Current State of Topographic Mapping of Ganymede: Squeezing the Most from JUICE

Abstract:

JUICE, ESA’s planned Ganymede orbiter, and NASA’s proposed Europa Clipper, won’t arrive for some time, and many issues concerning Ganymede’s geologic history and evolution remain. Topographic mapping will be a key component of JUICE orbital and Clipper flyby mapping and an understanding of Ganymede topography can also help guide instrument development. Topographic data for Ganymede are based almost entirely on Voyager and Galileo image analysis and are sparse. No more than 20% of the surface (which exceeds that of Mercury in area in total) is presently mappable. Both stereo (3D) and shape-from-shading (PC) are both possible (and nearly all possible DEM combinations have now been constructed). Unlike Europa, only an handful of sites are mappable using both techniques: these being mostly over the South Polar region with Voyager 2. Without stereo control, PC topography, while very useful, must be interpreted with caution. Only a handful of targeted stereo mosaics were possible from Galileo, but serendipitous Voyager-Galileo stereo greatly expands this data set. Topographic data allow determinations of RMS slope values for each terrain type, but currently only at length scales >100 m. Topographic amplitude can also be determined. Geologic units for which we have limited DEM data include: furrows, grooves, smooth and subdued grooved terrains, calderas, pit and dome craters, penepalimpsests, and palimpsests. Key science questions that can be guided by even the limited available topography include: relative elevations of smooth, grooved and dark terrains and the role of volcanic vs. tectonic resurfacing; relief of ancient degraded impact craters and the role of density and heat variations; density anomalies within or beneath the ice shell; the severity and history of thermal relaxation globally and the associated heat pulse. Here we focus on the issue of relaxation, where topographic evidence indicates that thermal relaxation reached a peak associated with bright terrain formation, reducing topography of earlier craters, furrows and all other features equally, and then decayed quickly, leaving more recently formed features mostly intact. Regional variations in heat flux may be evident in the distribution of relaxed craters, especially at the South Pole.