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USGS1996

Ida and Dactyl: Spectral reflectance and color variations

Galileo SSI color data between 0.4 and 1.0 ??m demonstrate that both Ida and Dactyl are S-type asteroids with similar, but distinct spectra. Small but definite color variations are also observed on Ida itself and involve both the blue part of the spectrum and the depth of the 1-??m pyroxene-olivine band. Ida's surface can be classified into two color terrains: Terrain A has a shallower 1-??m absorption and a steeper visible red slope than does Terrain B. Qualitatively, the color-albedo systematics of these two terrains follow those noted for color units on Gaspra and the variations in 1-??m band depth with weathering described by Gaffey et al. (Gaffey, M. J., J. F. Bell, R. H. Brown, T. H. Burbine, J. Piatek, K. L. Reed, and D. A. Chaky 1993. Icarus 106, 573-602). Terrain A, with its slightly lower albedo, its shallower 1-??m band, and its slightly steeper visible red slope relative to Terrain B could be interpreted as the "more processed," "more mature," or the "more weathered" of the two terrains. Consistent with this interpretation is that Terrain A appears to be the ubiquitous background on most of Ida, while Terrain B is correlated with some small craters as well as with possible ejecta from the 10-km Azzurra impact structure. Because of these trends, it is less likely that differences between Terrains A and B are caused by an original compositional inhomogeneity within the body of Ida, although they do fall within the range known to occur within the Koronis family. The spectrum of Dactyl is similar to, but definitely different from, that of Terrain B on Ida. It does not conform to the pattern that obtains between the colors and albedos of Terrains A and B: the satellite's 1-??m band is deeper than that of Terrain B, but its albedo is lower, rather than higher. By itself, the deeper band depth could be interpreted, following Gaffey et al., to mean that Dactyl is a less weathered version of Terrain B on Ida, but such an interpretation is at odds with Dactyl's redder spectral slope. Thus, the explanation for the color difference between Dactyl and Ida is likely to be different from that which accounts for the differences between the two terrains on Ida. Given that Dactyl and Ida have very similar photometric properties (Helfenstein, P., J. Veverka, P. C. Thomas, D. P. Simonelli, K. Klassen, T. V. Johnson, F. Fanale, J. Granahan, A. S. McEwen, M. J. S. Belton, and C. R. Chapman 1996 Icarus 120, 48-65), thus ruling out any dramatic texture differences between the two surfaces, the most likely explanation is that the satellite has a slightly different composition (more pyroxene?) than Ida. The spectral difference is within the range reported by Binzel et al. (Binzel, R. P., S. Xu, and S. J. Bus 1993. Icarus 106, 608-611.) for members of the Koronis family, and could be caused by compositional inhomogeneities of the Koronis parent body rather than by post-breakup regolith processes. ?? 1996 Academic Press, Inc.

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USGS2011

Hapke modeling of Rhea surface properties through Cassini-VIMS spectra

T The surface properties of the icy bodies in the saturnian system have been investigated by means of the Cassini-VIMS (Visual Infrared Mapping Spectrometer) hyperspectral imager which operates in the 0.35–5.1 μm wavelength range. In particular, we have analyzed 111 full disk hyperspectral images of Rhea ranging in solar phase between 0.08° and 109.8°. These data have been previously analyzed by Filacchione et al. (Filacchione, G. et al. [2007]. Icarus 186, 259–290; Filacchione, G. et al. [2010]. Icarus 206, 507–523) to study, adopting various “spectral indicators” (such as spectral slopes, band depth, and continuum level), the relations among various saturnian satellites. As a further step we proceed in this paper to a quantitative evaluation of the physical parameters determining the spectrophotometric properties of Rhea’s surface. To do this we have applied Hapke (Hapke, B. [1993]. Theory of Reflectance and Emittance Spectroscopy, Topics in Remote Sensing: 3. Springer, Berlin) IMSA model (Isotropic Multiple Scattering Approximation) which allow us to model the phase function at VIS–IR (visible–infrared) wavelengths as well as the spectra taking into account various types of mixtures of surface materials. Thanks to this method we have been able to constrain the size of water ice particles covering the surface, the amount of organic contaminants, the large scale surface roughness and the opposition effect surge. From our analysis it appears that wavelength dependent parameters, e.g. opposition surge width ( h ) and single-particle phase function parameters ( b , v ), are strongly correlated to the estimated single-scattering albedo of particles. For Rhea the best fit solution is obtained by assuming: (1) an intraparticle mixture of crystalline water ice and a small amount (0.4%) of Triton tholin; (2) a monodisperse grain size distribution having a particle diameter a m = 38 μm; and (3) a surface roughness parameter value of 33°. The study of phase function shows that both shadow hiding and coherent backscattering contribute to the opposition surge. This study represents the first attempt, in the case of Rhea, to join the spectral and the photometric analysis. The surface model we derived gives a good quantitative description of both spectrum and phase curve of the satellite. The same approach and model, with appropriate modifications, shall be applied to VIMS data of the other icy satellites of Saturn, in order to reveal similarities and differences in the surface characteristics to understand how these bodies interact with their environment.

