Ranjan Sukrit, Wordsworth Robin, Sasselov Dimitar D
1 Harvard-Smithsonian Center for Astrophysics , Cambridge, Massachusetts.
2 Harvard Paulson School of Engineering and Applied Sciences, Harvard University , Cambridge, Massachusetts.
Astrobiology. 2017 Aug;17(8):687-708. doi: 10.1089/ast.2016.1596. Epub 2017 May 24.
Recent findings suggest that Mars may have been a clement environment for the emergence of life and may even have compared favorably to Earth in this regard. These findings have revived interest in the hypothesis that prebiotically important molecules or even nascent life may have formed on Mars and been transferred to Earth. UV light plays a key role in prebiotic chemistry. Characterizing the early martian surface UV environment is key to understanding how Mars compares to Earth as a venue for prebiotic chemistry. Here, we present two-stream, multilayer calculations of the UV surface radiance on Mars at 3.9 Ga to constrain the surface UV environment as a function of atmospheric state. We explore a wide range of atmospheric pressures, temperatures, and compositions that correspond to the diversity of martian atmospheric states consistent with available constraints. We include the effects of clouds and dust. We calculate dose rates to quantify the effect of different atmospheric states on UV-sensitive prebiotic chemistry. We find that, for normative clear-sky CO-HO atmospheres, the UV environment on young Mars is comparable to young Earth. This similarity is robust to moderate cloud cover; thick clouds (τ ≥ 100) are required to significantly affect the martian UV environment, because cloud absorption is degenerate with atmospheric CO. On the other hand, absorption from SO, HS, and dust is nondegenerate with CO, meaning that, if these constituents build up to significant levels, surface UV fluence can be suppressed. These absorbers have spectrally variable absorption, meaning that their presence affects prebiotic pathways in different ways. In particular, high SO environments may admit UV fluence that favors pathways conducive to abiogenesis over pathways unfavorable to it. However, better measurements of the spectral quantum yields of these pathways are required to evaluate this hypothesis definitively. Key Words: Radiative transfer-Origin of life-Mars-UV radiation-Prebiotic chemistry. Astrobiology 17, 687-708.
最近的研究结果表明，火星可能曾是一个有利于生命出现的温和环境，在这方面甚至可能比地球更具优势。这些发现重新引发了人们对以下假说的兴趣：在火星上可能形成了对生命起源前具有重要意义的分子，甚至可能出现了原始生命，并被转移到了地球。紫外线在生命起源前的化学过程中起着关键作用。描绘早期火星表面的紫外线环境，对于理解火星作为生命起源前化学场所与地球相比的情况至关重要。在此，我们给出了39亿年前火星表面紫外线辐射率的二流、多层计算结果，以将表面紫外线环境作为大气状态的函数进行约束。我们探索了与符合现有约束条件的火星大气状态多样性相对应的广泛大气压力、温度和成分范围。我们考虑了云层和尘埃的影响。我们计算剂量率，以量化不同大气状态对紫外线敏感的生命起源前化学过程的影响。我们发现，对于标准的晴空CO-H₂O大气，年轻火星上的紫外线环境与年轻地球相当。这种相似性对于适度的云层覆盖具有鲁棒性；需要厚厚的云层（光学厚度τ≥100）才能显著影响火星的紫外线环境，因为云层吸收与大气中的CO是简并的。另一方面，SO₂、H₂S和尘埃的吸收与CO是非简并的，这意味着，如果这些成分积累到显著水平，表面紫外线通量可能会受到抑制。这些吸收体具有光谱可变的吸收特性，这意味着它们的存在会以不同方式影响生命起源前的化学途径。特别是，高SO₂环境可能允许有利于无生源论途径而非不利于该途径的紫外线通量。然而，需要对这些途径的光谱量子产率进行更好的测量，才能最终评估这一假说。关键词：辐射传输-生命起源-火星-紫外线辐射-生命起源前化学。天体生物学17，687 - 708。
