Sustainability of a Subsurface Ocean for Icy Satellite Volcanism

Images captured during the Voyager 2 mission to the outer planets depict various geological features on the surface of Neptune's icy satellite, Triton, that suggest recent geologic activity. The presence of volcanic plains, extensive ridge formations, and geysers indicate volcanism and possible tectonic activity at Triton's surface, estimated to be between 10 and 100 Myr old (Brown and Kirk, 1994; Schenk and Zahnle, 2007). The formation of a subsurface ocean early during Triton's evolution could provide an explanation for this activity if the layer has been sustained until present. The survival of the ocean will largely depend upon the dynamics influencing the satellite.

Voyager 2 image of smooth volcanic plains on Triton's surface (300 miles across). Volcanic flood regions on Triton likely correspond to a collapsed caldera. In some cases, volcanic flows may have been contributed from small vents as well. Image is borrowed from the NASA JPL Voyager Mission Website: http://voyager.jpl.nasa.gov/gallery/neptune.html

Triton was likely captured into orbit around Neptune about 4 Gyr ago from an initially heliocentric, elliptical orbit (Agnor and Hamilton, 2006). Once captured, the satellite's orbit would have circularized, contributing tidal stresses deforming the satellite and causing internal heating and subsequent ocean formation (Roberts and Nimmo, 2008; Ross and Schubert, 1990). We evaluate the ability of an ocean to survive until present by numerically modelling the cooling and crystallization of this ocean, incorporating the influence of internal heating from orbital evolution, and compositional constraints based on satellite observations.

Voyager 2 image of Triton's "cantaloupe" terrain (80,000 miles across), characterized by linear ridges and elliptical depressions thought to have formed as a result of diapirism within the crust. There are a various types of ridge formations found over Triton's surface. This image shows intersecting double ridges. Image borrowed from NASA JPL Voyager Mission Website:http://voyager.jpl.nasa.gov/gallery/neptune.html

Triton's crust and mantle are predominantly composed of H2O though observations suggest additional concentrations of ammonia and methane as well. For an entirely H2O composition, it would not be possible to maintain an ocean. Temperatures within Triton's interior would not likely exceed the melting temperature of H2O. Therefore, an early ocean or partially molten layer would solidify quickly. The presence of impurities significantly lowers the crystallization temperature of the system, however. This acts to lengthen the duration of an ocean. Indeed, earliest results from this study show that composition largely influences the evolution and sustainability of an ocean. These results have shown that it is possible to sustain an ocean until present given relatively large concentrations of impurities and tidal heating within Triton's crust.

For further information about the evolution of Triton, please refer to the references below.

References:

Agnor, C. B., and D. P. Hamilton, Neptune’s capture of its moon triton in a binary-planet gravitational encounter, Nature, 2006.

Brown, R. H., and R. L. Kirk, Coupling of volatile transport and internal heat flow on triton., Journal of Geophysical Research, 1994.

Roberts, J. H., and F. Nimmo, Tidal heating and the long-term thermal stability of a subsurface ocean on Enceladus, Icarus, 194, 675–689, doi:10.1016/j.icarus.2007.11.010, 2008.

Ross, M. N., and G. Schubert, The coupled orbital and thermal evolution of triton, Geophysical Research Letter, 1990.

Schenk, P. M., and K. Zahnle, On the negligible surface age of triton, Icarus, 2007.