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\begin{document}

\title{Photodissociation-Driven Mass Loss from Young and Highly-Irradiated
Exoplanets}



\author[1]{Alex Howe}%
\author[2]{Michael Meyer}%
\author[2]{Fred Adams}%
\affil[1]{NASA Goddard Space Flight Center}%
\affil[2]{University of Michigan}%


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  \date{May 21, 2024}


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\begin{abstract}
The most widely-studied mechanism of mass loss from extrasolar planets
is photoevaporation via XUV ionization, primarily in the context of
highly irradiated planets. However, the EUV dissociation of hydrogen
molecules can also theoretically drive atmospheric evaporation on
low-mass planets. For temperate planets such as the early Earth, impact
erosion is expected to dominate in the traditional planetesimal
accretion model, but it would be greatly reduced in pebble accretion
scenarios, allowing other mass loss processes to be major contributors.
We apply the same prescription for photoionization to this
photodissociation mechanism and compare it to an analysis of other
possible sources of mass loss in pebble accretion scenarios. We find
that there is not a clear path to evaporating the primordial atmosphere
accreted by an early Earth analog in a pebble accretion scenario. Impact
erosion could remove \textasciitilde{}2,300 bars of hydrogen if 1\% of
the planet's mass is accr eted as planetesimals, while the combined
photoevaporation processes could evaporate \textasciitilde{}750 bars of
hydrogen. Photodissociation is likely a subdominant, but significant
component of mass loss. Similar results apply to super-Earths and
mini-Neptunes. This mechanism could also preferentially remove hydrogen
from a planet's primordial atmosphere, thereby leaving a larger
abundance of primordial water compared to standard dry formation models.
We discuss the implications of these results for models of rocky planet
formation including Earth's formation and the possible application of
this analysis to mass loss from observed exoplanets.%
\end{abstract}%



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