Roussel Anaïs, McAdam Amy C, Pavlov Alex A, Knudson Christine A, Achilles Cherie N, Foustoukos Dionysis I, Dworkin Jason P, Andrejkovičová S, Bower Dina M, Johnson Sarah Stewart
Department of Biology, Georgetown University, Washington, District of Columbia, USA.
NASA Goddard Space Flight Center, Greenbelt, Maryland, USA.
Astrobiology. 2024 Jul;24(7):669-683. doi: 10.1089/ast.2023.0123. Epub 2024 Jul 9.
Mars has been exposed to ionizing radiation for several billion years, and as part of the search for life on the Red Planet, it is crucial to understand the impact of radiation on biosignature preservation. Several NASA and ESA missions are looking for evidence of ancient life in samples collected at depths shallow enough that they have been impacted by galactic cosmic rays (GCRs). In this study, we exposed a diverse set of Mars analog samples to 0.9 Megagray (MGy) of gamma radiation to mimic 15 million years of exposure on the Martian surface. We measured no significant impact of GCRs on the total organic carbon (TOC) and bulk stable C isotopes in samples with initial TOC concentration > 0.1 wt. %; however, diagnostic molecular biosignatures presented a wide range of degradation that didn't correlate to factors like mineralogy, TOC, water content, and surface area. Exposure dating suggests that the surface of Gale crater has been irradiated at more than five times our dose, yet using this relatively low dose and "best-case scenario" geologically recalcitrant biomarkers, large and variable losses were nevertheless evident. Our results empasize the importance of selecting sampling sites at depth or recently exposed at the Martian surface.
火星已经暴露在电离辐射下数十亿年了。作为在这颗红色星球上寻找生命的一部分，了解辐射对生物标志物保存的影响至关重要。美国国家航空航天局（NASA）和欧洲航天局（ESA）的几项任务正在寻找在足够浅的深度采集的样本中古代生命的证据，这些样本受到了银河宇宙射线（GCRs）的影响。在这项研究中，我们将一系列不同的火星模拟样本暴露在0.9兆戈瑞（MGy）的伽马辐射下，以模拟在火星表面1500万年的暴露情况。我们发现，对于初始总有机碳（TOC）浓度>0.1 wt.%的样本，GCRs对总有机碳和整体稳定碳同位素没有显著影响；然而，诊断性分子生物标志物呈现出广泛的降解，且与矿物学、TOC、含水量和表面积等因素无关。暴露年代测定表明，盖尔陨石坑表面受到的辐射剂量是我们实验剂量的五倍多，然而，使用这种相对较低的剂量和“最佳情况”下地质上难降解的生物标志物，仍然明显存在大量且可变的损失。我们的结果强调了在火星表面深处或最近暴露的地方选择采样点的重要性。
