Compaction and Melt Transport in Ammonia-Rich Ice Shells: Implications for the Evolution of Triton. (arXiv:1811.11257v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Hammond_N/0/1/0/all/0/1">Noah P. Hammond</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Parmentier_M/0/1/0/all/0/1">Marc Parmentier</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Barr_A/0/1/0/all/0/1">Amy C. Barr</a>
Ammonia, if present in the ice shells of icy satellites, could lower the
temperature for the onset of melting to 176 K and create a large temperature
range where partial melt is thermally stable. The evolution of regions of
ammonia-rich partial melt could strongly influence the geological and thermal
evolution of icy bodies. For melt to be extracted from partially molten
regions, the surrounding solid matrix must deform and compact. Whether
ammonia-rich melts sink to the subsurface ocean or become frozen into the ice
shell depends on the compaction rate and thermal evolution. Here we construct a
model for the compaction and thermal evolution of a partially molten,
ammonia-rich ice shell in a one-dimensional geometry. We model the thickening
of an initially thin ice shell above an ocean with $10%$ ammonia. We find that
ammonia-rich melts can freeze into the upper $5$ to $10$ kilometers of the ice
shell, when ice shell thickening is rapid compared to the compaction rate. The
trapping of near-surface volatiles suggests that, upon reheating of the ice
shell, eutectic melting events are possible. However, as the ice shell
thickening rate decreases, ammonia-rich melt is efficiently excluded from the
ice shell and the bulk of the ice shell is pure water ice. We apply our results
to the thermal evolution of Neptune’s moon Triton. As Triton’s ice shell
thickens, the gradual increase of ammonia concentration in Triton’s subsurface
ocean helps to prevent freezing and increases the predicted final ocean
thickness by up to $50$ km.
Ammonia, if present in the ice shells of icy satellites, could lower the
temperature for the onset of melting to 176 K and create a large temperature
range where partial melt is thermally stable. The evolution of regions of
ammonia-rich partial melt could strongly influence the geological and thermal
evolution of icy bodies. For melt to be extracted from partially molten
regions, the surrounding solid matrix must deform and compact. Whether
ammonia-rich melts sink to the subsurface ocean or become frozen into the ice
shell depends on the compaction rate and thermal evolution. Here we construct a
model for the compaction and thermal evolution of a partially molten,
ammonia-rich ice shell in a one-dimensional geometry. We model the thickening
of an initially thin ice shell above an ocean with $10%$ ammonia. We find that
ammonia-rich melts can freeze into the upper $5$ to $10$ kilometers of the ice
shell, when ice shell thickening is rapid compared to the compaction rate. The
trapping of near-surface volatiles suggests that, upon reheating of the ice
shell, eutectic melting events are possible. However, as the ice shell
thickening rate decreases, ammonia-rich melt is efficiently excluded from the
ice shell and the bulk of the ice shell is pure water ice. We apply our results
to the thermal evolution of Neptune’s moon Triton. As Triton’s ice shell
thickens, the gradual increase of ammonia concentration in Triton’s subsurface
ocean helps to prevent freezing and increases the predicted final ocean
thickness by up to $50$ km.
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