Mathematics – Logic
Scientific paper
Sep 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998icar..135..317p&link_type=abstract
Icarus, Volume 135, Issue Icarus, pp. 317-344.
Mathematics
Logic
39
Scientific paper
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 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 end-member 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
Belton Michael J. S.
Breneman Herbert H.
Chapman Clark R.
Gerhard Neukum
Giese Bernd
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