  Mars is believed to have had a substantial atmosphere in the past.
Atmospheric loss led to depressurization and cooling, and is thought to be the
primary driving force responsible for the loss of liquid water from its
surface. Recently, MAVEN observations have provided new insight into the
physics of atmospheric loss induced by ICMEs and solar wind interacting with
the Martian atmosphere. In addition to solar radiation, it is likely that its
atmosphere has been exposed to radiation bursts from high-energy astrophysical
sources which become highly probable on timescales of ~Gy and beyond. These
sources are capable of significantly enhancing the rates of photoionization and
charged particle-induced ionization in the upper atmosphere. We use Monte Carlo
simulations to model the interaction of charged particles and photons from
astrophysical sources in the upper Martian atmosphere and discuss its
implications on atmospheric loss. Our calculations suggest that the passage of
the solar system though dense interstellar clouds is the most significant
contributor to atmospheric loss among the sources considered here.
