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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.07.006 Byline: Andrew J. Dombard (a), Olivier S. Barnouin (a), Louise M. Prockter (a), Peter C. Thomas (b) Keywords: Asteroids, Surfaces; Asteroid Eros; Geological processes Abstract: There are [approximately equal to]300 features on the Asteroid 433 Eros that morphologically resemble ponds (flat-floored and sharply embaying the bounding depression in which they sit). Because boulders on Eros are apparently eroding in place and because ponds with associated boulders tend to be larger than ponds without blocks, we propose that ponds form from thermally disaggregated and seismically flattened boulder material, under the assumption that repeated day/night cycling causes material fatigue that leads to erosion of the boulders. Results from a simple boulder emplacement/thermal erosion model with boulders emplaced in a few discrete events (i.e., large impacts) match well the observed size distribution. Under this scenario, the subtle color differences of ponds (somewhat bluer than the rest of the surface) might be due to some combination of less space-weathered material and density stratification of silicate-rich chondrules and more metal-rich matrix from a disaggregated boulder. Volume estimates of ponds derived from NEAR Laser Rangefinder profiles are consistent with what can be supplied by boulders. Ponds are also observed to be concentrated in regions of low slope and high elevation, which suggests the presence of a less mobile regolith and thus a contrast in the resistance to seismic shaking between the pond material and the material that makes up the bounding depression. Future tests include shake-table experiments and temperature cycling (fatigue) of ordinary chondrites to test the thermal erosion mechanism. Author Affiliation: (a) The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Rd., Laurel, MD 20723, United States (b) Center for Radiophysics and Space Research, Cornell University, Ithaca, NY 14853, United States Article History: Received 28 January 2009; Revised 2 July 2010; Accepted 3 July 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.06.018 Byline: James H. Shirley (a), James B. Dalton (a), Louise M. Prockter (b), Lucas W. Kamp (a) Keywords: Europa; Satellites, Composition; Spectroscopy; Jupiter, Satellites Abstract: This investigation uses linear mixture modeling employing cryogenic laboratory reference spectra to estimate surface compositions and water ice grain sizes of Europa's ridged plains and smooth low albedo plains. Near-infrared spectra for 23 exposures of ridged plains materials are analyzed along with 11 spectra representing low albedo plains. Modeling indicates that these geologic units differ both in the relative abundance of non-ice hydrated species and in the abundance and grain sizes of water ice. The background ridged plains in our study area appear to consist predominantly of water ice ([approximately equal to]46%) with approximately equal amounts (on average) of hydrated sulfuric acid ([approximately equal to]27%) and hydrated salts ([approximately equal to]27%). The solutions for the smooth low albedo plains are dominated by hydrated salts ([approximately equal to]62%), with a relatively low mean abundance of water ice ([approximately equal to]10%), and an abundance of hydrated sulfuric acid similar to that found in ridged plains ([approximately equal to]27%). The model yields larger water ice grain sizes (100[mu]m versus 50-75[mu]m) in the ridged plains. The 1.5-[mu]m water ice absorption band minimum is found at shorter wavelengths in the low albedo plains deposits than in the ridged plains (1.498[+ or -].003[mu]m versus 1.504[+ or -].001[mu]m). The 2.0-[mu]m band minimum in the low albedo plains exhibits a somewhat larger blueshift (1.964[+ or -].006[mu]m versus 1.983[+ or -].006[mu]m for the ridged plains). The study area spans longitudes from 168[degrees] to 185[degrees]W, which includes Europa's leading side-trailing side boundary. A well-defined spatial gradient of sulfuric acid hydrate abundance is found for both geologic units, with concentrations increasing in the direction of the trailing side apex. We associate this distribution with the exogenic effects of magnetospheric charged particle bombardment and associated chemical processing of surface materials (the radiolytic sulfur cycle). However, one family of low albedo plains exposures exhibits sulfuric acid hydrate abundances up to 33% lower than found for adjacent exposures, suggesting that these materials have undergone less processing, thus implying that these deposits may have been emplaced more recently. Modeling identifies high abundances (to 30%) of magnesium sulfate brines in the low albedo plains exposures. Our investigation marks the first spectroscopic identification of MgSO.sub.4 brine on Europa. We also find significantly higher abundances of sodium-bearing species (bloedite and mirabilite) in the low albedo plains. The results illuminate the role of radiolytic processes in modifying the surface composition of Europa, and may provide new constraints for models of the composition of Europa's putative subsurface ocean. Author Affiliation: (a) Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Road, Pasadena, CA 91109, USA (b) Applied Physics Laboratory, Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, MD 20723, USA Article History: Received 27 October 2009; Revised 16 June 2010; Accepted 16 June 2010

