marcus, Nature 331, 693 (1988); T. E. Dowling and A. P. Ingersoll, J. Atmos. Sci., in press]. H. J. Reitsema et al., Science 215, 289 (1982); C. E. Couvault et al., Icarus 67, 126 (1986); W. B. Hubbard et al., Nature 319,636 (1986); P. D. Nicholson et al., in preparation. B. A. Smith et al., Science 212, 163 (1981); B. A. Smith et al., ibid. 215,504 (1982); J. N. Cuzzi et al., in Planetary Rings, R. Greenberg and A. Brahic, Eds. (Univ. of Arizona Press, Tucson, 1984), pp. 73199. A. L. Lane et al., Science 233, 65 (1986); J. Colwell et al., Icarus, in press. P. Goldreich and S. Tremaine, Annu. Rev. Astron. Astrophys. 20,249 (1982); T. G. Northrop and J. E. P. Connerney, Icarus 70, 124 (1987); L. R. Doyle et al., ibid. 80, 104 (1989); J. N. Cuzzi and R. H. Durisen, ibid., in press; L. W. Esposito and J. E. Colwell, Nature 339, 605 (1989). J. J. Lissauer, Nature 318, 544 (1985). P. Goldreich, S. Tremaine, N. Borderies, Astron. J. 92, 490 (1986). A. R. Dobrovolskis, Icarus 43, 222 (1980); N. Borderies, ibid. 77, 135 (1989); A. Dobrovolskis, in preparation. A. L. Lane et al., Science 246, 1450 (1989). Two sets of images were included in the search, yielding nearly full azimuthal coverage from an orbital radius of 34,000 km to 65,000 km, with partial coverage out to 90,000 km. The first set consisted of 94 narrow angle images, taken with 61s exposures and resolutions that varied from 76 to 88 km per line pair. The second set had 40 narrow angle images, taken with an exposure of 15 s and a resolution of 37 km per line pair. For a point source, the change in resolution for the second set of data nearly compensates for the reduced exposure time. This results in both sets yielding virtually the same limit of detectability. M. Ockert et al., J. Geophys. Res. 92, 14969 (1987); L. R. Doyle et al., Icarus 80, 104 (1989); H. C. Dones, thesis, University of California, Berkeley, 1989. M. Ockert et al., Bull. Amer. Astron. Soc. 20, 854 (1988); M. Showalter and J. Cuzzi, ibid., p. 855. The equivalent width is defined as E = f(I/F)dr = [4w0)P(a)/cosO]fJrdr, where (if the incident solar flux is irF) IIF is the ring reflectivity, equaling unity for a perfectly reflecting Lambert disk illuminated and viewed at normnal incidence. The IIF is defined in terms of the particle single scattering albedo wo and phase function P(a), the viewing angle from the ring normal 0, and the ring normal optical depth T. The quantity fTdr is often referred to as the equivalent depth. The optical depth and equivalent depth used here may differ by a factor of two from those determined from ground-based stellar occultation measurements because of diffraction corrections. [For further discussion, see J. Cuzzi et al., in Planetary Rings, R. Greenberg and A. Brahic, Eds. (Univ. of Arizona Press, Tucson, 1984) pp. 73-199; and IS DECEMBER I989 34. 35. 36. 37. 38. R. G. French etal., in Uranus, J. Bergstralh and M. S. Matthews, Eds. (Univ. of Arizona Press, Tucson, 1989), in press.] N. Divine and P. Nicholson, private communication. P. Nicholson et al., in preparation. S. Dermott et al., Nature 284, 309 (1980); A. Dobrovolskis, in preparation. A. W. Harris, in Planetary Rings, R. Greenberg and A. Brahic, Eds. (Univ. of Arizona Press, Tucson, 1984), pp. 641-659. P. Thomas, C. Weitz, J. Veverka, Icarus 81, 92 (1989). 