Stability of Amino Acids in Different Physical States under γ-Ray Irradiation and Implications for Mars Biosignature Detection

Searching for signs of past or present life is a central objective of Mars exploration. The detection of organic molecular biosignatures is highly desirable, as they can provide direct and reliable evidence of biological activity. However, Mars’s intense radiation environment poses a major challenge because it degrades or modifies these molecules, complicating their detection. To assess the stability of organic molecules, researchers have conducted various irradiation simulation experiments. However, the survivability and chemical reactivity of amino acids under Martian-like conditions remain controversial. One possible reason is the inconsistent physical states of amino acids used in these studies. Herein, we investigated the stability of glycine, cysteine, and histidine in three different physical states (i.e., powders, crystals, and solutions) under 60Co γ-ray irradiation. The results show that the stability of amino acids varies with their physical state. For glycine, the powders undergo a polymorphic transformation with a total dose of 2.39 MGy, while its single crystals remain largely intact at the same irradiation dose. In contrast, glycine in aqueous solution shows significant modification involving both decomposition and complex recombination or polymerization. Cysteine and histidine also display physical-state-dependent responses, with their powders being more stable than their aqueous counterparts but less resistant than their crystals. These results suggest that the preservation of organics on Mars may be biased by their physical states. Therefore, future Martian life-detection missions should consider not only the molecular identity but also the physical state of target compounds when interpreting data and designing sampling strategies.