During its first two flybys of Mercury, the MESSENGER spacecraft acquired images confirming that pervasive volcanism occurred early in the planet's history. MESSENGER's third Mercury flyby revealed a 290-kilometer-diameter peak-ring impact basin, among the youngest basins yet seen, having an inner floor filled with spectrally distinct smooth plains. These plains are sparsely cratered, postdate the formation of the basin, apparently formed from material that once flowed across the surface, and are therefore interpreted to be volcanic in origin. An irregular depression surrounded by a halo of bright deposits northeast of the basin marks a candidate explosive volcanic vent larger than any previously identified on Mercury. Volcanism on the planet thus spanned a considerable duration, perhaps extending well into the second half of solar system history. 10.1126/science.1188186

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.06.028 Byline: Debra L. Buczkowski, Olivier S. Barnouin-Jha, Louise M. Prockter Keywords: Asteroid Eros; Asteroids; surfaces; Geological processes; Impact processes Abstract: Using images and laser ranging data from the NEAR-Shoemaker mission, we map lineaments on the surface of Eros in order to investigate the relationship between surface morphology and interior structure. Several sets of lineations are clearly related to visible impact craters, while others suggest that different parts of the asteroid may have undergone different stress histories. Some of these sets infer internal structure, at least on a local level. This structure may derive from Eros' parent body and suggest, although largely coherent, Eros' interior may have portions that have not undergone a common history. Author Affiliation: Planetary Exploration Group, Space Department, Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Rd., Mailstop: MP3-E104, Laurel, MD 20723, USA Article History: Received 17 November 2006; Revised 25 May 2007

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.08.003 Byline: Louise Prockter (a), Paul Schenk (b) Keywords: Europa; Surfaces; Satellite; Volcanism; Satellites of Jupiter; Tectonics Abstract: Europa's Castalia Macula region was comprehensively imaged by the Galileo spacecraft on several orbits, at both local and regional resolutions and with different illumination geometries. Using these datasets we have mapped and identified the different geological units within the Castalia area, and derived digital elevation models (DEMs) of the topography within most of the Castalia Macula region. Using these data sets in combination allows us to map the geology and topography of this area in greater detail than perhaps any other site on Europa. Castalia Macula consists of unusually dark and reddish material, most of which is confined to a broad topographic depression 350 m deep located between two large uplifted domes 900 and 750 m high, to the north and south, respectively. The preservation of topography at the bottom of Castalia Macula indicates that dark material initially filled the depression to a certain depth but was subsequently removed via drainage, resulting in a dark stain up to the original equipotential surface. Superposition and topographic relationships suggest that the Castalia Macula plains deposit formed prior to uplift of both domes, and at least two distinct episodes of chaos formation have occurred near and on top of the northern dome. It appears that Castalia Macula is comparatively young and was active relatively recently, therefore it could provide an ideal place to sample material that has recently been erupted from the subsurface, and may have been in communication with Europa's ocean. These factors combine to make Castalia Macula a very attractive site for a future Europa lander. Author Affiliation: (a) Applied Physics Laboratory, Johns Hopkins University, MP3-E178, 11100 Johns Hopkins Road, Laurel, MD 20723, USA (b) Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, TX 77058, USA Article History: Received 9 September 2004; Revised 7 August 2005

Europa's Castalia Macula region was comprehensively imaged by the Galileo spacecraft on several orbits, at both local and regional resolutions and with different illumination geometries. Using these datasets we have mapped and identified the different geological units within the Castalia area, and derived digital elevation models (DEMs) of the topography within most of the Castalia Macula region. Using these data sets in combination allows us to map the geology and topography of this area in greater detail than perhaps any other site on Europa. Castalia Macula consists of unusually dark and reddish material, most of which is confined to a broad topographic depression 350 m deep located between two large uplifted domes 900 and 750 m high, to the north and south, respectively. The preservation of topography at the bottom of Castalia Macula indicates that dark material initially filled the depression to a certain depth but was subsequently removed via drainage, resulting in a dark stain up to the original equipotential surface. Superposition and topographic relationships suggest that the Castalia Macula plains deposit formed prior to uplift of both domes, and at least two distinct episodes of chaos formation have occurred near and on top of the northern dome. It appears that Castalia Macula is comparatively young and was active relatively recently, therefore it could provide an ideal place to sample material that has recently been erupted from the subsurface, and may have been in communication with Europa's ocean. These factors combine to make Castalia Macula a very attractive site for a future Europa lander. Keywords: Europa; Surfaces; Satellite; Volcanism; Satellites of Jupiter; Tectonics

