Surface-atmosphere coupling on Triton and Pluto.

Abstract

Sublimation of volatile ices and convection play important roles in determining the present and past climates of Neptune's large moon, Triton, and Pluto. I have developed models of these two processes and used the distribution of albedo on the surfaces of these two bodies to study surface temperatures, distribution of volatile ices, and lower atmospheric structure. My initial studies focused on Triton, which was encountered by Voyager 2 in 1989. One of the surprising results is that Triton's South Polar Cap is considerably larger than predicted by my model. Another basic result is that the volatile N₂ ice on Triton's surface has a low thermal emissivity (≃ 0.7) relative to canonical emissivity values, which are near unity. Some ambiguity in the thermal structure of Triton's atmosphere resulted from the encounter. By modeling the convective transport of heat between the surface and atmosphere I was able to show that the near-surface atmospheric temperature was close to the low end of the ra previous analyses of the occultation of a star by Pluto in 1988 may have erroneously concluded that Pluto's radius is approximately 1200 km. My results, while not ruling out that conclusion, show that Pluto could be much smaller than 1200 km and the atmosphere could still have produced the observed occultation lightcurve. A smaller surface radius, combined with the occultation lightcurve, implies that Pluto possesses a troposphere, which has never been considered before. The remaining piece of the Pluto atmosphere puzzle is the somewhat anomalous atmospheric composition required to explain the temperature structure derived from the occultation results. By expanding my earlier Triton work on the distribution N₂ ice to include the physics of simultaneous sublimation of N₂ and CH₄, I have been able to show that the required "anomalous" atmospheric composition is totally reasonable. Synthesizing these results with other recent work, I propose a new and testable paradigm for Pluto's atmosphere.