Water/Icy Super-Earths: Giant Impacts and Maximum Water Content

TL;DR

This study uses simulations to show that giant impacts between water-rich super-Earths do not increase their water content and often decrease it, providing insights into planetary formation and composition evolution.

Contribution

The paper introduces a comprehensive set of equations describing how giant impacts affect the water content of differentiated super-Earths, applicable across various compositions.

Findings

Giant impacts between similar composition bodies do not decrease water fraction.

Impacts can either leave water content unchanged or decrease it.

The disruption criteria are similar across different planetary compositions.

Abstract

Water-rich super-Earth exoplanets are expected to be common. We explore the effect of late giant impacts on the final bulk abundance of water in such planets. We present the results from smoothed particle hydrodynamics simulations of impacts between differentiated water(ice)-rock planets with masses between 0.5 and 5 M_Earth and projectile to target mass ratios from 1:1 to 1:4. We find that giant impacts between bodies of similar composition never decrease the bulk density of the target planet. If the commonly assumed maximum water fraction of 75wt% for bodies forming beyond the snow line is correct, giant impacts between similar composition bodies cannot serve as a mechanism for increasing the water fraction. Target planets either accrete materials in the same proportion, leaving the water fraction unchanged, or lose material from the water mantle, decreasing the water fraction. The criteria for catastrophic disruption of water-rock planets are similar to those found in previous work on super-Earths of terrestrial composition. Changes in bulk composition for giant impacts onto differentiated bodies of any composition (water-rock or rock-iron) are described by the same equations. These general laws can be incorporated into future N-body calculations of planet formation to track changes in composition from giant impacts.