Modeling the satellite particles in planetary exospheres : application to Titan, Earth and Mars

The planetary exospheres are poorly known in their outer parts, since neutral densities are low compared with the instruments detection capabilities. 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. satellite particles, which is usually neglected in the models. These particles are indeed produced through rare collisions in the exospheres (elastic, charge exchange, etc …) at low altitudes 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. At Titan, such calculations using the Boltzmann equation suggest a negligible contribution of H2 satellite populations compared to H2 ballistic populations by one or two orders of magnitude, in contradiction with conclusions inferred from energetic neutral atom images by the Cassini MIMI/INCA imager. The application to Earth predicts that H satellite particles, produced in their majority by elastic collisions with O atoms, are negligible by two orders of magnitude too, with a total density in good agreement with recent IBEX observations. We finally show the first results for cold H and cold O satellite particles in the Martian environment.