Matthew H. Burger
Morgan State University/GESTAR and NASA/GSFC
MESSENGER Observations of Mercury's Tenuous Atmosphere
Mercury is surrounded by a tenuous, collisionless atmosphere known to contain hydrogen, helium, oxygen, sodium, potassium, calcium, and magnesium. The Mercury Atmospheric and Surface Composition Spectrometer (MASCS) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft has made high spatial resolution observations of emission from neutral atoms in Mercury’s atmosphere both near Mercury’s surface and in the extended, anti-sunward-directed tail. MASCS began routine orbital observations of both the dayside and nightside atmosphere on March 29, 2011. In addition to making the first definitive detection of oxygen on the dayside, UVVS regularly measures the altitude profiles of H, Na, Ca, and Mg, the last of which was discovered by MASCS during MESSENGER’s second flyby of Mercury on October 6, 2008.
The most striking feature of the MASCS data is the substantial differences among species’ distributions. Because the atmosphere is collisionless, its composition reflects the balance of active source and loss processes. The loss of neutrals due to photoionization and solar radiation-pressure effects that push material down the tail is well understood and cannot fully explain the differences in the observed spatial distributions of Na, Ca, and Mg. Instead, there must be substantial differences in the source mechanisms and/or the regions on the surface from which each species originates. Sodium is thought to arise primarily from the dayside as a result of photon-stimulated desorption (PSD); solar wind and magnetospheric ion precipitation onto the poles and nightside both increase the PSD flux (in sunlit regions) and directly sputter Na into the exosphere. Calcium originates predominantly from the dawn equatorial region in a location fixed in local time, but the source mechanism is not known. The magnesium may be similar spatially and energetically to calcium in its source distribution, but its emissions are weak and more data and modeling analysis are needed to place definitive limits on the source. However, the data suggest a mix of hot and cold magnesium sources, whereas calcium shows evidence only of a single hot source.