Other
Scientific paper
Feb 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996icar..119..405j&link_type=abstract
Icarus, Volume 119, Issue 2, pp. 405-426.
Other
7
Scientific paper
Ices in the atmospheres and on the surfaces of planets and moons are thought to play important roles in the evolution and stability of, and in radiative transfer in, planetary atmospheres. In this paper, the capability of far-infrared spectral observations to determine the composition and characteristics of planetary ices is investigated with particular application to martian H_2O and CO_2 ices. Thin film transmission spectra of crystalline (Ic) and amorphous H_2O ice and crystalline CO_2 ice were measured using a Fourier transform spectrometer. The far-infrared refractive indices of these ices at temperatures from 77 to 150 K over the spectral range 50 to 500 cm^-1 were derived. These data are in generally good agreement with previously published indices. The refractive index data were incorporated into a radiative transfer model used to study the far-infrared properties of cloud and surface ices on Mars. Typical mid-latitude H_2O ice clouds on Mars have vertical far-infrared optical depths on the order of 10^-4, precluding their detection using an earth-based remote sensing instrument. However, model calculations of polar condensates showed observable H_2O ice cloud spectral features near the 225 cm^-1 lattice absorption band. The presence of a CO_2 ice haze lowered the apparent surface brightness temperature by 10 to 20 K. Theoretical emission by CO_2 frost showed strong spectral contrast in the surface brightness temperature of 20 to 30 K near the 66 and 110 cm^-1 lattice bands in solid CO_2. In the weakly absorbing inter-band region, CO_2 frost emissivity varied from approximately 0.4 to 0.7 with the incorporation of small amounts (0.1-1.0%) of dust or water ice. H_2O frost exhibited poor spectral contrast with an emissivity close to unity. The detection of polar hood condensates and of the presence of H_2O ice and dust in the CO_2 ice caps using an earth-based far-infrared instrument appears feasible, although a telescope with a mirror diameter on the order of 15 to 20 m or interferometric techniques are required to achieve adequate spatial resolution. It is speculated that these and other non-polar ices such as N_2 and CH_4, present in the outer solar system, will exhibit far-infrared spectral characteristics similar to CO_2 ice, making their detection possible, but technically challenging.
Atreya Sushil K.
Johnson Brian R.
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