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Published February 2015 | public
Journal Article Open

Atmospheric mass loss during planet formation: The importance of planetesimal impacts


Quantifying the atmospheric mass loss during planet formation is crucial for understanding the origin and evolution of planetary atmospheres. We examine the contributions to atmospheric loss from both giant impacts and planetesimal accretion. Giant impacts cause global motion of the ground. Using analytic self-similar solutions and full numerical integrations we find (for isothermal atmospheres with adiabatic index γ=5/3) that the local atmospheric mass loss fraction for ground velocities v_g ≲ 0.25v_esc is given by χ_loss = (1.71v_g/v_esc)^4.9, where v_esc is the escape velocity from the target. Yet, the global atmospheric mass loss is a weaker function of the impactor velocity v_Imp and mass m_Imp and given by X_loss≃0.4x + 1.4x^2 - 0.8x^3 (isothermal atmosphere) and X_loss≃0.4x + 1.8x^2 - 1.2x^3 (adiabatic atmosphere), where x=(v_Impm/v_escM). Atmospheric mass loss due to planetesimal impacts proceeds in two different regimes: (1) large enough impactors m≳√(2)ρ_0(πhR)^(3/2) (25 km for the current Earth), are able to eject all the atmosphere above the tangent plane of the impact site, which is h/2R of the whole atmosphere, where View the MathML sourceh,R and ρ0ρ0 are the atmospheric scale height, radius of the target, and its atmospheric density at the ground. (2) Smaller impactors, but above m>4πρ_(0)h^3(1 km for the current Earth) are only able to eject a fraction of the atmospheric mass above the tangent plane. We find that the most efficient impactors (per unit impactor mass) for atmospheric loss are planetesimals just above that lower limit (2 km for the current Earth). For impactor flux size distributions parametrized by a single power law, N(>r)∝r^(-q+1), with differential power law index q, we find that for 13 the mass loss is dominated by regime (2). Impactors with m≲4πρ_(0)h^3 are not able to eject any atmosphere. Despite being bombarded by the same planetesimal population, we find that the current differences in Earth's and Venus' atmospheric masses can be explained by modest differences in their initial atmospheric masses and that the current atmosphere of the Earth could have resulted from an equilibrium between atmospheric erosion and volatile delivery to the atmosphere from planetesimal impacts. We conclude that planetesimal impacts are likely to have played a major role in atmospheric mass loss over the formation history of the terrestrial planets.

Additional Information

© 2015 Elsevier B. V. Received 1 May 2014; Revised 25 September 2014; Accepted 29 September 2014; Available online 8 October 2014. We thank H.J. Melosh and the second anonymous referee for their constructive reviews and D. Jewitt, T. Grove, N. Inamdar for helpful comments and suggestions. R.S. dedicates this paper to the late Tom Ahrens, who initiated his interest in the problem of atmospheric escape and collaborated on related ideas.

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August 22, 2023
August 22, 2023