1Austrian Academy of Sciences, Space Research Institute, Graz, Austria.
2Institute of Physics, University of Graz, Graz, Austria.
Astrobiology. 2019 Jul;19(7):927-950. doi: 10.1089/ast.2018.1914.
Since the Archean, N has been a major atmospheric constituent in Earth's atmosphere. Nitrogen is an essential element in the building blocks of life; therefore, the geobiological nitrogen cycle is a fundamental factor in the long-term evolution of both Earth and Earth-like exoplanets. We discuss the development of Earth's N atmosphere since the planet's formation and its relation with the geobiological cycle. Then we suggest atmospheric evolution scenarios and their possible interaction with life-forms: first for a stagnant-lid anoxic world, second for a tectonically active anoxic world, and third for an oxidized tectonically active world. Furthermore, we discuss a possible demise of present Earth's biosphere and its effects on the atmosphere. Since life-forms are the most efficient means for recycling deposited nitrogen back into the atmosphere at present, they sustain its surface partial pressure at high levels. Also, the simultaneous presence of significant N and O is chemically incompatible in an atmosphere over geological timescales. Thus, we argue that an N-dominated atmosphere in combination with O on Earth-like planets within circumstellar habitable zones can be considered as a geo-biosignature. Terrestrial planets with such atmospheres will have an operating tectonic regime connected with an aerobic biosphere, whereas other scenarios in most cases end up with a CO-dominated atmosphere. We conclude with implications for the search for life on Earth-like exoplanets inside the habitable zones of M to K stars.
自太古代以来，氮一直是地球大气中的主要成分。氮是生命组成部分的基本元素；因此，地球和类地系外行星的地质生物氮循环是长期演化的基本因素。我们讨论了地球氮大气的形成及其与地质生物循环的关系。然后，我们提出了大气演化情景及其与生命形式的可能相互作用：首先是停滞盖缺氧世界，其次是构造活跃缺氧世界，最后是氧化构造活跃世界。此外，我们还讨论了目前地球生物圈可能的消亡及其对大气的影响。由于生命形式是目前将沉积氮循环回大气中最有效的手段，它们使氮的表面分压保持在高水平。此外，在地质时间尺度上，大气中的大量氮和氧同时存在在化学上是不相容的。因此，我们认为在恒星周围可居住区域内的类地行星上，氮占主导地位的大气与氧同时存在可以被视为一种地质生物特征。具有这种大气的类地行星将具有与有氧生物圈相关的运行构造机制，而其他情况下的大多数情景最终将以 CO 为主导的大气结束。最后，我们对在 M 到 K 恒星的可居住区域内寻找类地系外行星上的生命进行了探讨。
