Paper No. 3
Presentation Time: 1:40 PM
ELEMENTAL COMPOSITION OF THE SURFACE OF MERCURY FROM THE MESSENGER GAMMA-RAY SPECTROMETER
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.