The ultraviolet radiation environment and shielding in pit craters and cave skylights on Mars

Abstract

Caves and their entrances have been proposed as habitable environments and regions that could have preserved evidence of life, mostly due to their natural shielding from the damaging ionizing and non-ionizing radiation present on the surface. However, no studies to date have quantitatively determined the shielding offered by these voids on Mars. This paper describes the ultraviolet (UV) radiation environment in such environments by means of extensive radiative transfer (RT) model simulations applied to representative void geometries on Mars. The spatial and temporal characterization of the radiation field inside each void geometry is also performed. The RT model was previously tested against in situ Mars Science Laboratory UV observations on the Martian surface; the model showed excellent performance in terms of predicting the UV irradiances. The results presented here strongly suggest that pit craters and cave skylights are effectively shielded from the damaging UV radiation found on the Martian surface. In particular, locations on the floors of pit craters and skylights without cavernous spaces strongly attenuate UV irradiance at latitudes higher than ~45°; observing in several cases irradiances of the order of ~2% of those values found on the surface. Numerical simulations of cave entrances show a reduction even more than two orders of magnitude in UV radiation, both in the maximum instantaneous and cumulative doses, throughout the year and at any location of the planet. The attenuated UV irradiance in cavernous spaces remains stable in a 10 Myr timescale, while at the same time the photosynthetically active radiation is higher than the minimum required for Earth-like phototrophs. The intermediate radiation environment between the damaging radiation on the surface and the permanent darkness of a hypothetical cave offered by voids on Mars may represent favorable environments for habitability without constraining the type of energy source for potential as-yet unknown Martian organisms.