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USGS1994

Discovery of Grooves on Gaspra

We report the discovery of grooves in Galileo high-resolution images of Gaspra. These features, previously seen only on Mars' satellite Phobos, are most likely related to severe impacts. Grooves on Gaspra occur as linear and pitted depressions, typically 100-200 m wide, 0.8 to 2.5 km long, and 10-20 m deep. Most occur in two major groups, one of which trends approximately parallel to the asteroid's long axis, but is offset by some 15??; the other is approximately perpendicular to this trend. The first of these directions falls along a family of planes which parallel three extensive flat facets identified by Thomas et al., Icarus 107. The occurrence of grooves on Gaspra is consistent with other indications (irregular shape, cratering record) that this asteroid has evolved through a violent collisional history. The bodywide congruence of major groove directions and other structural elements suggests that present-day Gaspra is a globally coherent body. ?? 1994 Academic Press. All rights reserved.

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USGS1996

The volcanic and tectonic history of Enceladus

Enceladus has a protracted history of impact cratering, cryo-volcanism, and extensional, compressional, and probable strike-slip faulting. It is unique in having some of the outer Solar System's least and most heavily cratered surfaces. Enceladus' cratering record, tectonic features, and relief elements have been analyzed more comprehensively than done previously. Like few other icy satellites, Enceladus seems to have experienced major lateral lithospheric motions; it may be the only icy satellite with global features indicating probable lithospheric convergence and folding. Ridged plains, 500 km across, consist of a central labyrinthine ridge complex atop a broad dome surrounded by smooth plains and peripheral sinuous ridge belts. The ridged plains have few if any signs of extension, almost no craters, and an average age of just 107 to 108 years. Ridge belts have local relief ranging from 500 to 2000 m and tend to occur near the bottoms of broad regional troughs between swells. Our reanalysis of Peter Thomas' (Dermott, S. F., and P. C. Thomas, 1994, The determination of the mass and mean density of Enceladus from its observed shape, Icarus, 109, 241-257) limb profiles indicates that high peaks, probably ridge belts, also occur in unmapped areas. Sinuous ridges appear foldlike and are similar to terrestrial fold belts such as the Appalachians. If they are indeed folds, it may require that the ridged plains are mechanically (perhaps volcanically) layered. Regional topography suggests that folding may have occurred along zones of convective downwelling. The cratered plains, in contrast to the ridged plains, are heavily cratered and exhibit extensional structures but no obvious signs of compression. Cratered plains contain a possible strike-slip fault (Isbanir Fossa), along which two pairs of fractures seem to have 15 km of right-lateral offset. The oldest cratered plains might date from shortly after the formation of the saturnian system or the impact disruption and reaccretion of Enceladus. Another area of cratered plains has modified craters (e.g., Ali Baba and Aladdin), which some workers have explained by anomalous heat flow and viscous relaxation; lateral shear and shield-building volcanism also may have been important. A young rift-like structure (northern Samarkand Sulci) has few craters and a concentration of cracks or grabens and flattened, flooded, and rifted craters. Pit chains and cratered domes suggest explosive volcanism. Smooth plains may have formed by cryovolcanic equivalents of flood-basalt volcanism. Pure H2O would be difficult to extrude through an icy crust and is cosmochemically improbable as a cryovolcanic agent. Density relations rule out eutectic brine lavas on Enceladus, but NH3-H2O volcanism is possible. Current steady-state tidal dissipation may cause melting of ammonia hydrate at a depth of ???25 km if the crust is made of ammonia hydrate or ???100 km if it is made of water ice. ?? 1996 Academic Press, Inc.

