Mathematics – Logic
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p43e..05e&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P43E-05
Mathematics
Logic
[5410] Planetary Sciences: Solid Surface Planets / Composition, [5455] Planetary Sciences: Solid Surface Planets / Origin And Evolution, [6235] Planetary Sciences: Solar System Objects / Mercury
Scientific paper
Measurements of the surface composition of Mercury offers a window into the epoch of planet formation in the inner solar system. Mercury likely preserves a more complete record of early crustal formation than do Venus, Earth, or Mars, each of which experienced extensive and prolonged resurfacing and near-surface alteration since earliest crustal formation. The MErcury Surface, Space ENvironment, GEochemisty, and Ranging (MESSENGER) spacecraft was inserted into Mercury orbit on 18 March 2011 and carries a suite of instruments designed for remote sensing of elemental and mineralogical composition including a Gamma-Ray Spectrometer (GRS). We report measured surface abundances of radioactive elements on Mercury and their implications for hypotheses regarding the planet's formation and thermal evolution. The average surface abundances of radioactive elements over the region of Mercury measured by the GRS are 1150 ± 220 ppm K, 220 ± 60 ppb Th, and 90 ± 20 ppb U. Ratios of the moderately volatile incompatible element K to the refractory incompatible elements Th and U provide insights into the volatile inventory of planetary bodies. The measured K/Th ratio for Mercury (5200 ± 1800) is comparable to values for the other terrestrial planets. By contrast, the lunar K/Th value (360) is an order of magnitude lower, indicative of the depletion of lunar volatiles relative to Earth. Mercury's K/Th ratio, combined with the high abundance of the volatile element sulfur measured by the MESSENGER X-Ray Spectrometer, indicates that the planet has a volatile inventory similar to those of the other terrestrial planets. Hypotheses proposed to explain the unusually high ratio of metal to silicate on Mercury also carry predictions for the ratios of volatile to refractory elements that can be tested against the K, Th, and U abundances measured by MESSENGER. The abundance of K, relative to Th and U, is inconsistent with physical models for the formation of Mercury requiring extreme heating of the planet or its precursors and supports formation of the innermost planet from relatively volatile-enriched material comparable to known chondritic meteorites. Abundances of K, Th, and U indicate that heat production declined substantially in the past 4 Gy, consistent with widespread volcanism near the end of late heavy bombardment and only limited volcanic activity since.
Boynton Willam V.
Ebel Denton S.
Evans Larry G.
Goldsten John O.
Hamara David K.
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