The Penetration of Solar Radiation in Martian Ice Analogues

Abstract

The penetration depth of broad spectrum solar radiation (300 nm – 1100 nm) in ices relevant to Mars has been measured in the laboratory for the first time. These ices include carbon dioxide and water ices in the form of slab ice, ice of controlled grain sizes, and snow. All of these ice morphologies are observed on the surface of Mars today, and whilst some measurements of water ice have been made for terrestrial applications, many of these cannot be directly compared to carbon dioxide ice because of factors such as impurities in naturally occurring ice, or measurements covering only a narrow spectral range. The results presented here show that the penetration depth varies with both grain size and ice composition. Grain size (due to light scattering) is the dominant parameter at particle radii applicable to snow, and composition (therefore material-specific optical properties) dominates in larger grains and slab ice. An empirical model is presented, which can be used to predict penetration depth of a specific grain size and composition of ice. This has important applications for modelling surface processes on Mars, many of which have no terrestrial analogue. Relevant features include araneiforms, observed only in association with the seasonal carbon dioxide slab ice, or gullies, which vary in morphology depending on location, indicating varying formation mechanisms. By using more accurate penetration depths for ices on the Martian surface, radiative transfer models describing surface-atmosphere interactions could better simulate these formation processes. These results can further be applied to modelling icy moons and comets. Future work to expand on this thesis would involve determining the effect of impurities on the e-folding scale of CO2 ice as Mars’s seasonal polar caps contain <0.1% dust. Moreover, measuring the penetration depth in other extra-terrestrial ices would be benefit studies of comets and icy moons.