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USGS1997

Volcanic resurfacing of Io: Post-repair HST imaging

In March 1994, we used the newly refurbished Hubble Space Telescope (HST) to obtain global imaging of Io at five wavelengths between 0.34 and 1.02 μm, with a spatial resolution of 160 km. The images provided the clearest view of Io since Voyager and the first systematic observations in the wavelength range 0.7–1.0 μm. We have produced absolutely calibrated global mosaics of Io's reflectance in all our five wavelengths. The near-infrared images reveal that the 0.55- to 0.7-μm absorption edge seen in Io's disk-integrated spectrum has a very different spatial distribution from the better-known 0.40- to 0.50-μm absorption edge studied by Voyager, and must be generated by a different chemical species. The 0.55- to 0.7-μm absorption edge is strongly concentrated in the pyroclastic ejecta blanket of the volcano Pele, at a few much smaller discrete spots, and probably also in the polar regions. The Pele ejecta spectrum is consistent with the idea that S 2 O, partially decomposed to S 4 (and probably S 3 ), may be the species responsible for the 0.55- to 0.7-μm absorption edge at Pele and elsewhere on Io, though S 4 generated by other processes may also be a possibility. S 2 O can be produced by high-temperature decomposition of SO 2 gas, and the high temperature of the Pele volcano may account for its concentration there. Spectral anomalies of comparable size and prominence are not seen around the other “Pele-type” volcanos Surt and Aten (A. S. McEwen and L. A. Soderblom, 1983, Icarus 55, 191–217), suggesting that these volcanos, if chemically similar to Pele, are much less active. The spectrum of high-latitude regions is similar to that of quenched red sulfur glass, and if this similarity is not coincidental, the glass may be preserved here by the low polar surface temperatures. Alternatively, the low polar temperatures may preserve sulfur that has been reddened by radiation. There are many changes in albedo patterns in the 15 years between Voyager and these HST observations, but these are generally subtle at HST resolution and are not strongly concentrated in longitude; however there was a major brightening of a 400-km-diameter region centered on Ra Patera between March 1994 and repeat HST observations in July 1995, which was a larger albedo change than any seen in the previous 15 years. This was presumably due to a large eruption at Ra Patera, as confirmed by Galileo images. Long-exposure eclipse images of Io at 1.02 μm on March 6, 1994, place strong limits on the area of exposed silicate magma on Io at the time of the observations.

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USGS2000

Lunar Transient Phenomena: What do the Clementine Images Reveal?

Lunar Transient Phenomena (LTP) have been reported for at least 450 years. The events range from bright flashes, to reddish or bluish glows, to obscurations. Gaseous spectra and photometric measurements of the events have been obtained. Several theories have been offered as explanations for LTP, including residual volcanic activity or outgassing, bombardment by energetic particles, and piezoelectric effects. As the first set of digital multispectral images of the entire Moon, the Clementine data offer a unique opportunity to couple inferences of compositional relationships with lunar geomorphology in the regions of LTP. We have selected 11 regions from which numerous reliable historical reports of LTP exist. Our analysis of the Clementine multispectral images shows that many events occur in regions of bright, spectrally reddish deposits, which may be characteristic of volcanic ejecta. The events may be associated with outgassing of volatiles collected in or beneath mare basalt flows. We find that LTP tend to occur near the edges of maria, in agreement with a suggestion originally made by Cameron (1972. Icarus 16 , 339–387), and in other regions of crustal weakness. We also find that some of the reported events tend to be in craters with rims of distinctly different (bluer) composition. This compositional difference may result from recent slumping of the rim, accompanied by the appearance of fresher underlying material. In some cases, slumping may be triggered by the release of pockets of volatiles; in turn the slumping events may cause additional pockets of trapped material to be released. There are four instances in which Clementine multispectral images were acquired both before and after an event that was reported by a terrestrial team of amateur astronomers mobilized to observe the Moon during the mapping phase of Clementine. None of these four sets of images shows clear changes that could be attributed to the reported LTP.

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USGS1990

D/H on Mars: Effects of floods, volcanism, impacts, and polar processes

Water in the Martian atmosphere is 5.1 times more enriched in deuterium than terrestial water. The enrichment has been previously attributed to either a massive loss of water early in the planet's history or the presence of only a very small reservoir of water that has exchanged with the atmosphere over geologic time. Both these interpretations appear inconsistent with geologic evidence of large floods and sustained volcanism. Large floods are believed to have episodically introduced large amounts of water onto the surface. During a large flood roughly 10 17 g of water would almost immediately sublime into the atmospher and be frozen out on polar terrain, to form a new layer several centimeters thick. The long-term effect of a flood would depend on where the water pooled after the flood. If the water pooled at low latitudes, all the water would slowly sublime into the atmosphers and ultimately be frozen out at the poles, thereby adding several meters to the polar deposits for each flood. If the water pooled at high latitude, it would form a permanent ice deposit, largely isolated from further interchange with the atmosphere. Volcanism has also episodically introduced water into the atmosphere. Most of this water has become incorporated into the polar deposits. That released over the last 3.5 Ga could have added a few kilometers to the polar deposits, depending on the amount of dust incorporated along with the ice. Large cometary impacts would have introduced additional large amounts of water into the atmosphere. The long-term evolution of D/H in the atmosphere depends on the rate of exchange of water between the atmosphere and the polar deposits. If exchange is active, then loss rates of hydrogen from the upper atmosphere are substantially higher than those estimated by Y. L. Yung, J. Wen, J. P. Pinto, M. Allen, K. K. Pierce, and S. Paulsen [ Icarus 76, 146–159 (1988)]. More plausibly, exchange of water between the atmosphere and the polar deposits is limited, so that after eruptions, floods, and cometary impacts, the atmosphere soon becomes enriched in deuterium. According to this scenario, the atmospheric D/H is different from the bulk of the planet's water and so reveals little about the amount of water outgassed. The scenario implies, however, that the polar deposits are older and more stable than formerly thought.

