Upper Limit on the Fe Abundance at Mercury's Surface From MESSENGER'S First Mercury Flyby

Details Upper Limit on the Fe Abundance at Mercury's Surface From MESSENGER'S First Mercury Flyby Upper Limit on the Fe Abundance at Mercury's Surface From MESSENGER'S First Mercury Flyby

May 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusm.u23a..03f&link_type=abstract

American Geophysical Union, Spring Meeting 2008, abstract #U23A-03

Physics

0328 Exosphere, 5410 Composition (1060, 3672), 6235 Mercury

Scientific paper

The Neutron Spectrometer aboard MESSENGER successfully measured the leakage flux of neutrons from Mercury during the first flyby on 14 January 2008. The foreground-to-background signal ratio for neutrons spanning the energy range between 0 eV and 8 MeV was exceptionally high at closest approach, which occurred at 200 km altitude. A rotation of the spacecraft just prior to closest approach varied the angle between the normal to the outboard-facing thermal/epithermal neutron sensor and the instantaneous spacecraft velocity vector relative to Mercury, from near 90° to near 180° and then back to 90°. Variations in measured neutron currents responded to this rotation by developing a sharp peak in their intensity just prior to the main peak in counting rates due to the passage of closest approach. Such a peak is predicted for a gravitationally bound thermal-neutron exosphere at Mercury because the speed of the spacecraft was about 7 km/s, which is considerably larger than that of a thermal neutron, approximately 3 km/s. Preliminary analysis of these data shows that counting rates and their variations with spacecraft rotation are sufficiently robust to yield important upper limits for the Fe abundance of surface regolith. The dominant uncertainties for abundance estimates are the result of incomplete knowledge of the instrument response, which will be improved as calibration and modeling analyses are applied to the data. This technique will be applied to the next two flybys, as well as to the orbital phase of the MESSENGER mission to map the summed abundances of all highly absorbing elemental species, primarily Fe, Ti, Gd, and Sm.

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