The emissivity of volatile ices on Triton and Pluto

Abstract

The Hapke theory is used to calculate the emissivity of a semi-infinite layer of granular N 2 ice with CH 4 and CO as contaminants. It is assumed that the layer is composed of grains which can be characterized as having a single size, and that temperature gradients in the emitting layers of the surface are negligible. The emission spectrum for β-N 2, stable above 35.6 K, results from a very broad peak in the absorption spectrum centered at 154 μm, while two absorption peaks, at 143 and 204 μm, produce the emission spectrum of the lower temperature α-N 2 phase. For a grain size of 1 cm the Planck-mean bolometric emissivity calculated for the pure β-N 2 ice is 0.85. If the effective N 2 grain size is 1 mm the emissivity is 0.40. Both are low enough to significantly affect surface energy balance calculations. The very narrow absorption features of α-N 2 result in even smaller bolometric emissivities of only 0.11 and 0.30 for 1 mm and 1 cm grain sizes at 34 K. The effect of CH 4 and CO, in solid solution with N 2 or as separate, intimately mixed grains, on the emissivity is also estimated. It is found that the presence of either or both of these two molecules in solid solution with the N 2 ice on Triton and Pluto only slightly increases the β-N 2 emissivity. The emissivity of intimate mixtures of grains of CH 4 and CO with N 2 is much less certain, and probably much less applicable to Triton and Pluto. CH 4 and CO in solid solution with α-N 2 increase the emissivity by about 50%. For an α-N 2 grain size of 1 cm, the addition of 2% each CH 4 and CO in solid solution with the N 2 increases the emissivity from 0.30 to 0.48 at 34 K. For a 1 mm grain size the emissivity of such a solid solution changes to 0.16 from 0.11. However, the emissivity of α-N 2 even with CH 4 and/or CO in solution is still considerably lower than for β-N 2. Seasonal variations on Triton and Pluto could be strongly influenced by this emissivity contrast between the α and β phases. In the extreme case of pure N 2 ice, Pluto's atmosphere could be prevented from freezing out, even at aphelion.