Ranjan Sukrit, Sasselov Dimitar D
Harvard-Smithsonian Center for Astrophysics , Cambridge, Massachusetts.
Astrobiology. 2017 Mar;17(3):169-204. doi: 10.1089/ast.2016.1519.
The UV environment is a key boundary condition to abiogenesis. However, considerable uncertainty exists as to planetary conditions and hence surface UV at abiogenesis. Here, we present two-stream multilayer clear-sky calculations of the UV surface radiance on Earth at 3.9 Ga to constrain the UV surface fluence as a function of albedo, solar zenith angle (SZA), and atmospheric composition. Variation in albedo and latitude (through SZA) can affect maximum photoreaction rates by a factor of >10.4; for the same atmosphere, photoreactions can proceed an order of magnitude faster at the equator of a snowball Earth than at the poles of a warmer world. Hence, surface conditions are important considerations when computing prebiotic UV fluences. For climatically reasonable levels of CO, fluence shortward of 189 nm is screened out, meaning that prebiotic chemistry is robustly shielded from variations in UV fluence due to solar flares or variability. Strong shielding from CO also means that the UV surface fluence is insensitive to plausible levels of CH, O, and O. At scattering wavelengths, UV fluence drops off comparatively slowly with increasing CO levels. However, if SO and/or HS can build up to the ≥1-100 ppm level as hypothesized by some workers, then they can dramatically suppress surface fluence and hence prebiotic photoprocesses. HO is a robust UV shield for λ < 198 nm. This means that regardless of the levels of other atmospheric gases, fluence ≲198 nm is only available for cold, dry atmospheres, meaning sources with emission ≲198 (e.g., ArF excimer lasers) can only be used in simulations of cold environments with low abundance of volcanogenic gases. On the other hand, fluence at 254 nm is unshielded by HO and is available across a broad range of [Formula: see text], meaning that mercury lamps are suitable for initial studies regardless of the uncertainty in primordial HO and CO levels. Key Words: Radiative transfer-Origin of life-Planetary environments-UV radiation-Prebiotic chemistry. Astrobiology 17, 169-204.
紫外线环境是生命起源的关键边界条件。然而，关于行星条件以及因此生命起源时的表面紫外线，存在相当大的不确定性。在此，我们给出了39亿年前地球上紫外线表面辐射率的二流多层晴空计算结果，以限制作为反照率、太阳天顶角（SZA）和大气成分函数的紫外线表面通量。反照率和纬度（通过太阳天顶角）的变化可使最大光反应速率变化超过10.4倍；对于相同的大气，在雪球地球的赤道处光反应的进行速度比温暖世界的极地快一个数量级。因此，在计算前生物紫外线通量时，表面条件是重要的考虑因素。对于气候上合理的一氧化碳水平，波长小于189纳米的通量被屏蔽，这意味着前生物化学能够有效地抵御因太阳耀斑或变化导致的紫外线通量变化。一氧化碳的强屏蔽作用还意味着紫外线表面通量对合理水平的甲烷、氧气和臭氧不敏感。在散射波长处，随着一氧化碳水平的增加，紫外线通量下降相对缓慢。然而，如果二氧化硫和/或硫化氢能够如一些研究人员所假设的那样积累到≥1 - 100 ppm的水平，那么它们会显著抑制表面通量，进而抑制前生物光过程。对于波长小于198纳米的情况，羟基是一种强大的紫外线屏蔽物。这意味着无论其他大气气体的水平如何，波长小于198纳米的通量仅在寒冷、干燥的大气中存在，这意味着发射波长小于198纳米（例如ArF准分子激光器）的光源仅可用于模拟火山源气体含量低的寒冷环境。另一方面，254纳米处的通量不受羟基屏蔽，并且在广泛的[公式：见正文]范围内都可获得，这意味着汞灯适用于初始研究，而无需考虑原始羟基和一氧化碳水平的不确定性。关键词：辐射传输 - 生命起源 - 行星环境 - 紫外线辐射 - 前生物化学。天体生物学17，169 - 204。
