# Escape and fractionation of volatiles and noble gases from Mars-sized planetary embryos and growing protoplanets

## TL;DR

This study models the outgassing and hydrodynamic escape of steam atmospheres from Mars-sized planetary embryos, revealing their potential to lose volatiles and noble gases, influencing planetary evolution and atmospheric composition.

## Contribution

It provides a comprehensive model of volatile outgassing and escape from planetary embryos, incorporating various stellar EUV histories and initial conditions, to explain observed noble gas isotope ratios.

## Key findings

- Embryos typically lose their steam atmospheres within a few to tens of millions of years.
- Consistent noble gas isotope ratios are achievable with early, low-activity Sun scenarios.
- Late accretion scenarios require a highly active young Sun and massive atmospheres to match observations.

## Abstract

Planetary embryos form protoplanets via mutual collisions, which can lead to the development of magma oceans. During their solidification, large amounts of the mantles' volatile contents may be outgassed. The resulting H$_2$O/CO$_2$ dominated steam atmospheres may be lost efficiently via hydrodynamic escape due to the low gravity and the high stellar EUV luminosities. Protoplanets forming later from such degassed building blocks could therefore be drier than previously expected. We model the outgassing and subsequent hydrodynamic escape of steam atmospheres from such embryos. The efficient outflow of H drags along heavier species (O, CO$_2$, noble gases). The full range of possible EUV evolution tracks of a solar-mass star is taken into account to investigate the escape from Mars-sized embryos at different orbital distances. The envelopes are typically lost within a few to a few tens of Myr. Furthermore, we study the influence on protoplanetary evolution, exemplified by Venus. We investigate different early evolution scenarios and constrain realistic cases by comparing modeled noble gas isotope ratios with observations. Starting from solar values, consistent isotope ratios (Ne, Ar) can be found for different solar EUV histories, as well as assumptions about the initial atmosphere (either pure steam or a mixture with accreted H). Our results generally favor an early accretion scenario with a small amount of accreted H and a low-activity Sun, because in other cases too much CO$_2$ is lost during evolution, which is inconsistent with Venus' present atmosphere. Important issues are likely the time at which the initial steam atmosphere is outgassed and/or the amount of CO$_2$ which may still be delivered at later evolutionary stages. A late accretion scenario can only reproduce present isotope ratios for a highly active young Sun, but then very massive steam atmospheres would be required.