Cosmic ray ionisation of a post-impact early Earth atmosphere

Context. Cosmic rays, both solar and Galactic, have an ionising effect on the Earth’s atmosphere and are thought to be important in the production of prebiotic molecules. In particular, the H2-dominated atmosphere that follows an ocean-vaporising impact is considered favourable to prebiotic molecule formation. As a first step in determining the role that cosmic rays might have played in the origin of life, we need to understand the significance of their ionising effect. Aims. We model the transport of solar and Galactic cosmic rays through a post-impact early Earth atmosphere at 200 Myr. We aim to identify the differences in the resulting ionisation rates – particularly at the Earth’s surface during a period when the Sun was very active. Methods. We used a Monte Carlo model for describing cosmic ray transport through the early Earth atmosphere, giving the cosmic ray spectra as a function of atmospheric height. Using these spectra, we calculated the ionisation and ion-pair production rates as a function of height due to Galactic and solar cosmic rays. The Galactic and solar cosmic ray spectra are both affected by the Sun’s rotation rate, Ω, because the solar wind velocity and magnetic field strength both depend on Ω and influence cosmic ray transport. We considered a range of input spectra resulting from the range of possible rotation rates of the young Sun – from 3.5–15Ω⊙. To account for the possibility that the Galactic cosmic ray spectrum outside the Solar System is not constant over gigayear timescales, we compared the ionisation rate at the top of the Earth’s atmosphere resulting from two different scenarios. We also considered the suppression of the cosmic ray spectra by a planetary magnetic field. Results. We find that the ionisation and ion-pair production rates due to cosmic rays are dominated by solar cosmic rays in the early Earth atmosphere for most cases. The corresponding ionisation rate at the surface of the early Earth ranges from 5 × 10−21 s−1 for Ω = 3.5Ω⊙ to 1 × 10−16 s−1 for Ω = 15Ω⊙. Thus if the young Sun was a fast rotator (Ω = 15Ω⊙), it is likely that solar cosmic rays had a significant effect on the chemistry at the Earth’s surface at the time when life is likely to have formed. Conclusions. Cosmic rays, particularly solar cosmic rays, are a source of ionisation that should be taken into account in chemical modelling of the post-impact early Earth atmosphere. Modelling of cosmic ray transport and effects on chemistry will also be of interest for the characterisation of H2-dominated exoplanet atmospheres.