Stability of earth-like N2 atmospheres: Implications for habitability

Abstract

According to recent studies related to the EUV heating by the young Sun of Earth’s nitrogen atmosphere, upper atmosphere temperatures could rise up to several thousand Kelvin. For fluxes larger ≥ 7 times that of today’s Sun the thermosphere changes from a hydrostatic to a dynamically expanding non-hydrostatic regime, adiabatically cools but expands beyond the magnetopause so that the magnetosphere is not able to protect the upper atmosphere from solar wind erosion. A N2-rich terrestrial atmosphere would have been lost within a few million years during the EUV active period of the young Sun ≥ 4 Ga ago. These results indicate that a hydrogen-rich gaseous envelope, which could have remained from Earths protoatmosphere and/or higher atmospheric CO2 amounts may have protected Earth’s atmospheric nitrogen inventory against efficient escape to space. An alternative scenario would be that the nitrogen in Earth’s early atmosphere was degassed or delivered during the late heavy bombardment period, where the solar EUV flux decreased to values < 7 times of the modern value. Finally, we discuss how EUV heated and extended upper atmospheres and their interaction with the host star’s plasma environment could be observed around transiting Earth-like exoplanets at dwarf stars by space observatories such as the WSO-UV. Such future observations could be used to test the discussed atmospheric evolution scenarios and would enhance our understanding on the impact on the activity of the young Sun/star on the early atmospheres of Venus, Earth, Mars and exoplanets.