Abstract
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, are 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. (C) 1998 Academic Press.
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