Modeling the satellite particle population in the planetary exospheres: Application to Earth, Titan and Mars

• Modeling of the satellite particles in quasi-collisionless exopheres: application to Earth, Titan and Mars.
• New approach of the satellite particles density based on the Boltzmann equation.
• Comparison with the Chamberlain formalism.
• Satellite particles can represent a nonnegligible part of the exospheric populations.

The planetary exospheres are poorly known in their outer parts, since the low neutral densities are difficult to measure in situ. The exospheric models are thus often the main source of information at such high altitudes. We revisit here the importance of a specific exospheric population, i.e. the satellite particles, which is usually neglected in the models. These particles are indeed produced through rare collisions in the exospheres, and may either be negligible or dominate the exospheres of all planets with dense atmospheres in our Solar System, depending on the balance between their sources and losses. Richter et al. (Richter, E., Fahr, H.J., Nass, H.U. [1979]. Planet. Space Sci. 27, 1163–1173) were the first to propose, beyond the Chamberlain’s (Chamberlain, J.W. [1963]. Planet. Space Sci. 11, 901–901) rough approximation, a rigorous approach for these particles by using the Boltzmann equation in the Earth exosphere below 3000 km altitude. They pointed out their negligible presence at low altitudes without doing this calculation at higher altitudes. We further investigate this approach at Earth and apply it another planetary exospheres – Mars and Titan – thanks to improvements in the computing power and the collected planetary data. We determine the contribution of the satellite particles densities of light elements (H2 at Titan, H at Earth and Mars), and show in particular that the H satellite particles may contribute very significantly to the martian exospheric densities at high altitudes. The H2 satellite particles are also nonnegligible at Titan whereas the H satellite population represents only a small fraction of the total density at Earth. Considering collisionless exospheric profiles – such as the Chamberlain (Chamberlain, J.W. [1963]. Planet. Space Sci. 11, 901–901) approach including the ballistic and escaping populations only – could thus lead to significant underestimations of the total densities at high altitudes in some conditions.