The emissivity of volatile ices on Triton and Pluto

John A. Stansberry, D. J. Pisano, Roger V. Yelle

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26 Scopus citations


The Hapke theory is used to calculate the emissivity of a semi-infinite layer of granular N2 ice with CH4 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-N2, 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-N2 phase. For a grain size of 1 cm the Planck-mean bolometric emissivity calculated for the pure β-N2 ice is 0.85. If the effective N2 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-N2 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 CH4 and CO, in solid solution with N2 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 N2 ice on Triton and Pluto only slightly increases the β-N2 emissivity. The emissivity of intimate mixtures of grains of CH4 and CO with N2 is much less certain, and probably much less applicable to Triton and Pluto. CH4 and CO in solid solution with α-N2 increase the emissivity by about 50%. For an α-N2 grain size of 1 cm, the addition of 2% each CH4 and CO in solid solution with the N2 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-N2 even with CH4 and/or CO in solution is still considerably lower than for β-N2. 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 N2 ice, Pluto's atmosphere could be prevented from freezing out, even at aphelion.

Original languageEnglish (US)
Pages (from-to)945-955
Number of pages11
JournalPlanetary and Space Science
Issue number9
StatePublished - Sep 1996

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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