The Leaky Moon: Why Io’s Atmosphere is Always Trying to Escape

Io’s exosphere forms where its atmosphere transitions to a collisionless state. Most plume gases fail to escape due to Io’s gravity, unlike Enceladus, though volcanic escape pathways remain uncertain.

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Abstract

Appendix, Acknowledgements, and References

2.3 Exosphere and atmospheric escape

Escape takes place mostly above a certain level in the upper atmosphere known as the exobase where the transition from a collisional gas to a collisionless gas occurs (Figure 10). Below the exobase the atmosphere can be treated as a fluid, because the average distance a molecule or atom travels before making a collision – the mean free path – is shorter than the smallest macroscopic length scale. The latter is usually defined by the pressure scale height H which characterizes the exponential decay of pressure with altitude.

Above the exobase is a ...