Science Seminars

2/25/2010 – Exploring Mercury’s Surface-bound Exosphere With the Mercury Atmospheric and Surface Composition Spectrometer: Results from the Three MESSENGER Flybys

Speaker: Bill McClintock (LASP)
Date: Monday, Feb 15, 2010
Location: LSTB-299

Seminar Abstract:

The planet Mercury is surrounded by a tenuous surface-bounded exosphere whose composition and structure are controlled by interactions among the surface, magnetosphere, solar wind, and sunlight. One of the scientific goals of the MErcury: Surface, Space Environment, Geochemistry, Ranging (MESSENGER) mission is to understand the nature of those interactions in order to identify the important volatile species on or near Mercury and to determine their sources and sinks. MESSENGER is a NASA Discovery-class mission designed to orbit and explore the planet Mercury and its space environment. Launched on August 3, 2004, the spacecraft has flown past Mercury three times (M1 on January 14, 2008, M2 on October 6, 2008, and M3 on September 30, 2009) on its way to orbit insertion on March 18, 2011. It carries a suite of seven miniature scientific instruments. Included among them is the Mercury Atmospheric and Surface Composition Spectrometer (MASCS), provided by the Laboratory for Atmospheric and Space Physics. Prior to the MESSENGER flybys Mercury’s exosphere was known to contain H and He, observed by Mariner 10, as well as Na, K, and Ca, observed from ground-based telescopes. When species are liberated from the surface with sufficient energy, those with strong resonance lines in the visible can be accelerated by solar radiation pressure in some circumstances to form an anti-sunward tail. During the three MESSENGER flybys, MASCS mapped Na, Ca, and Mg in the planet’s anti-sunward tail and also detected Ca+ in a narrow region approximately 1-2 Mercury radii above the anti-sunward surface. Some of the MASCS data are consistent with the picture of Mercury’s exosphere developed from ground-based observations of Na and Ca. On the other hand, unexpected spatial distributions of Mg, Ca and Ca+, suggest that the exosphere is more complex previously thought.