Nonthermal escape of the atmospheres of Venus, Earth, and Mars

Abstract

Atmospheric loss from planetary atmospheres is an important geophysical problem with implications for planetary evolution. This is a multidisciplinary research field that requires an expertise in a wide range of subjects including statistical mechanics, fluid mechanics, plasma physics, collision theory, and surface science. This paper is a review of the current state of our understanding of atmospheric loss from the terrestrial planets. A detailed discussion is provided of the basic concepts required to understand the processes occurring in the high‐altitude portion of a planetary atmosphere referred to as the exosphere. Light atomic species with sufficient translational energy can escape from an atmosphere. The translational energy required for escape could be thermal energy and proportional to the ambient temperature or the result of some collisional processes energizing the species above thermal energies. These collisional processes, which include charge exchange and dissociative recombination between energetic ions, neutrals, and electrons, are referred to as nonthermal escape processes. We highlight the similarities and differences in the important escape mechanisms on the terrestrial planets and comment on application of these mechanisms to evolutionary theories of the terrestrial atmospheres. The emphasis in this paper is directed toward the need to consider the exosphere as collisional.

First observation of Mars with XMM-Newton

“…This is particularly interesting because the X-ray emission results directly from charge exchange interactions between atmospheric constituents and solar wind ions, a process which is considered as an important nonthermal escape mechanism and which may be responsible for a significant loss of the Martian atmosphere (Shizgal & Arkos 1996). Despite this importance, our observational knowledge of the Martian exosphere is still poor and is mainly based on detailed simulations (e.g., Chen & Cloutier 2003).…”

Mechanisms of ionospheric mass escape

“…First, we consider nonthermal forms of velocity distribution in addition to thermal Maxwellians [see also Shizgal and Arkos, 1996]. Table 1 provides a listing of the forms used.…”

Comment on "A Hydrogen-Rich Early Earth Atmosphere"

“…Helium continuously fluxes to Earth_s atmosphere from radioactive decay in Earth_s interior. Today_s exobase is sufficiently cold that thermal escape of helium accounts for less than one-millionth of the total helium escape flux (11). Thus, if we only considered thermal escape, we would incorrectly conclude that Earth_s present atmosphere should be relatively rich in helium.…”