Exospheres and Atmospheric Escape

Non‐thermal escape of molecular hydrogen from Mars

“…Moreover, the efficient escape of the Martian atmosphere to space is thought to be partly responsible for the current low atmospheric pressure [ Chassefière and Leblanc , 2004]. The present day degradation of the Martian atmosphere occurs mainly via non‐thermal escape processes induced by ion charge‐exchange, sputtering and ionospheric outflows driven by solar wind [ Chassefière and Leblanc , 2004; Johnson et al , 2008]. The dissociative recombination (DR) of O 2 + is a major source of hot O atoms in the upper atmosphere of Mars, responsible for the escape of oxygen and formation of a hot Martian corona [ Ip , 1988; Fox , 1993; Krest'yanikova and Shematovich , 2005].…”

Influence of Martian crustal magnetic anomalies on the emission of energetic neutral hydrogen atoms

“…However, these enhancements are expected to be negligible since the local cusp regions are much smaller in comparison to the total area of the magnetic anomalies [Brain et al, 2007], and instances of large scale heights are rare according to radio occultation observations [Ness et al, 2000]. Enhancements of the neutral density due to sputtering are also expected to be negligible for nominal solar conditions at Mars [Johnson et al, 2008;Wang et al, 2014]. Another source of the exospheric density enhancement is via the modification of chemical reactions in the magnetic anomaly regions with closed field lines.…”

Escape from Titan's Atmosphere: Kenetic Monte Carlo Simulations

“…Hence on average a molecule that suffered its previous collision near the exobase can travel a distance over which the density will decrease by a factor 1/e without suffering another collision. In terms of the hydrostatic column density, N, this critical level occurs at the altitude where N ~ c/σ, for the collision cross-section between molecules, σ with c of the order of unity (Johnson et al, 2008).…”

“…These molecules as well as those in the tail of a Maxwellian distribution, referred to as suprathermal, can have kinetic energies much greater than the gravitational binding energy. Johnson (1990Johnson ( , 1994 used realistic potentials to determine the mean free path when considering the deflections of suprathermal molecules within a background gas and analytically derived an average escape column resulting in c~1.3, with σ equal to the diffusion (momentum transfer) cross section (e.g., Johnson et al, 2008). Therefore, it should be kept in mind that escape occurs from a broad region of altitudes in real planetary atmospheres with the most energetic molecules being able to escape from depths several scale heights below the exobase as shown by the mass obtain escaping trajectories primarily through thermal collisions (Lebonnois et al, 2003), and the N atoms ( Figure 2-1(b)) are suprathermal particles produced from non-thermal interactions (Johnson et al, 2008;Shematovich et al, 2003).…”

“…Therefore to find a solution one varies the parameters  and (dT/dr) 0 to find a solution with the right asymptotic behavior, zero total energy flux at r  ∞ and consistent with density data, when available. Strobel (2008)…”