Ganymede Dark Terrain: Early Results from the Galileo Mission

Details Ganymede Dark Terrain: Early Results from the Galileo Mission Ganymede Dark Terrain: Early Results from the Galileo Mission

Sep 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.2127h&link_type=abstract

American Astronomical Society, DPS meeting #28, #21.27; Bulletin of the American Astronomical Society, Vol. 28, p.1138

Astronomy and Astrophysics

Astronomy

Scientific paper

Dark terrain comprises about one-half of the surface of Ganymede and on the basis of Voyager data is interpreted to be a nearly-primordial impact-generated surface, with the lower density of craters relative to Callisto attributed to viscous relaxation in an early warm lithosphere. Voyager data also reveal evidence for some resurfacing by low-albedo volcanism, but the source and detailed distribution of darkening agents is unknown. Determination of the significance of viscous relaxation and the amount of early volcanism are critical to understanding the early thermal evolution of Ganymede and comparison to Callisto. The first two Galileo encounters targeted dark terrain in several areas to image furrow sets, impact craters, dark plains, palimpsests, dark-bright terrain relationships and to obtain stereo. These data show evidence for an ancient regional structural deformation fabric in impact crater interiors, details of furrow rim structure and stratigraphy, bright plains of possible volcanic origin in a large old impact crater, and crater size-frequency distributions overlapping in age with the oldest bright terrain. At high resolution (approx. 80m/pixel) dark terrain is very heterogeneous in terms of its albedo; bright material is exposed in crater walls, massifs, furrows, and abundant knobs and pits. Dark plains occur preferentially in lows adjacent to ridges, massifs and crater rims and often show embayment relations which in some places have sharp flow-like contacts with adjacent terrain. Dark material is also observed streaming down the walls of craters and massifs in a manner similar to erosional debris on slopes; here the dark deposits are clearly erosional and reveal brighter ice deposits just below the dark veneer. Accumulating evidence for the mode of occurrence of these two types of dark terrain deposits among the several targets is providing insight into the source of the darkening agent and processes of dark terrain evolution.

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