The mass of a planet is the quantity that determines the formation of an atmosphere. The massive outer planets of our solar system have huge atmospheres of hydrogen, helium, ammonia and methane. The inner planets, on the other hand, have weakly formed atmospheres, with the heavier gases oxygen, nitrogen and carbon dioxide playing the main role. In the case of even smaller objects, such as the Earth’s moon, the gravitational force is no longer sufficient to bind gases permanently.

However, there is a volatile atmosphere of atoms that could have been ejected from the rock on the surface of the moon by micrometeorites, solar wind ions or photons. It could also have been formed by the vaporisation of atoms from the hotter, illuminated surface.

Observations in the wavelength range of the elements sodium and potassium can be used to trace the properties of this thin atmosphere, which can provide important insights into its formation and preservation. Although spectroscopic images of the low atmosphere are possible at any phase of the moon, images of the extended atmosphere are only possible under special circumstances. Near new moon the sky is still too bright at observation time, near full moon the scattered light of the covered moon disc outshines the actual observation object. A total lunar eclipse offers a favourable opportunity here, when the moon is completely immersed in the earth’s shadow but the moon’s atmosphere is still illuminated by the sun.

In order to obtain the circumlunar sodium distribution, several images are required: one image in the sodium wavelength, which also contains interfering scattered light, and one outside this wavelength band. By subtraction, an image of the sodium distribution from lunar and terrestrial sources is obtained. A third image, in which the telescope is not pointing at the moon, provides the contribution from the terrestrial sources. A final subtraction gives the desired data.

The sodium atmosphere observed by M. Mendillo and J. Baumgardner (Nature 377, 404 [1995]) in November 1993 during a total lunar eclipse was considerably wider than expected at full moon - it extended to at least nine lunar radii. The measured brightness distribution now allows conclusions to be drawn about the formation of the atmosphere. According to the authors, it is incompatible with sources that include either solar wind or micrometeorites. This leaves photon sputtering or thermal evaporation as the favoured explanations for the lunar atmosphere. This suggests that the volatile atmospheres of other simple bodies, such as Mercury, are also generated by sunlight.