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USGS1990

Comment on "The surface of lo: A new model" by Bruce Hapke

Hapke (1989, Icarus 79, 56–74) proposed that the surface of Io is dominantly basaltic with thin coatings of polysulfur oxide, S2O, ad SO,2. However, observations and models of the active volcanism indicate that volatiles such as sulfur and SO2 must be more abundant than envisioned by Hapke.

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USGS2004

Surface changes on Io during the Galileo mission

A careful survey of Galileo SSI global monitoring images revealed more than 80 apparent surface changes that took place on Io during the 5 year period of observation, ranging from giant plume deposits to subtle changes in the color or albedo of Patera surfaces. Explosive volcanic activity was discovered at four previously unrecognized centers: an unnamed patera to the south of Karei that produced a Pele-sized red ring, a patera to the west of Zal that produced a small circular bright deposit, a large orange ring detected near the north pole of Io, and a small bright ring near Io's south pole. Only a handful of Io's many active volcanoes produced large scale explosive eruptions, and several of these erupted repeatedly, leaving at least 83% of Io's surface unaltered throughout the Galileo mission. Most of the hot spots detected from SSI, NIMS and ground-based thermal observations caused no noticeable surface changes greater than 10 km in extent over the five year period. Surface changes were found at every location where active plumes were identified, including Acala which was never seen in sunlight and was only detected through auroral emissions during eclipse. Two types of plumes are distinguished on the basis of the size and color of their deposits, confirming post-Voyager suggestions by McEwen and Soderblom [Icarus 55 (1983) 191]. Smaller plumes produce near-circular rings typically 150-200 km in radius that are white or yellow in color unless contaminated with silicates, and frequently coat their surroundings with frosts of fine-grained SO2. The larger plumes are much less numerous, limited to a half dozen examples, and produce oval, orange or red, sulfur-rich rings with maximum radii in the north-south direction that are typically in the range from 500 to 550 km. Both types of plumes can be either episodic or quasi-continuous over a five year period. Repeated eruptions of the smaller SO2-rich plumes likely contribute significantly to Io's resurfacing rate, whereas dust ejection is likely dominated by the tenuous giant plumes. Both types of plume deposits fade on time-scales of months to years through burial and alteration. Episodic seepages of SO2 at Haemus Montes, Zal Montes, Dorian Montes, and the plateau to the north of Pillan Patera may have been triggered by activity at nearby volcanic centers.

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USGS2004

Lava lakes on Io: Observations of Io's volcanic activity from Galileo NIMS during the 2001 fly-bys

Galileo's Near-Infrared Mapping Spectrometer (NIMS) obtained its final observations of Io during the spacecraft's fly-bys in August (I31) and October 2001 (I32). We present a summary of the observations and results from these last two fly-bys, focusing on the distribution of thermal emission from Io's many volcanic regions that give insights into the eruption styles of individual hot spots. We include a compilation of hot spot data obtained from Galileo, Voyager, and ground-based observations. At least 152 active volcanic centers are now known on Io, 104 of which were discovered or confirmed by Galileo observations, including 23 from the I31 and I32 Io fly-by observations presented here. We modify the classification scheme of Keszthelyi et al. (2001, J. Geophys. Res. 106 (E12) 33 025-33 052) of Io eruption styles to include three primary types: promethean (lava flow fields emplaced as compound pahoehoe flows with small plumes < 200 km high originating from flow fronts), pillanian (violent eruptions generally accompanied by large outbursts, > 200 km high plumes and rapidly-emplaced flow fields), and a new style we call "lokian" that includes all eruptions confined within paterae with or without associated plume eruptions). Thermal maps of active paterae from NIMS data reveal hot edges that are characteristic of lava lakes. Comparisons with terrestrial analogs show that Io's lava lakes have thermal properties consistent with relatively inactive lava lakes. The majority of activity on Io, based on locations and longevity of hot spots, appears to be of this third type. This finding has implications for how Io is being resurfaced as our results imply that eruptions of lava are predominantly confined within paterae, thus making it unlikely that resurfacing is done primarily by extensive lava flows. Our conclusion is consistent with the findings of Geissler et al. (2004, Icarus, this issue) that plume eruptions and deposits, rather than the eruption of copious amounts of effusive lavas, are responsible for Io's high resurfacing rates. The origin and longevity of islands within ionian lava lakes remains enigmatic. ?? 2003 Elsevier Inc. All rights reserved.