39. Compare Table 5; Table 3 of B. A. Smith et al., Science 233, 43 (1986); Table 2 of B. A. Smith et al., ibid. 215, 504 (1982). 40. J. A. Burns et al., Icarus 44, 339 (1980); J. A. Bums et al., in Planetary Rings, R. Grecnberg and A. Brahic, Eds. (Univ. of Arizona Press, Tucson, 1984), pp. 200-272. 41. A. L. Broadfoot et al., Science 233, 74 (1986). 42. W.-H. Ip, Icarus 60, 547 (1984); J. N. Cuzzi and R. H. Durisen, ibid., in press. 43. J. N. Cuzzi and J. A. Burns, ibid. 74, 284 (1988); J. E. Colwell and L. E. Esposito, in preparation. 44. L. W. Esposito and J. E. Colwell, Nature 339, 605 (1989); J. E. Colwell and L. W. Esposito, in preparation. 45. C. C. Porco and P. Goldreich, Astron. J. 93, 724 (1987); R. French et al., in Uranus, J. Bergstralh and M. S. Matthews, Eds. (Univ. Arizona Press, Tucson, 1989), in press. 46. D. N. C. Lin et al., Mon. Not. Roy. Astron. Soc. 227, 75 (1987); P. Goldreich, private communication. 47. M. S. Marley, W. B. Hubbard, C. C. Porco, Bull. Amer. Astron. Soc. 21, 913 (1989). 48. B. A. Smith, in Uranus and Neptune NASA CP-2330, J. T. Bergstralh, Ed. (National Aeronautics and Space Administration, Washington, DC, 1984), pp. 213-223. 49. D. P. Cruikshank and R. H. Brown, in Satellites, J. A. Bums and M. S. Matthews, Eds. (Univ. of Ariz. Press, Tucson, 1986), pp. 836-873. 50. D. P. Cruikshank, R. H. Brown, R. N. Clark, Icarus 58,293 (1984); D. P. Cruikshank, R. H. Brown, L. P. Giver, A. T. Tokunaga, Science 245, 283 (1989). 51. R. H. Brown, T. V. Johnson, J. B. Pollack, in Uranus, J. T. Bergstralh and M. S. Matthews, Eds. (Univ. of Ariz. Press, Tucson, in press). 52. D. J. Tholen and M. W. Buie, Bull. Amer. Astron. Soc. 20, 807 (1988). 53. T. V. Johnson, R. H. Brown, J. B. Pollack, J. Geophys. Res. 92, 14884 (1987); W. B. McKinnon and S. Mueller, Geophys. Res. Lett. 16, 591 (1989); D. P. Sirnonelli and J. B. Pollack, Icarus, in press. 54. M. J. Lupo, Icarus 52, 40 (1982). 55. J. B. Pollack, J. A. Burns, M. E. Tauber, ibid. 37, 587 (1979); W. B. McKinnon and A. C. Leith, ibid., in press. 56. P. Goldreich, N. Murray, P. Y. Longaretti, D. Banfield, Science 245, 500 (1989). 57. E. Anders and N. Grevesse, Geochem. Cosmochimica Acta 53, 197 (1989). 58. D. G. Jankowski, C. F. Chyba, P. D. Nicholson, Icarus 80, 211 (1989). 59. 0. G. Franz, ibid. 45, 602 (1981). 60. J. Goguen, H. B. Hammel, R. H. Brown, ibid. 77, 239 (1989). 61. B. J. Hapke, J. Geophys. Res. 86, 3039 (1981); Icanrs 59, 41 (1984); ibid. 67, 264 (1986). A nonlinear, least-squares approach was used to obtain best-fit Hapke parameters [P. Helfenstein, ibid. (in press)] 6(violet) = 30; w(green) = 0.996. It should also be noted that the Voyager data indicate that Triton's surface is photometrically heterogeneous and these represent global averages. 62. B. Buratti, Icarus 61, 208 (1985). 63. A. Verbiscer and J. Veverka, Bull. Amer. Astron. Soc. 20, 872 (1988). 64. D. J. Tholen, personal communication. 65. J. F. Bell, R. N. Clark, T. B. McCord, D. P. Cruikshank, Bull. Amer. Astron. Soc. 11, 570 (1979). 