About the loss of a primordial atmosphere of super-Earths by planetesimal impacts

TL;DR

This study examines how impacts during planet formation influence the loss of primordial water atmospheres on super-Earths, revealing that most vapor escapes and the remaining water can form ice layers, affecting planetary composition.

Contribution

It introduces a discrete impact model to analyze water vapor loss during planet formation, highlighting the roles of impact velocities and initial composition.

Findings

Most water vapor escapes during impacts.

Impact velocities significantly influence water retention.

Planetesimal size distribution has negligible effect.

Abstract

We consider planets composed of water ice and rock, located far from a central star. In an earlier study, computing the growth of planets by continuous accretion, we found that a large fraction of the ice evaporates upon accretion, creating a water vapor atmosphere. Here we consider accretion as a discrete series of planetesimal impacts (of order $10^8$), at the same time-dependent accretion rate, and investigate the fate of the vapor, as a result of its interaction with the accreting planetesimals. We find that a large fraction of the vapor escapes. The remaining fraction may form an outer layer of ice after the termination of accretion and cooling of the surface. The escaped water mass may significantly alter the ice-to-rock ratio of the planet. We investigate the effect of different choices of parameters such as the ice-to-rock ratio, the planetesimal size distribution, and the impact velocities. We find that the planetesimal size distribution has a negligible effect and explain why. By contrast, the ice-to-rock ratio and impact velocities affect the fraction of retained water masses considerably.