Editorial to “Surface-Bounded Exospheres and Interactions in the Inner Solar System”

Milillo, Anna; Sarantos, Menelaos; Murakami, Go; Teolis, Ben D.; Wurz, Peter (2023). Editorial to “Surface-Bounded Exospheres and Interactions in the Inner Solar System”. Space science reviews, 219(7) Springer 10.1007/s11214-023-00998-4

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Studying the evolution of the surfaces and atmospheres of planetary bodies in the solar sys-
tem is fundamental to our understanding of the present state of the solar system. Exospheres
are the interfaces between the planetary body and the open space, so that, studying the ex-
ospheric filling and loss processes is the way to expand knowledge of the body’s evolution.
This endeavour entails finding variation of the rates of the ongoing processes as a function
of the space environment, or, in other words, how the planetary space weather affects these
bodies. Aside from occasional catastrophic events, such as volcanic eruptions and geysers
in a few bodies or occasional impacts of comets and asteroids, surface and atmospheric
changes are caused predominantly by the continuous bombardment of the bodies by pho-
tons, energetic ions, and micrometeoroids.
While the exospheres are present around any kind of planetary body, they are quite dif-
ferent if we consider the bodies with an atmosphere and those without a collisional gas en-
velope. In fact, in the former case the exosphere is the upper part of the gas envelope where
collisions become less and less frequent with altitude, so that, the boundary, the exobase, is a
thick shell only conventionally defined as the surface where Knudsen number, Kn (the ratio
of the mean free path over the atmospheric scale height), is equal to unity. On the contrary,
in the latter case the exosphere is directly connected to the surface, thus, it is called surface-
bounded exosphere, since the surface release processes are also the exospheric filling ones
and atoms and molecules collide with the surface far more frequently than collisions with
each other. In this case, the exobase is considered the surface itself, but it has quite different
characteristics from the exosphere – atmosphere boundary.

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