Solar Gardening and the Seasonal Evolution of Nitrogen Ice on Triton and Pluto

Abstract

Nitrogen ice, probably the dominant high albedo surface material on Pluto and Triton, has optical constants that are more strongly absorbing in the thermal infrared than at visible wavelengths. This optical behavior results in absorption of sunlight, on average, at greater depths than the depths from which thermal emission emerges, producing a vertical imbalance between heating and cooling of a sunlit nitrogen ice surface. Assuming that the ice is sufficiently permeable to gas flow that the bulk ice is in vapor pressure equilibrium with the nitrogen atmosphere, this vertical distribution of heating and cooling drives net sublimation at depth and condensation closer to the surface. Over time scales much shorter than seasonal time scales on Triton and Pluto, this process of solar gardening will tend to produce a characteristic textural and compositional distribution which has important photometric, spectroscopic, and seasonal consequences. Solar gardening may help explain several previously puzzling observations including the anomalously strong spectral absorption by methane ice in spectra of Triton and Pluto in the 0.7- to 1.1-μm range and the surprising persistence of Triton's south polar cap at low latitudes during southern summer.