Paper No. 3
Presentation Time: 1:40 PM


PEPLOWSKI, Patrick N.1, EVANS, Larry G.2, LAWRENCE, David J.1, GOLDSTEN, John O.3, STOCKSTILL-CAHILL, Karen R.4, MCCOY, Timothy J.5, NITTLER, Larry R.6, SOLOMON, Sean C.7, STARR, Richard D.8 and WEIDER, Shoshana Z.6, (1)Space Department, Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, (2)NASA, Goddard Space Flight Center, Code 691/CSC, Greenbelt, MD 20771, (3)Space Department, Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, (4)Space Department, Johns Hopkins University Applied Physics laboratory, Laurel, MD 20723, (5)National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, (6)Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Rd NW, Washington, DC 20015, (7)Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, (8)Physics Department, Catholic University of America, Washington, DC 20064,

Orbital measurements obtained by the MESSENGER Gamma-Ray Spectrometer (GRS) have been analyzed to determine the surface abundances of Na, Al, Si, S, K, Ca, Fe, Th, and U on the northern hemisphere of Mercury. Where measurements were made by both the GRS and the X-Ray Spectrometer (XRS), the gamma-ray results (Al/Si = 0.29 +0.05/-0.13, S/Si = 0.092 ± 0.015, Ca/Si = 0.24 ± 0.05, and Fe/Si = 0.077 ± 0.013) are consistent with those obtained by the XRS. This agreement includes the previously-reported high sulfur and low iron abundances, an important result as the GRS and XRS sample to different depths in the regolith. The Na/Si (0.12 ± 0.01), K (1228 ± 234 ppm), Th (155 ± 54 ppb), and U (90 ± 20 ppb) results are unique to the GRS and offer insight into the volatile inventory of the planet. The S, Na, K/Th, and Th/U measurements have revealed that the surface of Mercury has a higher volatile content than was expected on the basis of pre-MESSENGER theories for Mercury’s formation. Spatially resolved maps of the gamma-ray emission from the surface reveal variations in the K abundance ranging from 300 to 2400 ppm and variations in the Na/Si abundance ratio ranging from 0.10 to 0.20 (~2.5 to 5 wt% Na). The highest Na abundances are seen in the north polar region. A comparison of the distribution of K and Na abundances with models for the maximum surface temperature suggest the possibility that a temperature-dependent process is controlling the abundances of these volatile elements on the surface, and may provide a mechanism for their release into the exosphere. Taken together, these results rule out a global, lunar-like feldspar-rich crust and are consistent with previously suggested analogs for surface material on Mercury, including terrestrial komatiities, low-iron basalts, partial melts of CB chondrites, and partial melts of enstatite chondrites. Petrologic modeling of these compositions indicates a substantial amount of Na-bearing plagioclase feldspar is present.