Summers David P, Khare Bishun
SETI Institute, c/o NASA Ames Research Center, Moffett Field, California 94035-1000, USA.
Astrobiology. 2007 Apr;7(2):333-41. doi: 10.1089/ast.2006.0032.
Understanding the abiotic fixation of nitrogen is critical to understanding planetary evolution and the potential origin of life on terrestrial planets. Nitrogen, an essential biochemical element, is certainly necessary for life as we know it to arise. The loss of atmospheric nitrogen can result in an incapacity to sustain liquid water and impact planetary habitability and hydrological processes that shape the surface. However, our current understanding of how such fixation may occur is almost entirely theoretical. This work experimentally examines the chemistry, in both gas and aqueous phases, that would occur from the formation of NO and CO by the shock heating of a model carbon dioxide/nitrogen atmosphere such as is currently thought to exist on early terrestrial planets. The results show that two pathways exist for the abiotic fixation of nitrogen from the atmosphere into the crust: one via HNO and another via NO(2). Fixation via HNO, which requires liquid water, could represent fixation on a planet with liquid water (and hence would also be a source of nitrogen for the origin of life). The pathway via NO(2) does not require liquid water and shows that fixation could occur even when liquid water has been lost from a planet's surface (for example, continuing to remove nitrogen through NO(2) reaction with ice, adsorbed water, etc.).
了解氮的非生物固定对于理解行星演化以及类地行星上生命的潜在起源至关重要。氮作为一种必需的生化元素，对于我们所知的生命起源而言无疑是必不可少的。大气中氮的流失会导致无法维持液态水，并影响塑造行星表面的行星宜居性和水文过程。然而，我们目前对这种固定作用可能如何发生的理解几乎完全是理论性的。这项工作通过实验研究了在气相和水相中，由模拟二氧化碳/氮大气（如目前认为早期类地行星上存在的大气）的激波加热形成一氧化氮和一氧化碳时所发生的化学反应。结果表明，大气中的氮非生物固定到地壳存在两条途径：一条通过亚硝酸，另一条通过二氧化氮。通过亚硝酸的固定需要液态水，这可能代表了在有液态水的行星上的固定作用（因此也将是生命起源的氮源）。通过二氧化氮的途径不需要液态水，这表明即使行星表面的液态水已经流失，固定作用仍可能发生（例如，通过二氧化氮与冰、吸附水等的反应继续去除氮）。
