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Many objects in the solar system are suspected to have experienced reorientation of their spin axes. As their rotation rates are slow and their shapes are nearly spherical, the formation of mass anomalies, by either endogenic or exogenic processes, can change objects’ moments of inertia. Therefore, the objects reorient to align their largest moment of inertia with their spin axis. Such a phenomenon...
Enceladus is one of the most remarkable satellites in the solar system, as revealed by Cassini's detection of active plumes erupting from warm fractures near its south pole. This discovery makes Enceladus the only icy satellite known to exhibit ongoing internally driven geological activity. The activity is presumably powered by tidal heating maintained by Enceladus' 2:1 mean-motion resonance with...
We have mapped the locations of over 100,000 ice blocks across the south polar region of Saturn’s moon Enceladus, thus generating the first quantitative estimates of ice-block number density distribution in relation to major geological features. Ice blocks were manually identified and mapped from twenty of the highest resolution (4–25m per pixel) Cassini Imaging Science Subsystem (ISS) narrow-angle...
The Hapke (Hapke, B. [1981]. J. Geophys. Res. 86, 3039–3054) photometric model and its modifications are widely used to characterize telescopic, spacecraft, and laboratory observations of the bidirectional reflectance of particulate surfaces. Following work and methods laid out in a companion paper (Helfenstein, P., Shepard, M.K. [2011]. Icarus, in press), we deconstruct the Hapke model and, separating...
In conjunction with a companion paper (Shepard, M.K., Helfenstein, P. [2011]. Icarus, submitted for publication), we derive, test, and apply a detailed approach for visualizing the phase angle dependence of light scattering in particulate soils from both whole-disk and disk-resolved observations. To reduce the number of model parameters and provide stronger constraints on model fits, we combine Hapke’s...
We describe in-flight calibration of the Cassini Imaging Science Sub-system narrow- and wide-angle cameras using data from 2004 to 2009. We report on the photometric performance of the cameras including the use of polarization filters, point spread functions over a dynamic range greater than 10 7 , gain and loss of hot pixels, changes in flat fields, and an analysis of charge transfer efficiency...
We have used Cassini stereo images to study the topography of Iapetus' leading side. A terrain model derived at resolutions of 4–8 km reveals that Iapetus has substantial topography with heights in the range of −10 km to +13 km, much more than observed on the other middle-sized satellites of Saturn so far. Most of the topography is older than 4 Ga [Neukum, G., Wagner, R., Denk, T., Porco, C.C., 2005...
We present individual spectra 0.8–2.5 μm of the leading and trailing hemispheres of Enceladus obtained with the CorMASS spectrograph on the 1.8 m Vatican Advanced Technology Telescope (VATT) at the Mount Graham International Observatory. While the absorption bands of water ice dominate the spectrum of both hemispheres, most of these bands are stronger on the leading hemisphere than the trailing hemisphere...
We present a new heuristic model combining the coherent-backscatter and shadow-hiding opposition effects. The model considers the aggregate structure of regolith in a realistic way and accounts for the fractal architecture of planetary surfaces both on microscopic particulate and macroscopic, texturally faceted size scales. It describes how the shadow-hiding opposition effect is manifested in a fractally...
We have applied computer stereophotogrammetry to Apollo Lunar Surface Closeup Camera (ALSCC) pictures of the lunar surface to construct the first-ever digital topographic relief maps of undisturbed lunar soil over spatial scales from 85 μm to 8.5 cm. Using elevation histograms, fractal analysis, and Hapke's photometric roughness model we show that Apollo 14 (Fra Mauro) Imbrium ejecta is rougher than...
The unique high-resolution digital terrain data set available for Ganymede's Uruk Sulcus area and Galileo Regio is analyzed by quantitative image analysis techniques to identify relationships between surface brightness, elevation, and slopes. Broad and narrow ridges and grooves of the Uruk Sulcus terrain model are found to be aligned with bright and dark lineament patterns visible in images. These...
The Galileo mission has revealed remarkable evidence of mass movement and landform degradation on the icy Galilean satellites of Jupiter. Weakening of surface materials coupled with mass movement reduces the topographic relief of landforms by moving surface materials down-slope. Throughout the Galileo orbiter nominal mission we have studied all known forms of mass movement and landform degradation...
The multispectral imager on the Near-Earth Asteroid Rendezvous spacecraft has been subjected to a comprehensive series of inflight tests to validate its radiometric characteristics measured onground and to characterize instrument stability, pointing, geometric distortion, coalignment with other instruments, and light-scattering characteristics under flight conditions. The results of these tests, described...
We have combined spectral reflectance data from the Solid State Imaging (SSI) experiment, the Near-Infrared Mapping Spectrometer (NIMS), and the Ultraviolet Spectrometer (UVS) in an attempt to determine the composition and implied genesis of non-H 2 O components in the optical surface of Europa. We have considered four terrains: (1) the “dark terrains” on the trailing hemisphere, (2) the “mottled...
Using clear-filter images from Voyager 2 (effective wavelength 0.48 μm), we have constructed the first-ever digital albedo map of Saturn's moon Phoebe. Most normal reflectances in this new map are between 0.07 and 0.11; the albedo histogram is largely bimodal, suggesting that the satellite is covered predominantly by two different types of surface materials. The highest albedos are confined to isolated,...
Galileo's Solid State Imaging camera recorded six images at wavelengths from 0.41 to 0.99 μm of Europa's trailing hemisphere (∼1.6 km/pixel resolution) during the G1 orbit (1st orbit—target Ganymede) of the nominal mission. We have photometrically corrected these data and extracted spectra representing Europa's diverse geologic terrains. The goals of the analysis of these spectra are (1) to determine...
During its first two encounters with Ganymede, the Galileo spacecraft obtained images of a 16,500 km 2 portion of Galileo Regio, a large expanse of dark terrain, at high resolution (76–86 m/pixel). Through mapping of the G1 and G2 target sites within Galileo Regio, we are able to characterize geological units based on their morphology and relative albedo. We find three generally low albedo...
High-resolution Galileo imaging has provided important insight into the origin and evolution of grooved terrain on Ganymede. The Uruk Sulcus target site was the first imaged at high resolution, and considerations of resolution, viewing geometry, low image compression, and complementary stereo imaging make this region extremely informative. Contrast variations in these low-incidence angle images are...
Clear-filter Viking images, and an accurate numerical model of the shape of Phobos, have been used to determine this satellite's photometric properties. A global-average Hapke function derived from disk-resolved data confirms previous indications that Phobos has a strong opposition surge. Photometrically corrected images were mosaicked into an albedo map; most of the resulting normal reflectances...
We test the hypotheses that the lunar opposition effect is due to shadow-hiding, coherent backscatter, or some combination of the two phenomena. Hapke's photometric model is extended to include975726description of the coherent backscatter opposition effect. The model was fit simultaneously toV-filter (λ = 0.55 μm) whole-disk photometry of the Moon over many phase angles (2° ≤ α ≤ 143°) and to disk-resolved...
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