Satellites

Abstract

The presence of an atmosphere on planetary satellites is an anomaly. The relatively low masses of the moons result in low values of gravity and escape velocity. As a consequence, any volatiles outgassed during and after the accretionary phase of the moons would be rapidly lost. Since the conventional dynamo mechanism does not operate on the moons, they do not possess intrinsic magnetic fields. Therefore, they are likely to be stripped of any remaining atmosphere by interaction with charged particles of the solar wind or planetary magnetospheres origin. The only exceptions to the above scenario are: Saturn’s moon Titan — which definitely has a massive atmosphere, Jupiter’s moon Io — which appears to possess a tenuous atmosphere, and Neptune’s moon Triton — which has some promise of having an atmosphere with pressures comparable to those in the troposphere of the Earth. In addition, extremely tenuous atmospheres might also exist on some of the Galilean moons (Europa, Ganymede, and Callisto), and above the rings of Saturn. Io’s atmosphere seems to result from its perpetual volcanic activity caused by its unique tidal interaction in the Jovian system. The ability of Titan to retain an atmosphere appears to be the result of its comparatively greater mass, combined with its location in the Saturn system and in the primitive solar nebula. The reason for the existence of an atmosphere on Triton is similar to that for Titan, along with the fact that Triton should have strong tidal interaction with Neptune due to its 160° orbital inclination.