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USGS2005

Evidence for subsurface water ice in Korolev crater, Mars

Following the work of Kieffer and Titus (2001, Icarus 154, 162–180), we present results of thermal IR observations of Korolev crater, located at ∼ 73 ° "> ∼73° latitude in the martian northern polar region. Similar to techniques employed by Titus et al. (2003, Science 299, 1048–1050), we use infrared images from the Thermal Emission Imaging System (THEMIS) aboard Mars Odyssey to identify several regions within the crater basin with distinct thermal properties that correlate with topography. The THEMIS results show these regions exhibit temperature variations, spatially within the crater and throughout the martian year. In addition to the variations identified in the THEMIS observations, Mars Global Surveyor Thermal Emission Spectrometer (TES) observations show differences in albedo and temperature of these regions on both daily and seasonal cycles. Modeling annual temperature variations of the surface, we use TES observations to examine the thermal properties of these regions. This analysis reveals the crater interior deposits are likely thick layers (several meters) of high thermal inertia material (water ice, or extremely ice-rich regolith). Spatial variations of the physical properties of these regions are likely due to topography and possibly variations in the subsurface material itself. The nature of these deposits may help constrain polar processes, as well as provide context for the polar lander mission, Phoenix.

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USGS2005

The rayed crater Zunil and interpretations of small impact craters on Mars

A 10-km diameter crater named Zunil in the Cerberus Plains of Mars created ∼ 10 7 "> ∼107 secondary craters 10 to 200 m in diameter. Many of these secondary craters are concentrated in radial streaks that extend up to 1600 km from the primary crater, identical to lunar rays. Most of the larger Zunil secondaries are distinctive in both visible and thermal infrared imaging. MOC images of the secondary craters show sharp rims and bright ejecta and rays, but the craters are shallow and often noncircular, as expected for relatively low-velocity impacts. About 80% of the impact craters superimposed over the youngest surfaces in the Cerberus Plains, such as Athabasca Valles, have the distinctive characteristics of Zunil secondaries. We have not identified any other large (⩾10 km diameter) impact crater on Mars with such distinctive rays of young secondary craters, so the age of the crater may be less than a few Ma. Zunil formed in the apparently youngest (least cratered) large-scale lava plains on Mars, and may be an excellent example of how spallation of a competent surface layer can produce high-velocity ejecta (Melosh, 1984, Impact ejection, spallation, and the origin of meteorites, Icarus 59, 234–260). It could be the source crater for some of the basaltic shergottites, consistent with their crystallization and ejection ages, composition, and the fact that Zunil produced abundant high-velocity ejecta fragments. A 3D hydrodynamic simulation of the impact event produced 10 10 rock fragments ⩾10 cm diameter, leading to up to 10 9 secondary craters ⩾10 m diameter. Nearly all of the simulated secondary craters larger than 50 m are within 800 km of the impact site but the more abundant smaller (10–50 m) craters extend out to 3500 km. If Zunil is representative of large impact events on Mars, then secondaries should be more abundant than primaries at diameters a factor of ∼1000 smaller than that of the largest primary crater that contributed secondaries. As a result, most small craters on Mars could be secondaries. Depth/diameter ratios of 1300 small craters (10–500 m diameter) in Isidis Planitia and Gusev crater have a mean value of 0.08; the freshest of these craters give a ratio of 0.11, identical to that of fresh secondary craters on the Moon (Pike and Wilhelms, 1978, Secondary-impact craters on the Moon: topographic form and geologic process, Lunar Planet. Sci. IX, 907–909) and significantly less than the value of ∼0.2 or more expected for fresh primary craters of this size range. Several observations suggest that the production functions of Hartmann and Neukum (2001, Cratering chronology and the evolution of Mars, Space Sci. Rev. 96, 165–194) predict too many primary craters smaller than a few hundred meters in diameter. Fewer small, high-velocity impacts may explain why there appears to be little impact regolith over Amazonian terrains. Martian terrains dated by small craters could be older than reported in recent publications.

