Studying the evolution of the surfaces and atmospheres of planetary bodies in the solar system 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 exospheric 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 photons, energetic ions, and micrometeoroids.

While the exospheres are present around any kind of planetary body, they are quite different if we consider the bodies with an atmosphere and those without a collisional gas envelope. 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.

Since the exospheric constituents experience small interaction among them in a non-collisional condition, we can talk about separate exospheres for different species that can overlap to each other. In fact, some surface-release processes and some exospheric processes are active only for specific species. For example, the observed distributions of refractory species in the exospheres are quite different from the volatile ones and the distributions of specific molecules are expected to be even more peculiar. At the same time, the exospheres at different bodies have different characteristics depending on the external environment, such as the presence of a magnetospheric cavity, the intensity of the solar radiation and solar wind, the micrometeoroid population, as well as on surface composition and mineralogy.

The present collection of papers focuses on the large subset of planetary objects (planets, moons, and small bodies) that are not protected by either strong magnetic fields or thick atmospheres in the inner solar system where the solar influence is stronger. The alteration of the solid surface and the production of the surface-bounded exospheres by the impacts of meteoroids and of the time-varying solar wind over the last 4.54 Gy constitute an essential component of space weathering of exposed bodies such as Mercury, Moon, and asteroids. Furthermore, the detailed investigation of this subject is a paramount element in exo-planet studies. In fact, the observations performed with the new generation of very powerful telescopes could allow to obtain the exospheric composition and shape of exo-planet exospheres.

In the last decade, several space missions provided important new findings for many airless bodies. The missions SELENE, Chandrayaan-1, LADEE and LRO provided important results about the solar wind and Moon surface interaction, MESSENGER provided many important findings about the neutral and ionized environment at Mercury. Furthermore, new ground-based imaging techniques offered the possibility of improved exosphere observations. For example, the use of solar telescopes, such as THEMIS, for imaging the sodium exosphere of Mercury, has allowed to have high spatial resolution images for an observation time of several hours, while with classic nocturnal telescopes it was possible to observe only for a few hours at sunrise or sunset.

In the next decade, the ESA-JAXA BepiColombo mission to Mercury (orbit insertion is scheduled for December 2025) and various orbiters and landers to the Moon are in the space programmes of almost all space agencies in the worldwide. Therefore, this collection is the occasion to gather the present state of knowledge on this subject in preparation for the interpretation of the data to be received from the next generation of missions.

The contributions of this collection deal with different themes related to exospheric processes for different species and bodies in a comparative view.

In the Wurz et al. paper (Particles and photons as drivers: Comparison between Moon and Mercury) the external environment of Mercury and the Moon are described and the processes of particle release from the surface as driven by external drivers like thermal radiation, photons, electrons, ions, and dust are detailed.

In the Teolis et al. paper (Surface Exospheric Interactions) the surface effects related to exospheric generation processes are described in detail. Current understanding, latest developments, and future directions on studies of sticking and accommodation of exospheric species onto the surface, of diffusion in the regolith and ultimate escape of species from the regolith back into space are reviewed with focus on the Moon and Mercury.

In the Janches et al. paper (Meteoroids as one of the sources for exosphere formation on airless bodies in the inner solar system) the meteoroids and dust distributions in the inner Solar System are described by placing them in the context of the exosphere of the Moon, Mercury and other airless bodies. Effects onto the exosphere components of meteoroid streams and large impactors are also discussed.

In the Grava et al. paper (Volatiles and refractories in surface-bounded exospheres in the inner Solar System) the exospheres of extreme volatile species, like noble gas, and those constituted by refractory elements or refractory bearing molecules are considered in a comparative way. The different properties of these two groups, i.e., how they are bounded to the surface, how they are released, and how they migrate or stick onto the regolith, result in exospheres generated by different main agents and having totally different distributions and variabilities.

The Leblanc et al. paper (Comparative Na and K Mercury and Moon exospheres) treats in a comparative way the alkali exospheres at Mercury and Moon. These two main air-less bodies in the inner Solar System have surface bounded exospheres with similarities and important differences when observed in the two alkali main components, Na and K, i.e., the brightest elements that can be observed from the Earth and in space in the visible range. The observations and models are discussed in the frame of the different environments of these bodies with and without a magnetosphere.

The Schorghöfer et al. paper (Water Group Exospheres and Surface Interactions on the Moon, Mercury, and Ceres) focuses on the special case of water groups in the exospheres linked to the likely-present water ice in the cold traps of different bodies. In fact, surprisingly cold traps holding water ice are present from the distant dwarf planet of the main belt, like Ceres, to the innermost planet Mercury. Their formation mechanism is still unclear but future observations of water group species in the exosphere could provide important information on their transport.

In the Lammer et al. paper (The Exosphere as a Boundary: Origin and Evolution of Airless Bodies in the Inner Solar System and Beyond Including Planets with Silicate Atmospheres) the exospheric processes are considered from an historical point of view in relation to the parent stars, estimating the history of source and loss rates and surface alterations of the Moon and Mercury. Different possible environments as expected in exoplanets are also discussed and new targeted observations, that will shed light in these topics, are suggested.

Finally, the last paper by Milillo et al. (Future directions for the investigation of surface-bounded exospheres in the inner Solar System) reconsiders the main points of the whole collection with a special focus on open questions, expected results from the next space missions and recommended directions for modelling, laboratory experiments and future space and ground observations.

This collection intends to summarize the current state of knowledge on surface-bounded exospheres for the next-generation of scientists that will mainly be involved in the exploration of Mercury, Moon and exoplanets.