A thermal model for the seasonal nitrogen cycle on Triton

Abstract

A Mars thermal model has been adapted to study the seasonal nitrogen cycle on Triton. Unlike other models published to date, it incorporates diurnal and seasonal subsurface heat conduction, and accounts for the heat capacity of N 2 frost deposits. Key observables that this model attempts to reproduce include Triton's atmospheric pressure and surface albedo features at the time of the Voyager encounter, as well as changes in Triton's disk-integrated spectral properties between 1977 and 1989. Subsurface heat conduction is found to have an important effect on the behavior of Triton's polar caps and atmospheric pressure fluctuations. The results of this model differ from those of previous studies in that they generally do not predict the catastrophic freeze-out of Triton's atmosphere on a seasonal basis. The model results indicate that even for a wide range of possible input parameters, it is unlikely that Triton's bright southern polar cap is a seasonal nitrogen deposit, because it would have largely sublimated before the Voyager encounter. However, these results do not rule out the possibility of a large bright permanent polar cap in Triton's southern hemisphere. The best agreement between the model results and the available observations is obtained when Triton's seasonal nitrogen frost deposits are assumed to be darker than the undelying substrate. Assuming a constant nitrogen frost albedo of 0.62, a frost emissivity of 0.5, a substrate albedo of 0.8, and a substrate thermal inertia of 7 × 10 −3 cal cm −2 K −1 sec −1 2 yields good agreement with the surface pressure and global albedo patterns observed during the Voyager encounter, as well as a rapid change in disk-integrated albedo from 1977 to 1989 due to the retreat and disappearance of a dark south seasonal polar cap. These results lend support to microphysical arguments for the presence of dark, or smooth, translucent nitrogen frost on the surface of Triton.