66. D. P. Crnikshank, A. Stockton, H. M. Dyck, E. E. Becklin, W. Macy, Icarus 40, 104 (1979). 67. L. M. Trafton, in Uranus and Neptune NASA CP2330, J. T. Bergstralh, Ed. (National Aeronautics and Space Administration, Washington, DC, 1984), pp. 481-496. 68. L. Broadfoot et al., Science 246, 1459 (1989). 69. W. R. Thompson, B. Murray, B. N. Khare, C. Sagan, J. Geophys. Res. 92, 14933 (1987); L. J. Lanzerotti and W. L. Brown, in Proceedings of the NATO Advanced Research Workshop on Ices in the Solar System, Nice, France (1984); G. Strazzulla, L. Calcagno, G. Foti, Mon. Nat. Roy. Astron. Soc. 204, 59 (1983). 70. J. S. Kargel and S. K. Croft, Lunar Planet. Sci. Conf: XX, 500 (1989); D. G. Jankowski and S. W. Squyres, Science 241, 1322 (1989); D. J. Stevenson and J. I. Lunine, Nature 323, 46 (1986). 71. S. K. Croft and L. A. Soderblom, in Uranus, J. T. Bergstralh and M. S. Matthews, Eds. (Univ. ofAriz. Press, Tucson, in press). 72. C. P. McKay, J. B. Pollack, A. P. Zent, D. P. Cruikshank, R. Courtin, Geophys. Res. Lett. 16, 973 (1989). 73. J. B. Pollack and J. N. Cuzzi, J. Atmos. Sci. 37, 868 (1980); K. N. Liou, Q. Cai, J. B. Pollack, J. N. Cuzzi, Appl. Opt. 22, 3001 (1983). 74. M. W. Schaefer and B. E. Schaefer, Nature 333, 436 (1988). 75. T. V. Johnson and T. R. McGetchin, Icarus 18, 612 (1973). 76. E. M. Shoemaker and R. F. Wolfe, in Satellites of Jupiter, D. Morrison, Ed. (Univ. of Ariz. Press, Tucson, 1982), pp. 277-339. 77. , C. S. Shoemaker, Lunar Planet. Sci. Conf: XVII, 799 (1986). 78. E. Everhart, Astrophys. J. 10, L131 (1972); E. Everhart, in Comets-Asteroids-Meteorites, A. H. Delsemme, Ed. (Univ. of Toledo Press, Toledo, 1977), pp. 99-104. 79. M. Duncan, T. Quinn, S. Tremaine, Astrophys. J. 328, L169 (1988); M. Duncan, T. Quinn, S. Tremaine, Icarus, in press. 80. E. M. Shocmaker, C. S. Shoemaker, R. F. Wolfe, in Global Catastrophes in Earth History, Lunar and Planetary Science Institute Cont. 673, 174 (1988). 81. We gratefully acknowledge the skill, hard work, and good cheer of all our colleagues who helped us during this final Voyager encounter, especially the Voyager Spacecraft Team, for providing us with a healthy, reliable, obedient, and stable spacecraft with which to perform our imaging experiments. Special thanks go to H. Marderness, G. Hanover, and E. Wahl for designing the image motion compensation; G. Masters, M. Urban, B. Cunningham, and D. Rice for extended exposure capability; and our own E. Simien for making every image available. We thank our fellow scientists J. Bums, C. Ferrari, D. M. Janes, F. Rocques, and the analysts at MIPL, coordinated and directed by C. Avis and S. Lavoie: G. Gameau, H. Mortensen, C. Stanley, L. Wynn, L. Wainio, G. Yagi, D. Alexander, C. Levine, E. Runkle, J. Yoshimizu, R. Mortensen, and D. Jensen. We thank those at the USGS in Flagstaff for their help: K. Edwards, E. Eliason, T. Becker, J. Swann, K. Hoyt, R. Batson, J. Inge, P. Bridges, and H. Morgan. We also thank P. Goldreich, P. Nicholson, D. Stevenson, and R. West for careful and thoughtful reviews of this manuscript. 6 November 1989; accepted 15 November 1989 REPORTS 1449