HR: 0800h
AN: P31A-0965    [Abstracts]
TI: Tectonic history of the icy satellites: Discussions on the internal evolution and its surface manifestation
AU: * Kimura, J
EM: junkim@eps.s.u-tokyo.ac.jp
AF: Department of Earth and Planetary Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-Ku, Tokyo, 113-0033 Japan
AU: Yamagishi, Y
EM: yamagisi@jamstec.go.jp
AF: IFREE, JAMSTEC, 2-15 Natsushima-Cho, Yokosuka-city, Kanagawa, 237-0061 Japan
AU: Kurita, K
EM: kurikuri@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-Ku, Tokyo, 113-0032 Japan
AB: Most eminent feature of tectonic activity on the icy satellites is extensional signatures, which have been formed by tensional stress. The examples are band-shaped terrain on Europa, and groove on Ganymede. Various origins for the stress have been discussed. The plausible candidates are solid state convection, tidal deformation, and volume change associated with internal structural evolution. Convective stress (e.g. Squyres and Croft [1986]) and tidal stress (e.g. Greenberg et al. [1998]) have been evaluated as an order of 0.1 MPa at maximum, which is insufficient to induce the surface fracture. Among the source of the internal volume change, differentiation and thermal stress (due to temperature change) could have induced large volume expansion but these must have been exerted only at very early stage of the satellites. So the traces of tectonics would be erased, and these processes are not likely to contribute to the present features. Here we have focused on the possibility of phase changes (i.e. the solidification of liquid water) as a source of the internal volume change. The volume increase due to the solidification of liquid water to the ice-Ih is as large as about 10%, which is much larger than that due to the temperature change. To evaluate stress associated with the solidification of liquid water, it is necessary to determine thermal/structural evolution of the satellites. In this work, the evolution of the internal structure for Europa and Ganymede case have been numerically solved by considering heat transfer based on the mixing length theory. Starting from the water-covered silicate-iron core, the solidification of the liquid layer proceeds from the surface and the bottom of the layer, which is treated as Stefan problem. In the Europan case, the solidification only proceeds from the surface because of its small size and negative pressure gradient of the melting curve. Associated with this structural evolution, we have evaluated the stress accumulation by this phase change. The icy lithosphere is modeled as a viscoelastic medium. The stress accumulation is governed by competition between solidification rate and viscous relaxation. We have found the distinct difference in history of the internal structure and the surface stress between Europa and Ganymede. In Europa, the ice shell grows slowly and the internal liquid layer survives until today. Slow solidification rate is mostly due to the negative slope of the melting curve of ice Ih. Phase change from liquid water to ice Ih can create sufficient tensile stress to induce the surface fracture. In Ganymede, the liquid layer solidifies rapidly due to growth of ice shell and high-pressure ice layer, so the liquid layer has disappeared within an order of 0.1 Gyr. Phase change from liquid water to high-pressure ice with volume reduction is dominant, so compressional stress is generated at the surface. Surface tectonics of Ganymede, which has formed grooved terrain, may have been controlled due to another event rather than the solidification of liquid layer.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8130 Heat generation and transport
DE: 8149 Planetary tectonics (5475)
DE: 8164 Stresses--crust and lithosphere
DE: 6218 Jovian satellites
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting