DPS 2001 meeting, November 2001
Session 35. Icy Galilean Satellites I: Geology and Geophysics
Oral, Chairs: W. Moore, J. Moore, Thursday, November 29, 2001, 4:40-6:10pm, Regency GH

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[35.07] Topographic Features of Bright and Dark Terrains on Ganymede: Implications from Galileo-G28 Stereo Images

B. Giese (DLR-Institute of Space Sensor Technology and Planetary Exploration, Berlin), R .T. Pappalardo (Univ. of Colorado), J. W. Head III (Brown University), G. Neukum (DLR, Berlin)

Stereo images of Ganymede obtained during Galileo's 28th orbit around Jupiter have provided important constraints on the formation of bright, resurfaced terrains and their relation to dark terrain. Topographic models in Nicholson Regio (NR) show that bright, grooved terrain has an undulatory topography with characteristc lengths of 10-15 km and typical amplitudes of about 400m. The undulations are high-standing with respect to dark terrain and affect also the dark terrain boundaries that are elevated or flexed in some places. Small-scale sawtooth-shaped blocks are superimposed on the undulations. These observations are suggestive of a tectonic origin of grooved terrain, specifically a process of extensional necking of the dark, brittle surface layer atop a ductile substrate. The brittle layer thickness predicted by this model is about 3-4 km.

High-standing undulations suggest an isostatic response of the ductile layer. Arbela Sulus, a bright 20km-wide band located in NR, shows an entirely different topography. It is much smoother than the surrounding terrain (typical elevations across are less than 100m) and at about constant elevation level over more than 100km along the band. Most parts of the sulcus are at about or slightly lower than the topographic level of adjoining dark terrain. There are no apparent embayment relations. This suggests a formational process in which low-viscosity, about equal density subsurface material welled up isostatically in a pre-existing gap. Analysis of rifted terrains in NR shows (i) portions of these areas elevated against surrounding unrifted, primarily dark areas (ii) troughs with elevated flanks, and (iii) an old crater where its rifted center is higher than the rim. These features also imply a ductile layer at depth and isostatic uplift in response to unloading caused by extension. Old 15km-impact craters show strong evidence for viscous relaxation. Their rims are partly narrow and well preserved while their bottoms are about level with the surrounding terrain.

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