Theoretical models of nonthermal processes in the atmospheres of Venus, Earth and Mars
Creators: name: Arkos, Gregory G
Publisher: name: University of British Columbia
Year: 2009

description: The present state of planetary exospheres is determined largely by satellite and ground based observations which are predominantly measurements of the emissions of exospheric constituents. Such observations are responsible for the growing recognition of the importance of nonthermal collisional processes in determining the distribution and escape of species in planetary exospheres. Nonthermal processes provide an important enhanced escape mechanism for lighter species such as hydrogen. They may also make escape possible for heavier species, such as oxygen, nitrogen and carbon, for which thermal escape is very small. Nonthermal processes have been employed in order to understand discrepancies in the terrestrial helium budget. They have also been used to reconcile discrepancies between observed and calculated escape fluxes for hydrogen on Earth. Nonthermal processes have also been utilized to explain features observed in the exospheres of other planets. These include the measured deuterium-to-hydrogen ratio on Venus and the extended hot oxygen corona on Mars. Given the importance of nonthermal processes it is clear that exospheric conditions are determined to a large extent by collisional processes and that the collisionless model must be reconsidered. The formation of translationally energetic (or hot) oxygen coronae via the nonthermal process of dissociative recombination of 0₂⁺ in the atmospheres of Venus and Mars is examined using both hydrodynamic and kinetic theory approaches. Of interest is the distribution of hot oxygen at altitude resulting from production and transport from lower altitudes. It is found that an extended hot oxygen corona can be predicted from either approach, although the magnitude and extent of the predicted coronae vary significantly. Product velocity distribution functions describing the rate of production of hot atoms for the atomic systems H-H+, D-H-f, O-H, and O-D are calculated for a variety of nonthermal processes, including direct-elastic and charge-exchange collisions. The calculations are carried out using a kinetic theory approach, and utilize direct numerical integration techniques. The calculations incorporate realistic, quantum-mechanical collision cross sections for each system so as to accurately describe the kinematics of the collision process. Energy exchange rate coefficients for each of the atomic systems are calculated and compared with results obtained using a more complicated Monte Carlo approach. The product velocity distribution functions are also used to estimate the escaping fractions of H and D as a result of nonthermal direct elastic energization by hot oxygen atoms. These kinetic theory calculations are compared to work done by other workers using Monte Carlo methods incorporating approximate and quantum mechanical cross sections. The calculations show that the fraction of hot deuterium produced via direct energization by hot oxygen, while less than the fraction of hot hydrogen, is not negligible as previously believed. An altitude dependent, kinetic theory approach is used to calculate the rate of escape of atmospheric constituents, in the context of escape resulting from energization of neutral atmospheric species via nonthermal processes. The reduction of the escape rate by the ambient atmosphere is included through an altitude dependent parameter describing the probability of escape, although the effect of thermalization via collisions with the background is neglected. Temperature and density profiles used in the calculations are taken from available atmospheric data and from atmospheric models, and escape fluxes of hydrogen and deuterium are estimated for Venus and Earth.