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.

Articles you may be interested inNonequilibrium contributions to the rate of chemical reaction in the Lorentz gas: A comparison of perturbation and numerical solutions of the Boltzmann equation J. Chem. Phys. 89, 197 (1988); 10.1063/1.455504 Nonequilibrium velocity distribution and reaction rate in ion-molecule reactions J. Chem. Phys. 74, 6742 (1981); 10.1063/1.441131 Nonequilibrium velocity distribution and reaction rate in the hot 18F+H2 reaction J. Chem. Phys. 66, 4078 (1977); 10.1063/1.434480 Nonequilibrium velocity distribution and reaction rate in hotatom reactions J. Chem. Phys. 65, 3883 (1976); 10.1063/1.432905 Nonequilibrium velocity distributions and reaction rates in fast highly exothermic reactionsThe correction to the equilibrium rate of reaction in a one-component reactive system is calculated from the perturbation of the velocity distribution function obtained by solving the Chapman-Enskog and Burnett equations. A method of solution of the Chapman-Enskog equation with a Sonine polynomial expansion is described that is not limited by the number of terms retained and is applicable to realistic systems characterized by elastic and reactive cross sections which may be available only in tabulated form. The convergence of the Sonine polynomial expansion is demonstrated for a variety of model reactions with and without activation energy and for a set of cross sections obtained with a semiempirical potential for the reaction H2 (i) +H 2 (j) ..... (products), where i and j denote the vibrational quantum numbers. The Sonine polynomial method is compared with the recent variational solutions of Present and Morris. It is shown that the extent of the departure from equilibrium is due to the deviation of the reactive collision number R(O)(c) from a special form R,(O)(c). It was found that for the realistic systems considered here the decrease in the equilibrium rate of reaction due to a perturbation of the velocity distribution function by reaction is very small ( ;S 1 %) and that the Burnett correction to the distribution function can be neglected.For the set of (H 2 , H 2 ) reactions, the greatest decrease in the equilibrium rate of reaction is 0.66% at 6400 0 K for (i, j) = (3, 2). A formal transport theory description of reaction rates analogous to the description of heat conduction and viscous flow by Chapman and Cowling is also presented.

A review and critique of present‐day kinetic theory models of planetary exospheres is presented. Models of ionized exospheres, specifically the solar and the terrestrial polar wind, are also discussed. The objective of the paper is to point out the need for a rigorous kinetic theory treatment of the atmosphere in the altitude region between the thermosphere and the exosphere. This is the region where the atmosphere undergoes a transition from a collision‐dominated to a collisionless situation. The various aspects of the exospheric problem are introduced and developed around this main theme. The exospheric problem and its considerable overlap with several related problems in physics and chemistry are noted. The calculation of the velocity distribution function of exospheric constituents with the spherically symmetric collisionless model is then presented. Various modifications of this standard model with regard to planetary rotation, nonuniform exobasic density and temperature distributions, and time‐dependent effects are subsequently discussed and compared. The extent to which collisionless models suffice to explain observational data of planetary exospheres is assessed. This assessment is given in terms of a comparison of density profiles or reduction factors determined with hydrogen and/or helium cells. A comparison of experimental measurements of the diurnal variation of terrestrial atomic hydrogen densities and theoretical models to explain the asymmetry is presented. The collisionless approach is criticized on theoretical grounds for the neglect of a consideration of the transition region and the resulting artificial boundary between the thermosphere and the exosphere. The aim of collisional models with regard to escape‐induced non‐Maxwellian effects, modifications of the temperature structure, nonthermal escape processes, and satellite particle populations from Monte Carlo simulations and/or kinetic theory models based on a Boltzmann equation is discussed, and the various results are compared. The various attempts to introduce collisional concepts are shown to be incomplete, each considering the problem with different objectives. Throughout, the importance of a more rigorous collisional treatment is stressed, in particular with respect to the description of nonthermal escape processes and satellite particle populations which cannot be considered in a collisionless context. The effects of charge exchange reactions, photoionization, and solar radiation pressure on satellite particles are examined. With regard to nonthermal escape processes, the absence of a detailed kinetic theory of product velocity distributions based on a collisional theory is also noted.

No abstract