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We present global-scale maps of Titan from the Visual and Infrared Mapping Spectrometer (VIMS) instrument on Cassini. We map at 64 near-infrared wavelengths simultaneously, covering the atmospheric windows at 0.94, 1.08, 1.28, 1.6, 2.0, 2.8, and 5 [micro]m with a typical resolution of 50 km/pixel or a typical total integration time of 1 s. Our maps have five to ten times the resolution of ground-based maps, better spectral resolution across most windows, coverage in multiple atmospheric windows, and represent the first spatially resolved maps of Titan at 5 [micro]m. The VIMS maps provide context and surface spectral information in support of other Cassini instruments. We note a strong latitudinal dependence in the spectral character of Titan's surface, and partition the surface into 9 spectral units that we describe in terms of spectral and spatial characteristics. Keywords: Titan

Observations of the inner coma of Comet 19P/Borrelly with the camera on the Deep Space 1 spacecraft revealed several highly collimated dust jets emanating from the nucleus. The observed jets can be produced by acceleration of evolved gas from a subsurface cavity through a narrow orifice to the surface. As long as the cavity is larger than the orifice, the pressure in the cavity will be greater than the ambient pressure in the coma and the flow from the geyser will be supersonic. The gas flow becomes collimated as the sound speed is approached and dust entrainment in the gas flow creates the observed jets. Outside the cavity, the expanding gas loses its collimated character, but the density drops rapidly decoupling the dust and gas, allowing the dust to continue in a collimated beam. The hypothesis proposed here can explain the jets seen in the inner coma of Comet 1P/Halley as well, and may be a primary mechanism for cometary activity. Keywords: Comets; Comae; Jets

Stereo analysis of images obtained during the 2001 flyby of Comet Borrelly by NASA's Deep Space 1 (DS1) probe allows us to quantify the shape and photometric behavior of the nucleus. The shape is complex, with planar facets corresponding to the dark, mottled regions of the surface whereas the bright, smooth regions are convexly curved. The photometric as well as textural differences between these regions can be explained in terms of topography (roughness) at and below the image resolution, without invoking significant variations in single-particle properties; the material on Borrelly's surface could be quite uniform. A statistical comparison of the digital elevation models (DEMs) produced from the three highest-resolution images independently at the USGS and DLR shows that their difference standard deviation is 120 m, consistent with a matching error of 0.20 pixel (similar to reported matching accuracies for many other stereo datasets). The DEMs also show some systematic differences attributable to manual versus automatic matching. Disk-resolved photometric modeling of the nucleus using the DEM shows that bright, smooth terrains on Borrelly are similar in roughness (Hapke roughness [theta] = 20[degrees]) to C-type asteroid Mathilde but slightly brighter and more backscattering (single-scattering albedo w = 0.056, Henyey-Greeustein phase parameter g = -0.32). The dark, mottled terrain is photometrically consistent with the same particles but with roughnesses as large as 60[degrees]. Intrinsically darker material is inconsistent with the phase behavior of these regions. Many local radiance variations are clearly related to topography, and others are consistent with a topographic explanation; one need not invoke albedo variations greater than a few tens of percent to explain the appearance of Borrelly. Published by Elsevier Inc. Keywords: Borrelly; Small bodies; Topography: Photogrammetry; Photometry

The highest resolution images of Comet 19P/Borrelly show many dark features which, upon casual inspection, appear to be low albedo markings, but which may also be shadows or other photometric variations caused by a depression in the local topography. In order to distinguish between these two possible interpretations we conducted a photometric analysis of three of the most prominent of these features using six of the highest quality images from the September 22, 2001 Deep Space 1 (DS1) flyby. We find that: 1. The radiance in the darkest parts of each feature increases as phase angle decreases, similarly to the radiance behavior of the higher albedo surrounding terrain. The dark features could be either fully illuminated low albedo spots or, alternatively, they could be depressions. No part of any of the three regions was in full shadow. 2. One of the regions has a radiance profile consistent with a rimmed depression, the second, with a simple depression with no rim, and the third with a low albedo spot. 3. The regolith particles are backscattering and carbon black is one of the few candidate regolith materials that might explain this low albedo. We conclude that Borrelly's surface is geologically complex to the limit of resolution of the images with a combination complex topography, pits, troughs, peaks and ridges, and some very dark albedo markings, perhaps a factor of two to three darker than the average 3-4% albedo of the surrounding terrains. Our technique utilizing measured radiance profiles through the dark regions is able to discriminate between rimmed depressions, rimless depressions and simple albedo changes not associated with topography. Keywords: Comets: Surface morphology Surface processes; Photometry; Albedo