During the Near-Earth Asteroid Rendezvous (NEAR) spacecraft's investigation of asteroid 433 Eros, inflight calibration measurements from the multispectral imager (MSI) have provided refined knowledge of the camera's radiometric performance, pointing, and light-scattering characteristics. Measurements while at Eros corroborate most earlier calibration results, although there appears to be a small, gradual change in instrument dark current and flat field due to effects of aging in the space environment. The most pronounced change in instrument behavior, however, is a dramatic increase in scattered light due to contaminants accumulated on the optics during unscheduled fuel usage in December 1998. Procedures to accurately quantify and to remediate the scattered light are described in a companion paper (Li et al. 2002, Icarus 155, 00-00). Acquisition of Eros measurements has clarified the relative, filter-to-filter, radiometric performance of the MSI. Absolute radiometric calibration appears very well constrained from flight measurements, with an accuracy of ~5%. Pointing relative to the spacecraft coordinate system can be determined from the temperature of the spacecraft deck with an accuracy of ~1 pixel. Key Words: asteroids; instrumentation; surfaces; Eros; NEAR.

Earth-based spectral measurements and NEAR Shoemaker magnetometer, X-ray, and near-infrared spectrometer data are all consistent with Eros having a bulk composition and mineralogy similar to ordinary chondrite meteorites (OC). By comparing the bulk density of 433 Eros (2.67 [+ or -] 0.03 g/[cm.sup.3]) with that of OCs (3.40 g/[cm.sup.3]), we estimate the total porosity of the asteroid to be 21-33%. Macro (or structural) porosity, best estimated to be ~20%, is constrained to be between 6 and 33%. We conclude that Eros is a heavily fractured body, but we find no evidence that it was ever catastrophically disrupted and reaccumulated into a rubble pile. Key Words: Eros; meteorites; surfaces, asteroids; interiors.

NEAR Shoemaker imaging of the asteroid Eros has revealed a variety of surface forms that probably reflect underlying structures. They range from global to local in scale. The most prominent positive topographic feature is a long ridge, Rahe Dorsum, which extends over 18 km and has the morphology of a thrust fault. Two large, ancient trough systems are also present. Grooves are ubiquitous, commonly occur in suborthogonal sets, and have linear or scalloped outlines. The grooves on Eros are similar in morphology and scale to those on other small bodies such as Gaspra, Ida, and Phobos and are likely to have a similar mode of origin, namely drainage of regolith into underlying fractures. Models that use groove dimensions as indicators of regolith thickness imply depths of up to several tens of meters. Many craters have been subjected to structural control, resulting in squared outlines. We find no evidence that specific groove sets are associated with individual impact events; however, evidence exists that preexisting structures have been reactivated by later impact activity. The large scale of Rahe Dorsum and many grooves on Eros's surface suggests that the asteroid is a largely coherent, but fractured body. Key Words: asteroids; Eros; impact processes; regoliths; asteroid surfaces; tectonics.

