Radiation limits on amino acid preservation and chiral measurement at Oxia Planum

Abstract: Amino acid enantiomeric excess (EE) is a potential biosignature, but its measurement requires sufficient surviving material as well as preserved chirality. We derive a criterion that separates abundance-limited from racemization-limited preservation and apply it to the Mars Organic Molecule Analyzer (MOMA) on the ExoMars Rosalind Franklin rover. The model combines cosmic-ray and radiogenic doses with laboratory destruction kinetics and assumed ratios of interconversion to destruction rates. For an initially homochiral population of 1000~ppmw, the surviving EE is 58\% or 25\% when the total abundance reaches 1~ppmw for rate ratios of 0.04 or 0.10, respectively. This abundance criterion does not ensure that both enantiomers can be quantified. Applying provisional estimates of 1~ppmw per enantiomer and 10\% EE, serine at 2~m requires initial concentrations of 340 and 960~ppmw after cosmic-ray exposures of $10<sup>{7}$ and $10<sup>{8}$~yr, respectively, under transferred fused-silica--perchlorate kinetics and a 1.57-MGy radiogenic dose. These concentrations exceed measured meteoritic serine abundances, but do not predict the initial Oxia inventory. For organics retained in a closed host since deposition 3.9~Ga ago, the modeled radiogenic dose is 1.6--5.6~MGy across the adopted compositions. In the lower-dose case, drilling to 2~m reduces the total dose by only a factor of 1.5 after $10<sup>{7}$~yr. The boundaries depend on mineral kinetics and organic residence time; they distinguish preserved chirality from a measurable excess.