Impact events

Large Impacts Onto The Early Earth: Planetary Sterilization And Iron Delivery

By Keith Cowing
January 24, 2022
Filed under
Large Impacts Onto The Early Earth: Planetary Sterilization And Iron Delivery
Habitability of Earth in the aftermath of late accretion impacts depends strongly on the post-impact melt and iron distribution, and the energy deposited into the post-impact atmosphere. (a) In a nominal oblique impact a portion of the mantle is melted and ejecta heating also occurs downrange of the impact. The post-impact Earth has an ambiguous surface boundary of supercritical fluid, and the atmosphere consists of volatiles and a mix of vaporized silicate and projectile iron. Projectile iron is deposited in the melt/mantle/atmosphere, and some fraction also escapes the system with the impact ejecta. (b) In a head-on impact the projectile iron penetrates deep into the mantle, potentially reaching the core for larger impacts, and little material escapes the system. (c) In hit-and-run impacts, less melt is generated by the impact and a significant fraction of the projectile escapes the system.

Late accretion onto the Hadean Earth included large impacts that could have influenced early habitability, either by sterilizing the planet or alternatively catalyzing the origin of life by delivering iron required to create a reducing environment/atmosphere.

We present 3D numerical simulations of 1500-3400 km diameter impacts on the early Earth in order to quantify their effects on planetary habitability. We find sterilizing impact events require larger projectiles than previously assumed, with a 2000-2700 km diameter impactor required to completely melt Earth’s surface and an extrapolated >700 km diameter impactor required for ocean-vaporization.

We also find that reducing environments are less likely to arise following large impacts than previously suggested, because >70% of the projectile iron is deposited in the crust and upper mantle where it is not immediately available to reduce surface water and contribute to forming a reducing atmosphere. Although the largest expected late accretion impacts (~1 lunar mass) delivered sufficient iron to the atmosphere to have reduced an entire ocean mass of water, such impacts would also have melted the entire surface, potentially sequestering condensing iron that is not oxidized quickly.

The hypothesis that life emerged in the aftermath of large impacts requires an efficient mechanism of harnessing the reducing power of iron sequestered in the crust/mantle, or an origin of life pathway that operates in more weakly-reducing post-impact environments that require smaller quantities of impact-delivered iron.

Robert I. Citron, Sarah T. Stewart

Comments: 25 pages, 13 figures (accepted to PSJ)
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Geophysics (physics.geo-ph)
Cite as: arXiv:2201.09349 [astro-ph.EP] (or arXiv:2201.09349v1 [astro-ph.EP] for this version)
Submission history
From: Robert Citron
[v1] Sun, 23 Jan 2022 19:40:27 UTC (1,707 KB)

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