Spectral albedo and emissivity of CO2 in Martian polar caps: Model results

Abstract

A model originally developed to explain the spectral albedo and emissivity of terrestrial snow is extended to the case of carbon dioxide snow on Mars. The variation of albedo and emissivity with wavelength is caused by the spectral variation of the absorption coefficient of solid CO2. The most important variables controlling the radiative properties are grain size and contamination by dust or water. Solar zenith angle and snowpack thickness are of less importance. The observation that red albedo is higher than blue albedo in the Martian south polar cap indicates that the snow is contaminated with red dust. The interband absorption coefficient of CO2 ice in the thermal infrared is 2–3 orders of magnitude smaller than that of H2O ice, due to the absence of hydrogen bonding in CO2. This allows CO2 snow emissivity to be sensitive to grain size, emission angle, and impurities, in contrast to water snow which is nearly a blackbody under all conditions. The emissivity of CO2 snow varies substantially with wavelength, so energy budget modeling should be done in spectral detail. The addition of a thin layer of water frost over CO2 snow dramatically raises the thermal emissivity but causes little change to the spectrally averaged albedo unless the underlying CO2 snow is dirty. Remote sensing of CO2 grain size, H2O content, and dust content may be possible. However, the design of a remote‐sensing strategy awaits more accurate laboratory determination of the optical constants of CO2 ice.