Ice-wrapped Ocean Planets I

In the early 1970s John Lewis modelled the interiors of the outer planet satellites and discovered they might have sub-surface oceans of ammonia/water…

Lewis, J.S. (1971) Science, Volume 172, Issue 3988, pp. 1127-1128

Lewis, J.S. Icarus, Volume 15, Issue 2, October 1971, Pages 174-185

Steady-state thermal models for the icy satellites are constructed in which the energy released by radioactive decay in the interiors of the satellites is exactly balanced by the net radiative loss from their surfaces. It is shown that the Galilean satellites of Jupiter and the larger satellites of Saturn, Uranus, and Neptune very likely have extensively melted interiors, and most probably contain a core of hydrous silicates, an extensive mantle of ammonia-rich liquid water, and a relatively thin crust of ices. Consequences of this model relating to the Galilean satellites and the rings of Saturn are briefly described.

The atmospheric compositions and densities of the large icy satellites and certain features of the retention of volatiles during accretion are discussed.

Thus we’ve known about these dark, inner Oceans for over 3 decades or so. Opinion, in science, is never constant without data, and some estimates of the heat-transfer via solid-state convection of ice has meant the liquid interior models have fallen out of favour – at least until more data for the oceans came to hand from Galileo and Cassini. Galileo discovered that Ganymede, Callisto and Io had detectable magnetic fields – the case of Callisto, the field seemed localised to a thin layer of salt-containing fluid just under the outer ice, enwrapping the ‘mud’ mantle below. Cassini has discovered that Titan’s outer crust is decoupled from the inner layers, probably because of a liquid mantle of ammonia/water or ammonium sulfate.

But what about other moons? Europa very probably has an ocean as its crust looks like Arctic sea-ice, and Enceladus’s geysers are hard to explain via any other cause. Further afield? Here’s an interesting paper from Paul Schenk and Kevin Zahnle…

Schenk & Zahnle, Icarus, Volume 192, Issue 1, 1 December 2007, Pages 135-149

New mapping reveals 100 probable impact craters on Triton wider than 5 km diameter. All of the probable craters are within 90° of the apex of Triton’s orbital motion (i.e., all are on the leading hemisphere) and have a cosine density distribution with respect to the apex. This spatial distribution is difficult to reconcile with a heliocentric (Sun-orbiting) source of impactors, be it ecliptic comets, the Kuiper Belt, the scattered disk, or tidally-disrupted temporary satellites in the style of Shoemaker–Levy 9, but it is consistent with head-on collisions, as would be produced if a prograde population of planetocentric (Neptune-orbiting) debris were swept up by retrograde Triton. Plausible sources include ejecta from impact on or disruption of inner/outer moons of Neptune. If Triton’s small craters are mostly of planetocentric origin, Triton offers no evidence for or against the existence of small comets in the Kuiper Belt, and New Horizons observations of Pluto must fill this role. The possibility that the distribution of impact craters is an artifact caused by difficulty in identifying impact craters on the cantaloupe terrain is considered and rejected. The possibility that capricious resurfacing has mimicked the effect of head-on collisions is considered and shown to be unlikely given current geologic constraints, and is no more probable than planetocentrogenesis. The estimated cratering rate on Triton by ecliptic comets is used to put an upper limit of 50 Myr on the age of the more heavily cratered terrains, and of 6 Myr for the Neptune-facing cantaloupe terrain. If the vast majority of cratering is by planetocentric debris, as we propose, then the surface everywhere is probably less than 10 Myr old. Although the uncertainty in these cratering ages is at least a factor ten, it seems likely that Triton’s is among the youngest surfaces in the Solar System, a candidate ocean moon, and an important target for future exploration.

…which seems to indicate a very dramatic thermal history for Triton, with a more global melting of its crust than the apparently localised melt on Enceladus’s south pole. If so, then the sub-surface ocean is potentially very close to the surface and liable to burst through in cryovolcanic events, making the moon a very interesting target for future investigation.

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