Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.u21a0029s&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #U21A-0029
Physics
5410 Composition (1060, 3672), 5464 Remote Sensing
Scientific paper
Information on the surface elemental composition of Mercury is limited, but will be greatly expanded by observations to be made by instruments onboard the MESSENGER spacecraft. One of these instruments, the X-Ray Spectrometer (XRS), will determine elemental abundances on the surface of Mercury by measuring fluorescent X-ray emissions induced on the planet's surface by the incident solar flux. The most prominent fluorescent lines are the Kα lines from the elements Mg, Al, Si, S, Ca, Ti, and Fe (1-10 keV). The sampling depth is less than 100 microns. Prior to entering orbit about Mercury in March 2011, MESSENGER will have flown by the planet three times for spacecraft trajectory corrections and scientific observations. At the first Mercury flyby (14 January 2008), no X-ray fluorescence was detected from the planet during the 5 minutes of observing time available to the XRS. Calculations based on typical solar X-ray emissions and likely models for surface composition suggested that even for quiet Sun conditions (no flare activity) and limited viewing time, flux from the Mg and Si lines (1.25 and 1.74 keV, respectively) would be detectable above background. But solar X-ray emissions during the flyby were an order of magnitude less than expected and no X-ray signal was detected from the planet. Observing parameters for the second Mercury flyby (6 October 2008) will be similar to those in January with about 5 minutes of observing time and a sunlit portion of the planet filling the XRS field of view. More typical solar X-ray emissions should provide sufficient signal for a statistically significant measurement of surface composition.
McCoy Timothy James
Nittler Larry R.
Schlemm Charles E.
Solomon Stanley C.
Sprague Ann L.
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