On the Formation of Grooved Terrain on Ganymede by Extensional Instability

A.J. Dombard, W.B. McKinnon (Washington University)

Understanding the formation mechanics of grooved terrain on Ganymede is a vital clue to understanding the major event in that moon's geologic history: the replacement of over the half the surface by bright terrain. We have previously applied an analytical model (due to Herrick and Stevenson, Icarus 85, 191-204, 1990) in which necking-type instabilities arise when a strong brittle/plastic layer and a weaker viscous substrate undergo extension (Dombard and McKinnon, LPSC XXVII, 317-318, 1996). Recently, Goldsby and Kohlstedt (Scripta Met., submitted) have completed the analysis of their measurements of the low-temperature and low-strain-rate ductile behavior of ice. Incorporating the "final" dislocation-accommodated grain boundary sliding (GBS) flow mechanism into our model and, as before, recognizing lower surface temperatures on Ganymede at tex2html_wrap_inline15 3-4 Ga (the standard formation interval), we find that unstable extension is a viable means to produce the grooved terrain. As before, our conclusions reverse the negative judgement reached by Herrick and Stevenson. However, the sensitivity of the GBS mechanism to grain size is larger than originally thought, which renders the model problematic for coarse-grained ice (>1 mm grain diameter) at the equator. The mechanism remains quite viable at higher, cooler latitudes at all plausible grain sizes. The required strain rates are also geologically reasonable, but the thermal gradients need to be large (>10 K/km). Such large thermal gradients are logically due to the cooling of cryovolcanically emplaced bright terrain material or very near-surface hot ice plutons or diapirs. A caveat: the sensitivity to surface temperature means that the model is much less favored if grooved terrain formed at tex2html_wrap_inline15 1 Ga (should the impact cratering timescale be revised) or if bright terrain material was not cryovolcanically emplaced (i.e., no initially high albedos).