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USGS2009

The geology of Hotei Regio, Titan: Correlation of Cassini VIMS and RADAR

Joint Cassini VIMS and RADAR SAR data of ∼700-km-wide Hotei Regio reveal a rich collection of geological features that correlate between the two sets of images. The degree of correlation is greater than anywhere else seen on Titan. Central to Hotei Regio is a basin filled with cryovolcanic flows that are anomalously bright in VIMS data (in particular at 5 μm) and quite variable in roughness in SAR. The edges of the flows are dark in SAR data and appear to overrun a VIMS-bright substrate. SAR-stereo topography shows the flows to be viscous, 100–200 m thick. On its southern edge the basin is ringed by higher (∼1 km) mountainous terrain. The mountains show mixed texture in SAR data: some regions are extremely rough, exhibit low and spectrally neutral albedo in VIMS data and may be partly coated with darker hydrocarbons. Around the southern margin of Hotei Regio, the SAR image shows several large, dendritic, radar-bright channels that flow down from the mountainous terrain and terminate in dark blue patches, seen in VIMS images, whose infrared color is consistent with enrichment in water ice. The patches are in depressions that we interpret to be filled with fluvial deposits eroded and transported by liquid methane in the channels. In the VIMS images the dark blue patches are encased in a latticework of lighter bands that we suggest to demark a set of circumferential and radial fault systems bounding structural depressions. Conceivably the circular features are tectonic structures that are remnant from an ancient impact structure. We suggest that impact-generated structures may have simply served as zones of weakness; no direct causal connection, such as impact-induced volcanism, is implied. We also speculate that two large dark features lying on the northern margin of Hotei Regio could be calderas. In summary the preservation of such a broad suite of VIMS infrared color variations and the detailed correlation with features in the SAR image and SAR topography evidence a complex set of geological processes (pluvial, fluvial, tectonic, cryovolcanic, impact) that have likely remained active up to very recent geological time (<10 4 year). That the cryovolcanic flows are excessively bright in the infrared, particularly at 5 μm, might signal ongoing geological activity. One study [Nelson, R.M., and 28 colleagues, 2009. Icarus 199, 429–441] reported significant 2-μm albedo changes in VIMS data for Hotei Arcus acquired between 2004 and 2006, that were interpreted as evidence for such activity. However in our review of that work, we do not agree that such evidence has yet been found.

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USGS2010

Cassini spectra and photometry 0.25–5.1 μm of the small inner satellites of Saturn

The nominal tour of the Cassini mission enabled the first spectra and solar phase curves of the small inner satellites of Saturn. We present spectra from the Visual Infrared Mapping Spectrometer (VIMS) and the Imaging Science Subsystem (ISS) that span the 0.25-5.1 ??m spectral range. The composition of Atlas, Pandora, Janus, Epimetheus, Calypso, and Telesto is primarily water ice, with a small amount (???5%) of contaminant, which most likely consists of hydrocarbons. The optical properties of the "shepherd" satellites and the coorbitals are tied to the A-ring, while those of the Tethys Lagrangians are tied to the E-ring of Saturn. The color of the satellites becomes progressively bluer with distance from Saturn, presumably from the increased influence of the E-ring; Telesto is as blue as Enceladus. Janus and Epimetheus have very similar spectra, although the latter appears to have a thicker coating of ring material. For at least four of the satellites, we find evidence for the spectral line at 0.68 ??m that Vilas et al. [Vilas, F., Larsen, S.M., Stockstill, K.R., Gaffley, M.J., 1996. Icarus 124, 262-267] attributed to hydrated iron minerals on Iapetus and Hyperion. However, it is difficult to produce a spectral mixing model that includes this component. We find no evidence for CO 2 on any of the small satellites. There was a sufficient excursion in solar phase angle to create solar phase curves for Janus and Telesto. They bear a close similarity to the solar phase curves of the medium-sized inner icy satellites. Preliminary spectral modeling suggests that the contaminant on these bodies is not the same as the exogenously placed low-albedo material on Iapetus, but is rather a native material. The lack of CO 2 on the small inner satellites also suggests that their low-albedo material is distinct from that on Iapetus, Phoebe, and Hyperion. ?? 2009 Elsevier Inc.

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USGS2010

Saturn's icy satellites investigated by Cassini-VIMS. II. Results at the end of nominal mission

We report the detailed analysis of the spectrophotometric properties of Saturn's icy satellites as derived by full-disk observations obtained by visual and infrared mapping spectrometer (VIMS) experiment aboard Cassini. In this paper, we have extended the coverage until the end of the Cassini's nominal mission (June 1st 2008), while a previous paper (Filacchione, G., and 28 colleagues [2007]. Icarus 186, 259-290, hereby referred to as Paper I) reported the preliminary results of this study. During the four years of nominal mission, VIMS has observed the entire population of Saturn's icy satellites allowing us to make a comparative analysis of the VIS-NIR spectral properties of the major satellites (Mimas, Enceladus, Tethys, Dione, Rhea, Hyperion, Iapetus) and irregular moons (Atlas, Prometheus, Pandora, Janus, Epimetheus, Telesto, Calypso, Phoebe). The results we discuss here are derived from the entire dataset available at June 2008 which consists of 1417 full-disk observations acquired from a variety of distances and inclinations from the equatorial plane, with different phase angles and hemispheric coverage. The most important spectrophotometric indicators (as defined in Paper I: I/F continua at 0.55 ??m, 1.822 ??m and 3.547 ??m, visible spectral slopes, water and carbon dioxide bands depths and positions) are calculated for each observation in order to investigate the disk-integrated composition of the satellites, the distribution of water ice respect to "contaminants" abundances and typical regolith grain properties. These quantities vary from the almost pure water ice surfaces of Enceladus and Calypso to the organic and carbon dioxide rich Hyperion, Iapetus and Phoebe. Janus visible colors are intermediate between these two classes having a slightly positive spectral slope. These results could help to decipher the origins and evolutionary history of the minor moons of the Saturn's system. We introduce a polar representation of the spectrophotometric parameters as function of the solar phase angle (along radial distance) and of the effective longitude interval illuminated by the Sun and covered by VIMS during the observation (in azimuth) to better investigate the spatial distribution of the spectrophotometric quantities across the regular satellites hemispheres. Finally, we report the observed spectral positions of the 4.26 ??m band of the carbon dioxide present in the surface material of three outermost moons Hyperion, Iapetus and Phoebe. ?? 2009 Elsevier Inc.

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USGS2010

A new look at photometry of the Moon

We use ROLO photometry (Kieffer, H.H., Stone, T.C. [2005]. Astron. J. 129, 2887-2901) to characterize the before and after full Moon radiance variation for a typical highlands site and a typical mare site. Focusing on the phase angle range 45??. <. ??<. 50??, we test two different physical models, macroscopic roughness and multiple scattering between regolith particles, for their ability to quantitatively reproduce the measured radiance difference. Our method for estimating the rms slope angle is unique and model-independent in the sense that the measured radiance factor I/. F at small incidence angles (high Sun) is used as an estimate of I/. F for zero roughness regolith. The roughness is determined from the change in I/. F at larger incidence angles. We determine the roughness for 23 wavelengths from 350 to 939. nm. There is no significant wavelength dependence. The average rms slope angle is 22.2?? ?? 1.3?? for the mare site and 34.1?? ?? 2.6?? for the highland site. These large slopes, which are similar to previous "photometric roughness" estimates, require that sub-mm scale "micro-topography" dominates roughness measurements based on photometry, consistent with the conclusions of Helfenstein and Shepard (Helfenstein, P., Shepard, M.K. [1999]. Icarus 141, 107-131). We then tested an alternative and very different model for the before and after full Moon I/. F variation: multiple scattering within a flat layer of realistic regolith particles. This model consists of a log normal size distribution of spheres that match the measured distribution of particles in a typical mature lunar soil 72141,1 (McKay, D.S., Fruland, R.M., Heiken, G.H. [1974]. Proc. Lunar Sci. Conf. 5, Geochim. Cosmochim. Acta 1 (5), 887-906). The model particles have a complex index of refraction 1.65-0.003. i, where 1.65 is typical of impact-generated lunar glasses. Of the four model parameters, three were fixed at values determined from Apollo lunar soils: the mean radius and width of the log normal size distribution and the real part of the refraction index. We used FORTRAN programs from Mishchenko et al. (Mishchenko, M.I., Dlugach, J.M., Yanovitskij, E.G., Zakharova, N.T. [1999]. J. Quant. Spectrosc. Radiat. Trans. 63, 409-432; Mishchenko, M.I., Travis, L.D., Lacis, A.A. [2002]. Scattering, Absorption and Emission of Light by Small Particles. Cambridge Univ. Press, New York. <. http://www.giss.nasa.gov/staff/mmishchenko/books.html>) to calculate the scattering matrix and solve the radiative transfer equation for I/. F. The mean single scattering albedo is ??=0.808, the asymmetry parameter is ???cos. ?????=0.77 and the phase function is very strongly peaked in both the forward and backward scattering directions. The fit to the observations for the highland site is excellent and multiply scattered photons contribute 80% of I/. F. We conclude that either model, roughness or multiple scattering, can match the observations, but that the strongly anisotropic phase functions of realistic particles require rigorous calculation of many orders of scattering or spurious photometric roughness estimates are guaranteed. Our multiple scattering calculation is the first to combine: (1) a regolith model matched to the measured particle size distribution and index of refraction of the lunar soil, (2) a rigorous calculation of the particle phase function and solution of the radiative transfer equation, and (3) application to lunar photometry with absolute radiance calibration. ?? 2010 Elsevier Inc.

