Evolution of the Earth's Atmosphere during Late Veneer Accretion

TL;DR

This study models how late-stage impacts from leftover planetesimals, asteroids, and comets influenced Earth's early atmosphere, revealing stochastic erosion and growth patterns that tend to converge to current Earth-like conditions.

Contribution

It introduces a new statistical stochastic model combining impact dynamics and atmospheric processes to study Earth's atmospheric evolution during late veneer accretion.

Findings

Impacts cause moderate atmospheric erosion and occasional large growth events.

Atmosphere evolution is highly stochastic and depends on impactor properties.

Final atmospheric mass tends to resemble current Earth's atmosphere, regardless of initial conditions.

Abstract

Recent advances in our understanding of the dynamical history of the Solar system have altered the inferred bombardment history of the Earth during accretion of the Late Veneer, after the Moon-forming impact. We investigate how the bombardment by planetesimals left-over from the terrestrial planet region after terrestrial planet formation, as well as asteroids and comets, affects the evolution of Earth's early atmosphere. We develop a new statistical code of stochastic bombardment for atmosphere evolution, combining prescriptions for atmosphere loss and volatile delivery derived from hydrodynamic simulations and theory with results from dynamical modelling of realistic populations of impactors. We find that for an initially Earth-like atmosphere impacts cause moderate atmospheric erosion with stochastic delivery of large asteroids giving substantial growth ($\times 10$) in a few $\%$ of cases. The exact change in atmosphere mass is inherently stochastic and dependent on the dynamics of the left-over planetesimals. We also consider the dependence on unknowns including the impactor volatile content, finding that the atmosphere is typically completely stripped by especially dry left-over planetesimals ($<0.02 ~ \%$ volatiles). Remarkably, for a wide range of initial atmosphere masses and compositions, the atmosphere converges towards similar final masses and compositions, i.e. initially low mass atmospheres grow whereas massive atmospheres deplete. While the final properties are sensitive to the assumed impactor properties, the resulting atmosphere mass is close to that of current Earth. The exception to this is that a large initial atmosphere cannot be eroded to the current mass unless the atmosphere was initially primordial in composition.