How obliquity controls the surface appearance of Triton, Pluto and other volatile-rich Transneptunian objects

Triton is often seen as Pluto’s sibling, as both objects share similar sizes, densities, and atmospheric and surface ice composition. Yet Triton’s surface appearance, including its topography, surface albedo and volatile ice distribution, strongly differs from Pluto’s. For instance, Triton is relatively flat and uniformly bright, with permanent nitrogen ice deposits likely covering its entire southern hemisphere. In contrast, Pluto’s landscape includes tall mountains and deep basins, a surface with very bright and very dark features, and permanent nitrogen ice deposits located in the mid-latitudes and equatorial regions, and in particular in the topographic basin Sputnik Planitia.

These differences suggest a different geological history. In fact, Triton and Pluto are both thought to have formed beyond Neptune and then to have evolved differently. On the one side, Pluto remained in the Kuiper Belt and was hit by a twin to form the Pluto-Charon moon system. On the other side, Triton was captured by Neptune, as strongly suggested by its retrograde and highly inclined orbit around the Ice Giant planet, and its interior subsequently experienced intense tidal deformation and heating. Geological activity on Triton may still be powered today by tidal activity.  

Previous modeling studies also highlighted the importance of the Milankovitch parameters (obliquity, eccentricity, solar longitude of perihelion) on Pluto in controlling the surface temperatures and therefore the ice sublimation and condensation rates. In particular, the high obliquity of Pluto’s spin axis seems to explain the presence of massive volatile ice deposits in the equatorial regions. Could Triton’s and Pluto’s volatile ice distributions be distinct mainly because of differences in obliquity?

To answer this question, we performed new numerical simulations of Pluto’s and Triton’s volatile transport using the same climate model for both simulations, and the same initial states, but changing only the topography as well as the obliquity and orbital parameters specific to each object. The comparison of these simulations highlight the impact of obliquity in controlling the location of the permanent deposits of volatile ices on Pluto and Triton. At the conference, we will present these results and show that the impact of obliquity on Pluto and Triton, and on similar volatile-rich Transneptunian objects, goes beyond the volatile ice distribution.