We have examined the local base of the south polar layered deposits (SPLD) exposed in the bounding scarp near 72 [degrees]-74 [degrees] S, 215 [degrees]-230 [degrees] W where there is a clear unconformable contact with older units. Sections of layering up to a kilometer thick were examined along the bounding scarp, permitting an estimate of the thinnest individual layers yet reported in the SPLD. Rhythmic layering is also present locally, suggesting a similarly rhythmic variation in environmental conditions and a recorded climate signal at least in some SPLD strata. Locally, angular unconformities may be present, as has been reported for the north polar layered deposits (NPLD) and may likewise imply intervals of subaerial erosion in the SPLD. The outcropping layers display a broad range of weathering styles and may reflect more diverse conditions of depositions, erosion, and diagenesis than might have been expected from simple aeolian depostion modulated only by astronomically driven climatic fluctuations. An unexpected finding of our study is the presence of locally abundant small pits close to the bounding scarp. These quasi-circular, negative, rimless features probably originated as impact craters and were modified to varying degrees by local endogenic processes, as well as locally variable blanketing. A nominal exposure age for the most heavily cratered region in our study area is about 2 million years, and the crater statistics appear consistent with those for the overall SPLD, although there are large uncertainties in the absolute ages implied by the crater size-frequency statistics, as in all martian crater ages. Another new finding is the presence of mass wasting features along the steepest portion of the retreating bounding scarp as well as a number of examples of brittle fracture, consistent with large-scale slumping along the bounding scarp and probably also ancient basal sliding. Both subhorizontal and high angle faults appear to be exposed in the bounding scarp, but the dips of the faults are poorly constrained. These fractures, along with the relatively undeformed layers between them, suggest to us that whatever horizontal motion may have taken place outward from the central cap region was accomplished by ancient basal sliding rather than large-scale glacial-like flow or ice migration by differential ablation, as proposed recently for the northern permanent cap and underlying NPLD. We have also obtained the first direct estimate of the regional dip of the SPLD, around 2-3 [degrees] outward (northward) in one area. Key Words: Mars, surface; geological processes.

We present techniques that use only information contained within a raw, high-spectral-resolution, hyperspectral Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) scene to estimate and remove additive components (atmospheric scattering and instrument dark current). These techniques allow normalization of multiplicative components (instrument gain, topography, atmospheric transmission) and enhancement, extraction, and identification of relative-reflectance information related to surface composition and mineralogy. Our derivation of additive components from raw AVIRIS data is based on an adaptation of Crippen's 'regression intersection method (RIM).' As does RIM, we use pairs of surface units that are spectrally homogeneous, spatially extensive, and located in rugged terrain. However, our technique utilizes the long-wavelength spectral data of AVIRIS to derive and remove atmospheric scattering components for each unit. AVIRIS data from the Kelso Dunes and Granite Mountain areas of southern California served as spectrally contrasting, topographically modulated surfaces for illustration of this technique. For a given site and wavelength pair, subtraction of the wavelength-dependent additive component from individual bands will remove topographic shading in both sites in band-to-band ratio images. Normalization of all spectra in the scene to the average scene spectrum results in cancellation of multiplicative components and produces a relative-reflectance scene. Absorption features due to mineral absorptions that depart from the average spectrum can be identified in the relative-reflectance AVIRIS product. The validity of these techniques is demonstrated by comparisons between relative-reflectance AVIRIS spectra derived from application of this technique and those derived by using the standard calibration techniques of JPL. Calibrated spectra were extracted from an AVIRIS scene of the Upheaval Dome area of Canyonlands National Park, UT. Results show that surface-reflectance information can be extracted and interpreted in terms of surface mineralogy after application of these techniques to AVIRIS data.

We analyze a collection of hyperspectral images of Io acquired by the near infrared mapping spectrometer (NIMS) of Galileo during the G2 to E16 orbits of Jupiter. This analysis leads to the geographical distribution and physical characterization of [SO.sub.2] frost deposits over about three-fourths of Io's surface. These deposits are excellent tracers of various phenomena, including volcanic production and emission, atmospheric transportation, condensation, metamorphism, irradiation, and sublimation, that occur throughout the [SO.sub.2] cycle. We assume that the deposits of solid [SO.sub.2] are optically thick and are geographically mixed with other sulfur-bearing compounds. We first assess the mean moderate backscattering behavior of the [SO.sub.2] frost (Henyey Greenstein phase function parameter g = -0.27 [+ or -] 0.05) using a sequence of spectra at two different locations over a large range of phase angles. This behavior may indicate a granular texture with many defects or a fluffy texture. Second, a more systematic inversion of the hyperspectral images is achieved based on a linear spectral model of pure [SO.sub.2] with variable grain size mixed with a spectrally neutral unit. As a result, we produce two global mosaics that map [SO.sub.2] frost coverage and mean grain size. [SO.sub.2] deposits are omnipresent on Io's surface at the spatial scale of this study ([approximately equals] 200 km), but the [SO.sub.2] frost is concentrated within several large areas centered at medium latitudes. These [SO.sub.2]-rich regions (surface coverage higher than 60%) show a longitudinal correlation with plumes located lower in latitude, suggesting that these plumes are the principal sources of [SO.sub.2] gas. After a possible dynamic condensation around the plumes or at the equator, the gas is remobilized by the solar or thermal fluxes and flows mostly latitudinally toward the coldest and nearest regions devoid of hot-spots. Third, the correlation of the distribution and grain size mosaics distinguishes four different [SO.sub.2] physical units that indicate relative regional variations of condensation, metamorphism, and sublimation. Finally, comparisons with Voyager ultraviolet (A. S. McEwen, T. V. Johnson, D. L. Matson, and L. A. Soderblom, 1988, Icarus, Vol. 75, pp. 450-478) and Galileo visible (P. Geissler, A. S. McEwen, L. Keszthelyi, R. M. C. Lopes-Gautier, J. Granahan, and D. P. Simonelli, 1999, Icarus, Vol. 140, pp. 256-282) observations demonstrate molecular contamination of [SO.sub.2] at medium and high latitudes and that these contaminated [SO.sub.2] deposits may be optically thin. [C] 2001 Academic Press Key Words: satellites of Jupiter; Io; infrared observations; ices; volcanism.