The Role of N2 as a Geo-Biosignature for the Detection and Characterization of Earth-like Habitats

TL;DR

This paper explores how N2 as a dominant atmospheric component, in conjunction with O2, can serve as a geo-biosignature for detecting Earth-like habitats and discusses atmospheric evolution scenarios related to life and planetary tectonics.

Contribution

It introduces the concept of N2 as a potential geo-biosignature and analyzes atmospheric evolution scenarios for Earth-like planets in habitable zones.

Findings

N2 and O2 coexistence suggests active biosphere and tectonics.

N2-dominated atmospheres are linked to specific planetary regimes.

Implications for detecting life on exoplanets in habitable zones.

Abstract

Since the Archean, N2 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 the Earth's N2 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: firstly, for a stagnant-lid anoxic world, secondly for a tectonically active anoxic world, and thirdly 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 nowadays, they sustain its surface partial pressure at high levels. Also, the simultaneous presence of significant N2 and O2 is chemically incompatible in an atmosphere over geological timescales. Thus, we argue that an N2-dominated atmosphere in combination with O2 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 aerobe biosphere, whereas other scenarios in most cases end up with a CO2-dominated atmosphere. We conclude with implications for the search for life on Earth-like exoplanets inside the habitable zones of M to K-stars.