Cryovolcanic and tectonic resurfacing mechanisms on Europa and Ganymede.

摘要

Cryovolcanic and tectonic resurfacing aspects on Jupiter's satellites Europa and Ganymede were studied using high resolution images from the Galileo spacecraft. Tidal flexing of Europa has created an intricate lineament network completely obliterating preexisting units, and has generated internal heat driving cryovolcanic reworking of crustal regions into ‘chaos’ terrain. Pole-to-pole geologic mapping across opposite hemispheres of Europa was undertaken to evaluate thermal and stress models of the lithosphere and to assess the relative roles of cryovolcanic and tectonic processes in space and time, reconstructing the resurfacing history of the satellite. It was found that crustal disruption occurred in areas of predicted high thermal gradient and that the change in lineament orientation with time reflects more than one rotation of Europa's outer shell around the tidally deformed interior. Resurfacing changed with time from tectonic- to cryovolcanic-dominated modes, consistent with a gradual thickening of the lithosphere.; Cryovolcanic resurfacing on Europa was investigated by establishing the origin of an unusual region of elevated chaos-like terrain, the “Mitten.” It was determined that the Mitten material is slightly elevated with respect to the surrounding terrain and appears to have extruded onto the plains and flowed for tens of kilometers. The area subsequently subsided by isostatic adjustment, viscous relaxation, and/or plains loading. The geologic evidence indicates an origin by ascent and extrusion of a diapir of warm ice. Thermal-mechanical modeling predicts that a Mitten-sized plume would have raised from an initial depth of 20–40 km. In this scenario, the Mitten represents the surface expression of the ascent of a large diapir, in contrast to typical chaos terrain, which may form by clustered small diapirs.; The initial stages of tectonic resurfacing on Ganymede were studied in order to establish controlling factors that may be obscured elsewhere. Detailed mapping and structural analysis revealed that tectonic deformation focused through visible and possibly eroded impact structures, reactivating radial and arcuate fractures. The size-frequency distribution of the arcs is consistent with a Ganymede-like crater population. It is concluded that tectonic resurfacing exposed hidden ancient craters, which may merit revision of existing crater counts and corresponding age estimates for Ganymede.