Energy-Limited Radiolytic Habitability in the Shallow Martian Subsurface: Implications for ExoMars Rosalind Franklin and Tianwen-3

The surface of Mars is sterilized by ionizing radiation and pervasive oxidants; its shallow subsurface, shielded from ultraviolet light and the most reactive oxidation, may instead preserve habitable conditions. The radiolytic habitable zone (RHZ) hypothesis holds that galactic cosmic rays can drive water and oxychlorine radiolysis there, generating H2 and oxidants that support chemolithotrophy without sunlight or geothermal heat. We develop a quantitative framework coupling Monte Carlo GCR transport, phase-resolved radiolysis chemistry, water-activity and H2-retention treatments, and microbial maintenance-power constraints for Gale Crater, Oxia Planum, southern Utopia, and Arabia/Mawrth, expressing radiolytic chemical energy as a depth-resolved redox power comparable to the power requirements of life. H2 retention is the dominant control. Sorbed or mineral-associated retention yields 4-6 x 10^-13 W kg^-1 at protected depth (>=10 cm), whereas connected-pore free-gas escape lowers redox power by two to four orders of magnitude. Gale remains inactive under both retention assumptions, consistent with published SAM evolved-gas analyses. Even in retained-H2 active terrains, supported cell density reaches only 10^3-10^5 cells cm^-3 at subseafloor maintenance powers. The framework predicts a spatially restricted, low-density RHZ, testable by stepped H2 evolved-gas analysis of protected-depth samples from ExoMars Rosalind Franklin, Tianwen-3, or Mars Sample Return.

Source: arXiv:2606.23483v1 - http://arxiv.org/abs/2606.23483v1 PDF: https://arxiv.org/pdf/2606.23483v1 Original Link: http://arxiv.org/abs/2606.23483v1