University of Maryland
Atlantic Building, Room 2400
4:30 PM Monday, November 11, 2013
Coffee, Tea & Snacks 4:15-4:30 PM

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.

Sponsored by: Department of Physics and the Institute for Physical Science and Technology, University of Maryland. For information call Catha Stewart at (301) 405-4811 or go to the UMD Space Physics group seminar web site.

There is free parking after 4:00 PM in lot B (the big parking garage across the street from the ATL building). There are a limited number of spaces in lot Q next to the new ATL wing with free parking after 4PM even when there is a basketball game on campus.