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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

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.