Origin & Evolution of Life

Ray Ionisation Of A Post-impact Early Earth Atmosphere: Solar Cosmic Ray Ionisation Must Be Considered In Origin-of-life Scenarios

By Keith Cowing
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astro-ph.EP
April 4, 2025
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Ray Ionisation Of A Post-impact Early Earth Atmosphere: Solar Cosmic Ray Ionisation Must Be Considered In Origin-of-life Scenarios
(a) Top-of-atmosphere Galactic cosmic ray spectrum at 1au at 200Myr. Spectra for a slow (black dotted line), medium (red dashed line) and fast (blue dot-dashed line) solar rotation rate. The LIS is shown by the solid grey line. The enhanced LIS shown by the dot-dashed grey line is discussed in Section 4. The spectrum for the fast rotation rate has been multiplied by 102 to allow all of the spectra to be shown within a reasonable range on the same axes. (b) Top-of-atmosphere solar cosmic ray spectrum at 1au at 200Myr for a slow, medium and fast solar rotation rate. The linestyles represent the same scenarios as in the Galactic cosmic ray spectra. The green line represents the fit of the observed spectrum Reeves et al. (1992) of the solar energetic particle event measured at Earth in October 1989. — astro-ph.EP

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 H2-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, ζ, 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 ζ 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, Ω, because the solar wind velocity and magnetic field strength both depend on Ω and influence CR transport.

We consider a range of input spectra resulting from the range of possible Ω, from 3.5−15Ω⊙. To account for the possibility that the Galactic CR spectrum outside the Solar System varies over Gyr timescales, we compare top-of-atmosphere ζ resulting from two different scenarios. We also consider the suppression of the CR spectra by a planetary magnetic field. We find that ζ and Q due to CR are dominated by solar CR in the early Earth atmosphere for most cases.

The corresponding ζ at the early Earth’s surface ranges from 5×10−21s−1 for Ω=3.5Ω⊙ to 1×10−16s−1 for Ω=15Ω⊙. 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.

S. R. Raeside, D. Rodgers-Lee, P. B. Rimmer

Comments: 13 pages, 11 figures, accepted for publication in A&A
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR); Space Physics (physics.space-ph)
Cite as: arXiv:2504.02596 [astro-ph.EP] (or arXiv:2504.02596v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2504.02596
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Submission history
From: Shauna Rose Raeside
[v1] Thu, 3 Apr 2025 13:59:09 UTC (128 KB)
https://arxiv.org/abs/2504.02596

Astrobiology, Space Weather,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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