Sodium D2 line profiles: Clues to the temperature structure of Mercury's exosphere

The velocity distribution of atoms in an exosphere can be used to constrain source and loss processes. The velocity distribution itself can be constrained by observing the line profile of an atmospheric emission line. The only atomic species known to exist in the Hermean exosphere that has a strong enough resonance transition in the visible spectrum that its line profile can be observed from the ground with existing instruments is sodium. Line profiles of the sodium D2 emission (5889 A) were obtained at two facilities. On 29 and 30 May 1997, we obtained line profiles with the 107 inch telescope at The McDonald Observatory at coude focus. On 6 January 1998, we obtained line profiles at the Anglo-Australian telescope. The line profiles show that the sodium in Mercury's atmosphere is at least 700 K hotter than the surface temperature throughout the atmosphere. This implies that the interaction of the Na atoms with the surface is governed by energetic processes.