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

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.