  This paper considers how planetesimal impacts affect planetary atmospheres.
Atmosphere evolution depends on the ratio of gain from volatiles to loss from
atmosphere stripping f_v; for constant bombardment, atmospheres with f_v<1 are
destroyed in finite time, but grow linearly with time for f_v>1. An impact
outcome prescription is used to characterise how f_v depends on planetesimal
impact velocities, size distribution and composition. Planets that are low mass
and/or close to the star have atmospheres that deplete in impacts, while high
mass and/or distant planets grow secondary atmospheres. Dividing these outcomes
is an fv=1 impact shoreline analogous to Zahnle & Catling's cosmic shoreline.
The impact shoreline's location depends on assumed impacting planetesimal
properties, so conclusions for the atmospheric evolution of a planet like Earth
with f_v~1 are only as strong as those assumptions. Application to the
exoplanet population shows the gap in the planet radius distribution at
~1.5R_earth is coincident with the impact shoreline, which has a similar
dependence on orbital period and stellar mass to the observed gap. Given
sufficient bombardment, planets below the gap would be expected to lose their
atmospheres, while those above could have atmospheres enhanced in volatiles.
The level of atmosphere alteration depends on the total bombardment a planet
experiences, and so on the system's (usually unknown) other planets and
planetesimals, though massive distant planets would have low accretion
efficiency. Habitable zone planets around lower luminosity stars are more
susceptible to atmosphere stripping, disfavouring M stars as hosts of
life-bearing planets if Earth-like bombardment is conducive to the development
of life.
