FEASIBILITY OF DETERMINING THE COMPOSITION OF PLANETARY ICES BY FAR-INFRARED OBSERVATIONS - APPLICATION TO MARTIAN CLOUD AND SURFACE ICES

Ices in the atmospheres and on the surfaces of planets and moons are t hought to play important roles in the evolution and stability of, and in radiative transfer in, planetary atmospheres. In this paper, the ca pability of far-infrared spectral observations to determine the compos ition and characteristics of planetary ices is investigated with parti cular application to martian H2O and CO2 ices. Thin film transmission spectra of crystalline (Ic) and amorphous H2O ice and crystalline CO2 ice were measured using a Fourier transform spectrometer. The far-infr ared 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 re fractive 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-i nfrared optical depths on the order of 10(-4), precluding their detect ion using an earth-based remote sensing instrument, However, model cal culations of polar condensates showed observable H2O ice cloud spectra l features near the 225 cm(-1) lattice absorption band, The presence o f a CO2 ice haze lowered the apparent surface brightness temperature b y 10 to 20 K. Theoretical emission by CO2 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 CO2. In the weakly absorbing inter-band region, CO2 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, H2O frost exhibited poor spectral contrast with an emissivi ty close to unity. The detection of polar hood condensates and of the presence of H2O ice and dust in the CO2 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 techniqu es are required to achieve adequate spatial resolution. It is speculat ed that these and other non-polar ices such as N-2 and CH4, present in the outer solar system, will exhibit far-infrared spectral characteri stics similar to CO2 ice, making their detection possible, but technic ally challenging. (C) 1996 Academic Press, Inc.