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Planetary embryos form protoplanets via mutual collisions, which can lead tothe development of magma oceans. During their solidification, large amounts ofthe mantles' volatile contents may be outgassed. The resulting H$_2$O/CO$_2$dominated steam atmospheres may be lost efficiently via hydrodynamic escape dueto the low gravity and the high stellar EUV luminosities. Protoplanets forminglater from such degassed building blocks could therefore be drier thanpreviously expected. We model the outgassing and subsequent hydrodynamic escapeof steam atmospheres from such embryos. The efficient outflow of H drags alongheavier species (O, CO$_2$, noble gases). The full range of possible EUVevolution tracks of a solar-mass star is taken into account to investigate theescape from Mars-sized embryos at different orbital distances. The envelopesare typically lost within a few to a few tens of Myr. Furthermore, we study theinfluence on protoplanetary evolution, exemplified by Venus. We investigatedifferent early evolution scenarios and constrain realistic cases by comparingmodeled noble gas isotope ratios with observations. Starting from solar values,consistent isotope ratios (Ne, Ar) can be found for different solar EUVhistories, as well as assumptions about the initial atmosphere (either puresteam or a mixture with accreted H). Our results generally favor an earlyaccretion 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 isinconsistent with Venus' present atmosphere. Important issues are likely thetime at which the initial steam atmosphere is outgassed and/or the amount ofCO$_2$ which may still be delivered at later evolutionary stages. A lateaccretion scenario can only reproduce present isotope ratios for a highlyactive young Sun, but then very massive steam atmospheres would be required.

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