Center for Star and Planet Formation, Globe Institute, University of Copenhagen, Copenhagen, Denmark.
Lund Observatory, Department of Physics, Lund University, Lund, Sweden.
Astrobiology. 2024 Sep;24(9):856-880. doi: 10.1089/ast.2023.0104.
Rocky planets may acquire a primordial atmosphere by the outgassing of volatiles from their magma ocean. The distribution of O between HO, CO, and CO in chemical equilibrium subsequently changes significantly with decreasing temperature. We consider here two chemical models: one where CH and NH are assumed to be irrevocably destroyed by photolysis and second where these molecules persist. In the first case, we show that CO cannot coexist with HO, since CO oxidizes at low temperatures to form CO and H. In both cases, H escapes from the thermosphere within a few 10 million years by absorption of stellar XUV radiation. This escape drives an atmospheric self-oxidation process, whereby rocky planet atmospheres become dominated by CO and HO regardless of their initial oxidation state at outgassing. HCN is considered a potential precursor of prebiotic compounds and RNA. Oxidizing atmospheres are inefficient at producing HCN by lightning. Alternatively, we have demonstrated that lightning-produced NO, which dissolves as nitrate in oceans, and interplanetary dust particles may be the main sources of fixed nitrogen in emerging biospheres. Our results highlight the need for origin-of-life scenarios where the first metabolism fixes its C from CO, rather than from HCN and CO.
类地行星可能通过其岩浆海洋中挥发物的逸出而获得原始大气。在化学平衡中，O 在 HO、CO 和 CO 之间的分布随后会随着温度的降低而显著变化。我们在这里考虑两种化学模型：一种假设 CH 和 NH 会被光解不可逆地破坏，另一种则假设这些分子会持续存在。在第一种情况下，我们表明 CO 不能与 HO 共存，因为 CO 在低温下会氧化形成 CO 和 H。在这两种情况下，H 都会通过吸收恒星 XUV 辐射在几百万年内从热层中逃逸。这种逃逸驱动了一个大气自氧化过程，使得岩石行星大气无论在逸出时的初始氧化状态如何，都主要由 CO 和 HO 主导。HCN 被认为是前生物化合物和 RNA 的潜在前体。氧化大气通过闪电产生 HCN 的效率很低。或者，我们已经证明，闪电产生的 NO 会溶解在海洋中的硝酸盐中，星际尘埃颗粒可能是新兴生物圈中固定氮的主要来源。我们的结果强调了需要有生命起源的情景，其中最初的新陈代谢从 CO 而不是 HCN 和 CO 中固定其 C。
