Spectral Albedo of Dusty Martian H2O Snow and Ice

Recent evidence of exposed H2O ice on Mars suggests that this ice was deposited as dusty (< ~1% dust) snow. This dusty snow is thought to have been deposited and subsequently buried over the last few million years. On Earth, freshly fallen snow metamorphoses with time into firn and, if deep enough, into glacier ice. While spectral measurements of martian ice have been made, no model of the spectral albedo of dusty martian firn or glacier ice exists at present. Accounting for dust and snow metamorphism is important because both factors reduce the albedo of snow and ice by large amounts. However, the dust content and physical properties of martian H2O ice are poorly constrained. Here we present a model of the spectral albedo of H2O snow and ice on Mars, which is based on validated terrestrial models. We find that small amounts (< 1%) of martian dust can lower the albedo of H2O ice at visible wavelengths from ~1.0 to ~0.1. Additionally, our model indicates that dusty (> 0.01% dust) firn and glacier ice have a lower albedo than pure dust, making them difficult to distinguish in visible/near-infrared images commonly used to detect H2O ice on Mars. Observations of excess ice at the Phoenix landing site are matched by 350-µm snow grains with 0.015% dust, indicating that the snow has not yet metamorphosed into glacier ice. Our model results can be used to characterize orbital observations of martian H2O ice and refine climate-model predictions of ice stability.