Wednesday, August 8, 2012

Triton’s Subsurface Ocean

Triton is the only large moon in orbit around the planet Neptune and it is also the seventh largest moon in the Solar System. Measuring 2700 km across and with an average density of 2065 kg/m3, the internal structure of Triton is consistent with an icy mantle surrounding a large rocky core. With a mean surface temperature of minus 235 degrees Centigrade, the surface of Triton is predominantly water ice with trace amounts of frozen nitrogen, carbon monoxide, carbon dioxide and methane. Triton orbits Neptune in a direction that is opposite to the planet’s spin, making it the only large moon in the Solar System with such an orbit. As a result, Triton could not have formed in situ around Neptune. The leading hypothesis suggests that Triton was once part of a binary system which came too close to Neptune, resulting in Triton being captured into orbit around Neptune.

Following capture, tidal interactions with Neptune over a few hundred million years led to the circularization of Triton’s initial orbit to its current near-circular orbit. This process dissipated a large amount of tidal energy in the form of heat within Triton’s interior. The amount of tidal heating is estimated to be sufficient to melt Triton entirely. Given that Triton has a thick icy mantle surrounding a rocky core; it is worth investigating if an ocean formed during this phase of tidal dissipation could be sustained for billions of years until the present time. As Triton cools, an ice shell will form over an underlying global ocean of liquid water. The rate at which this ice shell thickens depends on the amount of tidal dissipation in the ice shell and the amount of heat flux from the decay of radioactive nuclei within Triton’s large rocky core.

After the circularization of Triton’s initial high eccentricity orbit, Triton is still expected to maintain a small but non-zero orbital eccentricity. For this reason, the small variation in the distance of Triton from Neptune over each orbit causes the ice shell of Triton to be stretched differentially. This allows tidal dissipation to occur within the ice shell which tends to preferentially heat the base of the ice shell. A thin ice shell creates a larger amount of tidal dissipation since a thin shell is more easily deformed by tides. Tidal dissipation within the ice shell decreases as the ice shell thickens. For an ocean that is completely frozen, tidal dissipation decreases sharply as the ice shell becomes locked with the rocky core as it is no longer mechanically decoupled from the rocky core by an intervening layer of liquid water.

The basal heating of the ice shell due to tidal dissipation slows the thickening of the ice shell. Since tidal dissipation is directly proportional to the square of orbital eccentricity, a larger non-zero orbital eccentricity for Triton will generate a larger amount of tidal dissipation. The depth of Triton’s ocean measured from the moon’s surface to the rocky core is estimated to be about 400 km. In this study, a non-zero orbital eccentricity of just 0.00005 is sufficient to leave behind a 130 km thick ocean under a 270 km thick ice shell after 4.5 billion years. Even an orbital eccentricity of 0.00004 is just sufficient to prevent the ocean from completely freezing over 4.5 billion years.

Compared with the heat generated from the decay of radioactive elements, the heat produced from tidal dissipation is orders of magnitude less. Even so, tidal dissipation plays a fundamental role in determining the rate of thickening of the ice shell. Tidal dissipation within the ice shell heats the base of the ice shell and better insulates the rest of the underlying ocean from freezing. This is because warming the base of the ice shell reduces the thermal gradient which lowers the heat flux escaping through the ice shell from the underlying ocean. With an orbital eccentricity of only 0.00010, the thickening of the ice shell is stalled completely, allowing Triton to continuously maintain a 100 km thick ice shell overlying a 300 km thick global ocean of liquid water up to the present time. Such an ocean will contain over three times more water than all oceans on Earth combined.

J. Gaeman et al., “Sustainability of a Subsurface Ocean within Triton’s Interior”, Icarus 220 (2012) 339–347