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Sublimation of Ices
We provide a simple tool to calculate the sublimation of ices under various circumstances. The calculations are all based on the methods described by Cowan and A'Hearn (1979 Moon and Planets 21, 155-171), which in turn are based on earlier work by Delsemme and others as referenced in that paper. The calculations have not been substantially altered from the original publication and but updated vapor pressures and latent heats have been used. These changes to the input parameters lead to only small changes in the resultant sublimation rates. We note, as pointed out to us by W. Huebner and D. Boice, that our use of empirical results for heat of sublimation and vapor pressure leads our result to be inconsistent with the Clausius-Clapeyron equation, but this has negligible effect on the results for most (but not all) physical situations. Results are available for pure water, pure CO, pure CO2.
Normal Surfaces and Isothermal Spheres
Sublimation from a surface normal to the sun and from an isothermal sphere are relatively straightforward to calculate. We note that the isothermal sphere is sometimes incorrectly referred to as a fast-rotator (see below). When averaging over a real cometary nucleus, at least for simple, convex geometries, the normal surface represents the highest possible sublimation, while the isothermal sphere represents the lowest possible sublimation (all other parameters being equal). Tabulated results, for Bond albedo 5% and infrared emissivity 100%, are available for H2O, for CO2, and for CO.
Rapid Rotators
A rapidly rotating nucleus is one for which the thermal inertia is large enough that parallels of latitude become isotherms. The average sublimation over the nucleus is then a function of the obliquity, i.e., of the orientation of the rotation axis. The sublimation is relatively high if the nucleus is pole-on toward the sun (obliquity = 0°) and much smaller if the axis is perpendicular to the comet-sun line (obliquity = 90°). All calculations assume a spherical body.Note that a non-rotating comet is thus identical to a comet that is pole-on toward the sun. Our approach calculates the sublimation separately at each latitude (and for a rapid rotator the sublimation is constant all the way around the parallel of latitude, even on the night side) and then calculates the appropriate average over the entire surface, i.e., the average over all 4*pi*R2 of the surface, including areas where the actual sublimation is zero. Tabulated results are available for the pole-on case, which is identical both to the non-rotating case and to the case of zero thermal inertia, for H2O, for CO2, and for CO. As for the tables above, the visual Bond albedo is 5% and the thermal emissivity is 100%.
Source Code
Source code for the rapid rotator model in FORTRAN and Python versions is available at a GitHub repository: Small-Bodies-Node/ice-sublimation.
Created: 10 October 2003, mfa.
