Atmospheric Mass Loss During Planet Formation: The Importance of Planetesimal Impacts

Abstract

We quantify the atmospheric mass loss during planet formation by examining 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 ($\gamma=5/3$) that the local atmospheric mass loss fraction for ground velocities $v_g < 0.25 v_{esc}$ is given by $\chi_{loss}=(1.71 v_g/v_{esc})^{4.9}$, 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 $x=(v_{Imp}m/v_{esc}M)$. Atmospheric mass loss due to planetesimal impacts proceeds in two different regimes: 1) Large enough impactors $m > \sqrt{2} \rho_0 (\pi h R)^{3/2}$ (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 $h$, $R$ and $\rho_0$ are the atmospheric scale height, radius of the target, and its atmospheric density at the ground. 2) Smaller impactors, but above $m>4 \pi \rho_0 h^3$ (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 and that the current atmosphere of the Earth could have resulted from an equilibrium between atmospheric erosion and volatile delivery to the atmosphere from planetesimals. 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. (Abridged)