Matthew H. Burger

New Discoveries From MESSENGER and Insights into Mercury's Exosphere

For most of the orbital phase of the MESSENGER mission, a regular search for weakly emitting or less abundant species in Mercurys exosphere resulted in non-detections. However, during the final Earth year of the mission, emission from multiple lines of manganese, aluminum, and ionized calcium was detected. These observations validate the detection of a single line of ionized calcium during the third MESSENGER Mercury flyby, provide definitive confirmation for weak aluminum detections in ground-based observations, and represent the discovery of manganese in Mercurys exosphere. These detections occurred over a limited range of pre-dawn local times and Mercury true anomaly angles (0o-70o), and each has a distinct spatial distribution. Equally interesting is the absence of detectable emission from oxygen at limits well below the levels reported for Mariner 10.

A cold‐pole enhancement in Mercury's sodium exosphere

The Ultraviolet and Visible Spectrometer (UVVS) component of the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) on the MESSENGER spacecraft characterized the local-time distribution of the sodium exosphere over the course of its orbital mission. The observations show that the sodium exosphere is enhanced above Mercurys cold-pole longitudes. Based on previously published sodium exosphere models we infer that these regions act as nightside surface reservoirs, temporary sinks to the exosphere that collect sodium atoms transported anti-sunward. The reservoirs are revealed as exospheric enhancements when they are exposed to sunlight. As in the models the reservoir is depleted as the cold poles rotate from dawn to dusk, but unlike the models the depletion is only partial. The persistence of the reservoir means that it could, over the course of geologically long periods of time, contribute to an increase in the bulk concentration of sodium near the cold-pole longitudes.