Far-infrared spectroscopy of carbon dioxide clathrate hydrate with applications to the Martian northern polar region.

Abstract

Water ice in the Martian polar regions is believed to convert to CO$\sb2$ clathrate hydrate in winter. Surface temperatures are thought to drop to 148 K, while CO$\sb2$ hydrate should be stable below about 155 K. The thermal conductivity of structure I clathrate hydrates is much lower than that of water ice, and the latent heat of dissociation of the CO$\sb2$ hydrate is nearly half the latent heat of sublimation of water ice and nearly equal to that of CO$\sb2$ ice. A large deposit of hydrate would therefore affect the energy budgets of the polar regions. To confirm its presence with a remote sensing experiment, the dielectric properties of the hydrate must be significantly different from those of water ice, CO$\sb2$ ice and CO$\sb2$ gas. In this study we present the far-infrared spectrum of CO$\sb2$ hydrate at 150K and compare it to water ice spectra reported in the literature. The hydrate spectrum exhibits a broad absorption centered at 17 cm$\sp{-1}$, which is probably due to a rattling mode. Our dense medium radiative transfer model indicates that the absorption can give rise to a distinctive far-infrared emission spectral gradient that is not present in the ice spectrum. Our model of atmospheric transmission indicates that the spectral gradient should be clearly observable through windows in the Martian atmospheric absorption spectrum.