Feasibility of Determining the Composition of Planetary Ices by Far Infrared Observations: Application to Martian Cloud and Surface Ices
Abstract
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 H2O and CO2ices. Thin film transmission spectra of crystalline (Ic) and amorphous H2O ice and crystalline CO2ice 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-1were 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 H2O 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 H2O ice cloud spectral features near the 225 cm-1lattice absorption band. The presence of a CO2ice haze lowered the apparent surface brightness temperature by 10 to 20 K. Theoretical emission by CO2frost showed strong spectral contrast in the surface brightness temperature of 20 to 30 K near the 66 and 110 cm-1lattice bands in solid CO2. In the weakly absorbing inter-band region, CO2frost emissivity varied from approximately 0.4 to 0.7 with the incorporation of small amounts (0.1-1.0%) of dust or water ice. H2O frost exhibited poor spectral contrast with an emissivity close to unity. The detection of polar hood condensates and of the presence of H2O ice and dust in the CO2ice 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 N2and CH4, present in the outer solar system, will exhibit far-infrared spectral characteristics similar to CO2ice, making their detection possible, but technically challenging.- Publication:
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Icarus
- Pub Date:
- February 1996
- DOI:
- 10.1006/icar.1996.0027
- Bibcode:
- 1996Icar..119..405J