During its first two encounters with Ganymede, the Galileo spacecraft obtained images of a 16,500 [km.sup.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 units: an intermediate albedo plains unit, a lower albedo plains unit, and the lowest albedo unit which is found on furrow and crater floors. We also find high albedo units which include crater rims, furrow rims, and isolated knobs and massifs. Other features include an intermediate albedo lobate feature interpreted to be a palimpsest and a hummocky unit interpreted to be impact ejecta. Several processes are interpreted to have occurred within Galileo Regio. These include tectonic deformation, mass wasting, sublimation, resurfacing by impact ejecta, and possibly cryovolcanism and isostatic adjustment. We observe that the NW-SE trending furrows (Lakhmu Fossae) in Galileo Regio are degraded and are crosscut by the younger N-S trending furrows (Zu Fossae). We also find several other tectonic features which may be minor faults or fractures related to one or other of these systems. Through mapping and crater size-frequency distributions, we are able to propose a stratigraphy for the Galileo Regio target site. The oldest features in the area are high albedo knobs and massifs, which are interpreted to be remnants of early impact-related features and furrow rims. These may have formed at approximately the same time as the intermediate and low albedo plains units and the furrow systems. The lowest albedo unit of furrow floors probably subsequently evolved through sublimation and mass wasting. Much of the northeast portion of the target area was subsequently obscured by one of the youngest units, ejecta from an impact just to the north. We use our mapping of the high-resolution images of Galileo Regio to evaluate three endmember models for the formation of dark terrain: (1) the crust is dark throughout, (2) material on the surface is the result of a low albedo cryovolcanic layer over a higher albedo crust, and (3) dark material is distributed in small quantities throughout the crust, and geological processes have acted to concentrate low albedo material on the surface. Although it is possible that elements of more than one of these models are present within the dark terrain, we find that the third model, that of a thin veneer of low albedo material, best fits observations of Galileo Regio. Key Words: Ganymede; geological processes; satellites, general; satellites of Jupiter; surfaces, satellite.

Author(s): Louise M. Prockter (corresponding author) Jupiter has 16 known satellites, one of which, Ganymede, is a whopper -- it is the largest moon in the Solar System, and is [...]

During the Near Earth Asteroid Rendezvous (NEAR)--Shoemaker's low-altitude flyover of asteroid 433 Eros, observations by the NEAR Laser Rangefinder (NLR) have helped to characterize small-scale surface features. On scales from meters to hundreds of meters, the surface has a fractal structure with roughness dominated by blocks, structural features, and walls of small craters. This fractal structure suggests that a single process, possibly impacts, dominates surface morphology on these scales., The NEAR--Shoemaker mission (1) has measured the shape of asteroid 433 Eros from orbit with a laser altimeter (2), enabling quantitative assessments of the asteroid's surface morphology at scales of [...]

Regional-scale undulations with associated small-scale secondary structures are inferred to be folds on Jupiter's moon Europa. Formation is consistent with stresses from tidal deformation, potentially triggering compressional instability of a region of locally high thermal gradient. Folds may compensate for extension elsewhere on Europa and then relax away over time., Since 1979 when the two Voyager spacecraft began their survey of the outer solar system's icy satellites, abundant extensional tectonic structures but only tentative examples of compressional structures have been [...]

We investigate cryomagmatic mechanisms for the formation of the diffuse low-albedo margins of triple bands and haloes associated with elliptical lenticulae on Europa. One mechanism involves explosive venting of a gas-particle spray derived from a subsurface body of volatile-bearing water. Assuming the dark features represent cryoclastic deposits erupted from a medial or central vent, modeling the dynamics of the eruptive plumes indicates that eruption velocities of 30 to 250 [ms.sup.-1] and volatile contents of 0.1 to 20 wt% are required to produce deposits having the observed dimensions. Calculated heights for ballistic plumes range from [is less than] 1 to ~25 km. As an alternative mechanism, we consider conductive heating of lithospheric ice by a subsurface heat source (e.g., a warm ice diapir or cryomagmatic intrusion), which promotes enhanced sublimation of surface ice, producing local enrichment of nonice material. An analytical model describing a constant-temperature, infinite heat source places lower bounds on the time scales required to produce a detectable feature. A numerical treatment of more realistic geometries and boundary conditions allows cooling of the heat source to be modeled. We find that the thermal model satisfactorily explains features with the dimensions of minor lineaments and lenticulae haloes (a few kilometers) over time intervals of ~[10.sup.4] to [10.sup.5] years. The formation of a sublimation lag deposit proceeds rapidly once the ice temperature exceeds a threshold of ~130 K. However, for triple bands measuring up to 25 km in width, additional heat sources, such as repeated or multiple intrusions, are necessary to reproduce the observed dimensions. This is consistent with the observation that well-developed triple bands typically have multiple medial ridge sets, each of which may represent a discrete tabular heat source. Although neither the volcanic nor intrusive/thermal model can be ruled out at this stage, the latter is a more satisfactory explanation for some characteristics of the low-albedo features analyzed in this study. Key Words: Europa; volcanism; satellites of Jupiter; surfaces, satellite; ices.

Author(s): Louise Prockter [1] Wreathed in clouds and haze, Titan has long been the focus of Earth-based telescopes, and their probings have yielded tantalizing clues about the nature of the [...]