Statistics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p21a1194r&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P21A-1194
Statistics
[1027] Geochemistry / Composition Of The Planets, [5410] Planetary Sciences: Solid Surface Planets / Composition, [5494] Planetary Sciences: Solid Surface Planets / Instruments And Techniques, [6235] Planetary Sciences: Solar System Objects / Mercury
Scientific paper
The surface composition of Mercury provides critical clues to the origin and evolution of the planet. The MESSENGER Gamma-Ray Spectrometer uses a mechanically cryocooled germanium detector to measure spectra of gamma rays emanating from the planet’s surface as a result of cosmic-ray interactions. Elemental abundances can be determined by analyzing these spectra. The Mercury flybys have a closest approach of ~ 200 km near the equator and thus provide the opportunity to measure elemental abundances of surface equatorial regions, which will not be visited at low altitude during the mission orbital phase. The MESSENGER mission provides for three such flybys before the spacecraft is inserted into orbit around Mercury. The first flyby occurred on 14 January 2008, the second on 6 October 2008, and the third on 29 September 2009. The spectra are limited by low planetary photon flux. During the mission orbital phase, counting statistics will be improved by accumulating counts for numerous orbits over a given surface region, but each flyby is equivalent to only a single orbit. Sufficient counts have nonetheless been accumulated during the first two flybys to estimate bounds on abundances for some elements having relatively strong spectral peaks, including Si, Fe, and K. Relative to a Si abundance assumed to be 21.5 wt. %, the average abundance of Fe in Mercury’s equatorial regions is 7.0 ± 3.4 wt. %, where the uncertainty is one standard deviation, σ (68% confidence level). For K we find a 2-σ upper bound of 0.46 wt. %. Although the K signal is not sufficiently high to establish detection with the limited counting statistics of the flybys, the signal is expected to be strong enough that a quantitative abundance can be determined during the mission orbital phase. However the upper bound on K obtained from the flyby data already can rule out some suggested surface composition models. Planetary close-approach spectra were taken while the spacecraft was within 2500 km of Mercury (a total of 46 minutes for the first two flybys), and pre-approach background spectra were taken for a total of 66 hours. Amplification factors for the background spectra because of increased spacecraft activation by planetary neutrons during close approach were required, as well as spacecraft attenuation factors for planetary gamma-rays during close approach caused by large changes in spacecraft orientation. The uncertainties given above are based entirely on counting statistics; the uncertainties due to these other factors are not included. The results for the third flyby will be combined with those for the first two flybys to update the bounds on elemental abundances for the equatorial latitudes of Mercury.
Boynton Willam V.
Evans Larry G.
Goldsten John O.
Lawrence D. Jr. J.
Nittler Larry
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