The influence of thermal inertia on temperatures and frost stability on Triton

Abstract

Seasonal subsurface heat conduction can have a large influence on Triton's N 2 frost distribution. Increasing surface thermal inertia reduces the thickness and extent of seasonal N 2 frosts. If the thermal inertia of the nonvolatile substrate is greater than about 30% of the value for nonporous H 2O or CO 2, or if nonporous H 2O or CO 2 is overlaid by a porous regolith of low thermal inertia but less than a few meters thick, the northernmost latitudes visible to Voyager should have been frost-free at the time of the encounter, possibly accounting for their relatively low albedo. If the substrate in the northern hemisphere has sufficiently low albedo and/or emissivity and also has a thermal inertia comparable to that of nonporous H 2O or CO 2, there may be no seasonal or permanent N 2 deposits in the northern hemisphere at all. Because this model, like previous ones, predicts a monotonic recession of permanent N 2 deposits toward the poles and very limited seasonal N 2 frost in the southern hemisphere at Voyager time, and because of new spectroscopic evidence for nonvolatile CO 2 on Triton's bright southern hemisphere, we consider it possible that much of the bright material on Triton's southern hemisphere is not N 2. Bright nonvolatiles in the southern hemisphere may allow seasonal N 2 frosts to form there during the southern summer, possibly helping to explain Triton's spectroscopic changes during the past decade. All the models considered here predict 10-fold or greater seasonal variations in atmospheric pressure, with pressure currently increasing in high-thermal-inertia models and decreasing in models with low thermal inertia.