Abstract
Large regions of the jovian moon Ganymede have been resurfaced, but
the means has been unclear(1,2). Suggestions have ranged from volcanic
eruptions of liquid water(3-5) or solid ice(6) to tectonic deformation(7-9),
but definitive high-resolution morphological evidence has been lacking.
Here we report digital elevation models of parts of the surface of
Ganymede, derived from stereo pairs combining data from the Voyager
and Galileo spacecraft, which reveal bright, smooth terrains that
lie at roughly constant elevations 100 to 1,000 metres below the
surrounding rougher terrains. These topographic data, together with
new images that show fine-scale embayment and burial of older features(10),
indicate that the smooth terrains were formed by flooding of shallow
structural troughs by low-viscosity water-ice lavas. The oldest and
most deformed areas (the 'reticulate' terrains) in general have the
highest relative elevations, whereas units of the most common resurfaced
type-the grooved terrain-lie at elevations between those of the smooth
and reticulate terrains. Bright terrain, which accounts for some
two-thirds of the surface, probably results from a continuum of processes,
including crustal rifting, shallow flooding and groove formation.
Volcanism plays an integral role in these processes, and is consistent
with partial melting of Ganymede's interior(11,12).
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