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Atmospheric Escape Processes and Planetary Atmospheric Evolution
The habitability of the surface of any planet is determined by a complex
evolution of its interior, surface, and atmosphere. The electromagnetic and
particle radiation of stars drive thermal, chemical and physical alteration of
planetary atmospheres, including escape. Many known extrasolar planets
experience vastly different stellar environments than those in our Solar
system: it is crucial to understand the broad range of processes that lead to
atmospheric escape and evolution under a wide range of conditions if we are to
assess the habitability of worlds around other stars. One problem encountered
between the planetary and the astrophysics communities is a lack of common
language for describing escape processes. Each community has customary
approximations that may be questioned by the other, such as the hypothesis of
H-dominated thermosphere for astrophysicists, or the Sun-like nature of the
stars for planetary scientists. Since exoplanets are becoming one of the main
targets for the detection of life, a common set of definitions and hypotheses
are required. We review the different escape mechanisms proposed for the
evolution of planetary and exoplanetary atmospheres. We propose a common
definition for the different escape mechanisms, and we show the important
parameters to take into account when evaluating the escape at a planet in time.
We show that the paradigm of the magnetic field as an atmospheric shield should
be changed and that recent work on the history of Xenon in Earth's atmosphere
gives an elegant explanation to its enrichment in heavier isotopes: the
so-called Xenon paradox