The Nature of the Hydrogen Tori of Titan and Triton

Abstract

The natures of the circumplanetary hydrogen tori of Titan and Triton are explored. Two mechanisms are identified which dramatically alter the standard picture adopted for the spatial distribution of a gas torus that forms in the l/r central gravitational potential of a planet for a gas source escaping from an orbiting satellite. For the Titan hydrogen torus, atom-atom collisions are not important. In this situation, the perturbation of solar radiation pressure produced by the resonant scattering of Lyman-α photons by H atoms is a previously overlooked mechanism that destroys the normally assumed azimuthal symmetry of the torus. Atom orbits evolve inward with a preferred orientation of their longitude of pericenter, and a significant fraction of these atoms is lost from the torus by collision with the planet on its dusk side before they are otherwise lost by magnetospheric lifetime processes. This mechanism provides a natural physical basis for understanding the asymmetric distribution of hydrogen about Saturn recently reported by Shemansky and Hall (1992, J. Geophys. Res. 97, 4143-4161). For the Triton hydrogen torus, atom-atom collision times are important since, although they are much longer than a Kepler orbit time, they are much smaller than either the effective solar radiation pressure time scale for H or the time scale for magnetospheric and photoionization loss of H. In this situation, elastic collisions between gas atoms of the torus convert organized circular motion of the atom about the planet into random atom energy which rapidly expands the torus structure both inward and outward. This expansion reduces the torus density and in addition leads to inward collisional loss to the planet and outward escape loss from the planet. This expansion mechanism is a nonlinear effect that has been known for two decades in the field of Solar System and ring formation but has been previously overlooked as important for collisional gas tori about planets.