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

Abstract

AbstractCarbon 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 CO2 snow albedo across the ultraviolet, visible, and near‐IR spectra (0.2–5.0 µm). We apply recent laboratory‐derived refractive indices of CO2 ice, which produce higher broadband CO2 snow albedo (0.93–0.98) than previously estimated. Compared with H2O snow, we find that CO2 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 CO2 snow albedos by about 60% and 35%, respectively. The presence of small amounts of H2O snow on top of CO2 snow can substantially decrease the surface albedo. Whereas 2.5 cm of H2O snow can completely mask the impact of underlying CO2 ice or the surface, roughly twice as much overlying CO2 snow is required to mask underlying H2O snow. Similarly, a 10% mixing ratio of H2O ice embedded in CO2 snow decreases broadband albedo by 0.18, while 10% CO2 ice elevates H2O 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.