Exospheres of earth-like planets and moons

Surface-bounded exospheres have been detected at the Moon, Mercury and Europa and almost certainly exists about other objects. Historically, the first of these systems to be observed was the lunar exosphere, where He and Ar were detected by the Apollo spacecraft, but the most important discovery was the detection of Sodium and Potassium on 1988. The same discovery was made for Mercury on 1985. Soon after the discoveries several observers concentrate their efforts on the sodium emission lines, having a high cross section for resonant scattering making them very easy to detect, providing interesting data on the exosphere behaviour and its interaction with the surface and the interplanetary medium. The sources that maintain these exospheres are of considerable interest, but direct information on sources is difficult to obtain. In the last few years several efforts have been concentrated on the interaction of the meteor showers and the lunar exosphere and surface, we will report the most important observations and results on this particular source. We will review the current state of knowledge of the exospheres of Mercury and the Moon and discuss the important data that future space missions, as BepiColombo and Messenger, may provide. Then we will briefly describe the exosphere of Europa, discovered on 1996, more than 20 years after the study of the Io sodium cloud, having the same main element, as sodium, but being characterized by some different sources and processes.