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 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 1 Ga (should the
impact cratering timescale be revised) or if bright terrain material was
not cryovolcanically emplaced (i.e., no initially high albedos).