Cosmic ray ionisation of a post-impact early Earth atmosphere: Solar cosmic ray ionisation must be considered in origin-of-life scenarios

Abstract

Cosmic rays (CR), both solar and Galactic, have an ionising effect on the Earth's atmosphere and are thought to be important for prebiotic molecule production. In particular, the $\rm{H_2}$-dominated atmosphere following an ocean-vaporising impact is considered favourable to prebiotic molecule formation. We model solar and Galactic CR transport through a post-impact early Earth atmosphere at 200Myr. We aim to identify the differences in the resulting ionisation rates, $\zeta$, particularly at the Earth's surface during a period when the Sun was very active. We use a Monte Carlo model to describe CR transport through the early Earth atmosphere, giving the CR spectra as a function of altitude. We calculate $\zeta$ and the ion-pair production rate, $Q$, as a function of altitude due to Galactic and solar CR. The Galactic and solar CR spectra are both affected by the Sun's rotation rate, $\Omega$, because the solar wind velocity and magnetic field strength both depend on $\Omega$ and influence CR transport. We consider a range of input spectra resulting from the range of possible $\Omega$, from $3.5-15\, \Omega_{\rm{\odot}}$. To account for the possibility that the Galactic CR spectrum outside the Solar System varies over Gyr timescales, we compare top-of-atmosphere $\zeta$ resulting from two different scenarios. We also consider the suppression of the CR spectra by a planetary magnetic field. We find that $\zeta$ and $Q$ due to CR are dominated by solar CR in the early Earth atmosphere for most cases. The corresponding $\zeta$ at the early Earth's surface ranges from $5 \times 10^{-21}\rm{s^{-1}}$ for $\Omega = 3.5\,\Omega_{\rm{\odot}}$ to $1 \times 10^{-16}\rm{s^{-1}}$ for $\Omega = 15\,\Omega_{\rm{\odot}}$. Thus if the young Sun was a fast rotator, it is likely that solar CR had a significant effect on the chemistry at the Earth's surface at the time when life is likely to have formed.