An improved carbon dioxide snow spectral albedo model: Application to Martian conditions

Carbon dioxide ice is abundant on the Martian surface and plays an important role in the planet's energy budget due to its high reflectivity and seasonal variation. Here we adapt the terrestrial Snow, Ice, and Aerosol Radiation (SNICAR) model to simulate CO 2 snow albedo across the ultraviolet, visible, and near‐IR spectra (0.2–5.0 µm). We apply recent laboratory‐derived refractive indices of CO 2 ice, which produce higher broadband CO 2 snow albedo (0.93–0.98) than previously estimated. Compared with H 2 O snow, we find that CO 2 snow albedo is much higher in the near‐IR spectrum, less dependent on ice grain size, less dependent on solar zenith angle, and more susceptible to darkening from dust. A mass concentration of 0.01% Martian dust reduces visible and near‐IR CO 2 snow albedos by about 60% and 35%, respectively. The presence of small amounts of H 2 O snow on top of CO 2 snow can substantially decrease the surface albedo. Whereas 2.5 cm of H 2 O snow can completely mask the impact of underlying CO 2 ice or the surface, roughly twice as much overlying CO 2 snow is required to mask underlying H 2 O snow. Similarly, a 10% mixing ratio of H 2 O ice embedded in CO 2 snow decreases broadband albedo by 0.18, while 10% CO 2 ice elevates H 2 O snow broadband albedo by 0.10. We also present comparisons between hemispherical albedo produced by SNICAR and observations of directional reflectance of Martian polar ice caps. While imperfect, this best fit analysis provides general ranges of physical parameters in different Martian environments that produce reasonable model‐observation agreement.