Sodium winds on Mercury

Solar radiation acceleration imparts anti-sunward velocities to sodium atoms in the Mercury exosphere. The Earthward-directed vectors of the Sun-accelerated atom velocities can be observed from Earth as small Doppler shifts, either added to, or subtracted from the Earth–Mercury Doppler shifts. We measured these small Doppler shifts using high resolution spectrographs capable of detecting sodium velocity differences as small as 0.1 km/s. We report here four sets of observations performed at different Mercury true anomaly angles. For these measurements, the spectrograph slit was oriented first east–west, and then north–south on the planet so as to get east–west and north–south transects of the velocities. The velocity patterns in east–west transects could be explained in terms of sodium flows outwards from the subsolar point, except for unexpectedly large Earthward velocities observed above the dawn terminator, which we interpreted to be the result of evaporation of sodium as the cold surface is heated by the rising Sun. North–south transects also showed a general pattern consistent with sodium flows outwards from the subsolar point. However, in all cases, the velocities were higher in one hemisphere relative to the other. For two cases, excess sodium emission was observed in the same hemisphere as the velocity excess. We interpreted these results to mean that there existed sources of sodium at high latitudes, which could appear in either hemisphere.