Muséum National d'Histoire Naturelle, Institut de Minéralogie, Physique des Matériaux et Cosmochimie, CNRS UMR 7590, Sorbonne Université, Paris, France.
Ecole Normale Supérieure de Lyon, Université Claude Bernard Lyon 1, Lyon, France.
Astrobiology. 2021 May;21(5):566-574. doi: 10.1089/ast.2020.2340. Epub 2021 Mar 10.
Traces of life may have been preserved in ancient martian rocks in the form of molecular fossils. Yet the surface of Mars is continuously exposed to intense UV radiation detrimental to the preservation of organics. Because the payload of the next rovers going to Mars to seek traces of life will comprise Raman spectroscopy tools, laboratory simulations that document the effect of UV radiation on the Raman signal of organics appear critically needed. The experiments conducted here evidence that UV radiation is directly responsible for the increase of disorder and for the creation of electronic defects and radicals within the molecular structure of S-rich organics such as cystine, enhancing the contribution of light diffusion processes to the Raman signal. The present results suggest that long exposure to UV radiation would ultimately be responsible for the total degradation of the Raman signal of cystine. Yet because the degradation induced by UV is not instantaneous, it should be possible to detect freshly excavated S-rich organics with the Raman instruments on board the rovers. Alternatively, given the very short lifetime of organic fluorescence (nanoseconds) compared to most mineral luminescence (micro- to milliseconds), exploiting fluorescence signals might allow the detection of S-rich organics on Mars. In any case, as illustrated here, we should not expect to detect pristine S-rich organic compounds on Mars, but rather by-products of their degradation.
生命的痕迹可能以分子化石的形式保存在古代火星岩石中。然而，火星表面不断受到强烈的紫外线辐射的影响，这对有机物的保存不利。由于即将前往火星寻找生命痕迹的下一批漫游车的有效载荷将包括拉曼光谱仪工具，因此急需进行实验室模拟以记录紫外线辐射对有机物拉曼信号的影响。这里进行的实验证明，紫外线辐射直接导致有机物分子结构中无序程度增加，并产生电子缺陷和自由基，从而增强了光扩散过程对拉曼信号的贡献。目前的结果表明，长时间暴露在紫外线下最终会导致胱氨酸的拉曼信号完全降解。然而，由于紫外线引起的降解不是瞬间发生的，因此应该可以用漫游车上的拉曼仪器探测到新挖掘出的富含硫的有机物。或者，鉴于与大多数矿物发光（微秒至毫秒）相比，有机荧光的寿命（纳秒）非常短，因此利用荧光信号可能允许在火星上探测到富含硫的有机物。在任何情况下，正如这里所说明的，我们不应期望在火星上检测到原始的富含硫的有机化合物，而应检测到它们降解的产物。