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USGS2012

Searching for evidence of hydrothermal activity at Apollinaris Mons, Mars

A multidisciplinary approach involving various remote sensing instruments is used to investigate Apollinaris Mons, a prominent volcano on Mars, as well as the surrounding plains for signs of prolonged hydrologic and volcanic, and possibly hydrothermal activity. The main findings include (1) evidence from laser altimetry indicating the large thickness (1.5–2 km at some locations) of the fan deposits draping the southern flank contrary to previous estimates, coupled with possible layering which point to a significant emplacement phase at Apollinaris Mons, (2) corroboration of Robinson et al. (Robinson, M.S., Mouginis-Mark, P.J., Zimbelman, J.R., Wu, S.S.C., Ablin, K.K., Howington-Kraus, A.E. [1993]. Icarus 104, 301–323) hypothesis regarding the formation of incised valleys on the western flanks by density current erosion which would indicate magma–water interaction or, alternatively, volatile-rich magmas early in the volcano’s history, (3) mounds of diverse geometric shapes, many of which display summit depressions and occur among faults and fractures, possibly marking venting, (4) strong indicators on the flanks of the volcano for lahar events, and possibly, a caldera lake, (5) ubiquitous presence of impact craters displaying fluidized ejecta in both shield-forming (flank and caldera) materials and materials that surround the volcano that are indicative of water-rich target materials at the time of impact, (6) long-term complex association in time among shield-forming materials and Medusae Fossae Formation. The findings point to a site of extensive volcanic and hydrologic activity with possibly a period of magma–water interaction and hydrothermal activity. Finally, we propose that the mound structures around Apollinaris should be prime targets for further in situ exploration and search for possible exobiological signatures.

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USGS2015

Constraining the heat flux between Enceladus’ tiger stripes: numerical modeling of funiscular plains formation

The Cassini spacecraft’s Composite Infrared Spectrometer (CIRS) has observed at least 5 GW of thermal emission at Enceladus’ south pole. The vast majority of this emission is localized on the four long, parallel, evenly-spaced fractures dubbed tiger stripes. However, the thermal emission from regions between the tiger stripes has not been determined. These spatially localized regions have a unique morphology consisting of short-wavelength (&sim;1 km) ridges and troughs with topographic amplitudes of &sim;100 m, and a generally ropy appearance that has led to them being referred to as “funiscular terrain.” Previous analysis pursued the hypothesis that the funiscular terrain formed via thin-skinned folding, analogous to that occurring on a pahoehoe flow top (Barr, A.C., Preuss, L.J. [2010]. Icarus 208, 499–503). Here we use finite element modeling of lithospheric shortening to further explore this hypothesis. Our best-case simulations reproduce funiscular-like morphologies, although our simulated fold wavelengths after 10% shortening are 30% longer than those observed. Reproducing short-wavelength folds requires high effective surface temperatures (&sim;185 K), an ice lithosphere (or high-viscosity layer) with a low thermal conductivity (one-half to one-third that of intact ice or lower), and very high heat fluxes (perhaps as great as 400 mW m −2 ). These conditions are driven by the requirement that the high-viscosity layer remain extremely thin (≲200 m). Whereas the required conditions are extreme, they can be met if a layer of fine grained plume material 1–10 m thick, or a highly fractured ice layer >50 m thick insulates the surface, and the lithosphere is fractured throughout as well. The source of the necessary heat flux (a factor of two greater than previous estimates) is less obvious. We also present evidence for an unusual color/spectral character of the ropy terrain, possibly related to its unique surface texture. Our simulations demonstrate that producing the funiscular ridges via folding remains plausible, but the relatively extreme conditions required to do so leaves their origin open to further investigation. The high heat fluxes required to produce the terrain by folding, which equate to an endogenic blackbody temperature near 50 K, should be observable by future nighttime CIRS observations, if funiscular deformation is occurring today.

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