PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = TEXT INTERCHANGE_FORMAT = ASCII PUBLICATION_DATE = 2013-05-23 NOTE = "This text file describes non-mapped GRS results." END_OBJECT = TEXT END Written as part of the MESSENGER GRS Planetary Data Systems Submission 5/23/2013 GRS RESULTS The MESSENGER Gamma-Ray Spectrometer has characterized the abundances of stable and radioactive elements within the top tens of cm of Mercury's surface. These measurements are limited to the northern hemisphere due to MESSENGER's highly elliptical orbit, which has a periapse at mid-to-high northern latitudes. When this orbital profile is coupled with the altitude dependence of the gamma-ray signal (for which useful data aquisition is limited to that collected at altitudes of < ~2000 km), the resulting surface coverage is restricted to the northern hemipshere only [Peplowski et al., 2011a]. The procedure for deriving elemental abundance ratios for stable elements is presented in Evans et al., [2012] and Peplowski et al., [2012b]. The results from those publications are reproduced in the table below. Element Ratio Weighted* Average Weighted error# Ca/Si 0.24 0.05 S/Si 0.092 0.015 Fe/Si 0.077 0.013 Na/Si 0.12 0.01 Al/Si 0.29 +0.05/-0.13 * "Weighted" refers to the fact that most ratios were derived from multiple gamma-ray peaks, and that those measurements with the highest statistical significance were weighted accordingly. # Throughout this document, all listed errors are the one-standard deviation values. In each applicable case (Fe/Si, S/Si, Ca/Si, and Al/Si), these results are in agreement with those independently derived from the MESSENGER X-Ray Spectrometer [Nittler et al., 2011; Weider et al., 2012]. Peplowski et al. [2013] characterized spatially resolved measurements of variable Na/Si abundances in latitude bands in the northern hemisphere. These values are reproduced in the table below. Latitude Range Na/Si Abundance Ratio error 0-15 deg N 0.115 0.015 15-30 deg N 0.098 0.016 30-45 deg N 0.123 0.019 45-60 deg N 0.103 0.018 60-75 deg N 0.129 0.019 75-90 deg N 0.172 0.020 0-60 deg N 0.107 0.008 80-90 deg N 0.198 0.030 Finally, Peplowski et al. [2012a] produced a northern-hemisphere map of K abundances, which appears as a seperate PDS product. They also derived spatially-resolved measurements of K and Th abundances in specific regions of interest. Those values, along with the northern-hemisphere averaged K, Th, and U abundances published in Peplowski et al. [2011b], are reproduced below. The 2011 values were derived from the first 59 days of data aquisition. The 2012 values were derived using all data aquired prior to the 15 October 2011, at which point the detector began to operate in with reduced performance before the failure of the cryocooler in June 2012. Region K Abundundance Th Abundance U Abundance (ppm) (ppm) (ppm) N. Hemisphere (2011) 1150 +/- 220 0.175 +/- 0.064 0.090 +/- 0.020 N. Hemisphere (2012) 1288 +/- 224 0.155 +/- 0.054 N. volcanic Plains# 786 +/- 330 0.142 +/- 0.075 Caloris Basin 754 +/- 203 0.540 +/- 0.288 IcP/HCT* 952 +/- 176 0.168 +/- 0.072 # The northern volcanic plains unit is a large contigous unit located at the north pole that covers 6% of the planet's surface. See Head et al. [2011] and Denevi et al. [2013] for additional information. * IcP/HCT is the Intercrater Plains and Heavily Cratered Terrain, which for the purposes of this analysis was all data acquired outside of the northern volcanic plains unit. References: Denevi, B.W., C. M. Ernst, H. M. Meyer, M. S. Robinson, S. L. Murchie, T. R. Watter, S. C. Solomon, L. R. Ostrach, J. L. Whitten, J. W. Head, C. R. Chapman, P. K. Byrne, and P. N. Peplowski (2013), The Distribution and Origin of Smooth Plains on Mercury, Jour. Geophys. Res., (in press), doi:10.1029/2012JE004295. Evans, L. G., P. N. Peplowski, E. A. Rhodes, D. J. Lawrence, L. R. Nittler, T. J. McCoy, S. C. Solomon, A. L. Sprague, K. R. Stockstill-Cahill, R. D. Starr, S. Z. Weider, W. V. Boynton, and D. K. Hamara (2012), Major-element abundances on the surface of Mercury: Results from the MESSENGER Gamma-Ray Spectrometer, J. Geophys. Res. 117, E00L07, doi:10.1029/2012JE004178. Head, J. W., C. R. Chapman, R. G. Strom, C. I. Fassett, B. W. Denevi, D. T. Blewett, C. M. Ernst, T. R. Watters, S. C. Solomon, S. L. Murchie, L. M. Prockter, N. L. Chabot, J. J. Gillis-Davis, J. L. Whitten, T. A. Goudge, D. M. H. Baker, D. M. Hurwitz, L. R. Ostrach, Z. Xiao, W. J. Merline, L. Kerber, J. L. Dickson, J. Oberst, P. K. Byrne, C. Klimczak, and L. R. Nitter (2013), Flood Volcanism in the Northern High Latitudes of Mercury Revealed by MESSENGER, Science, 333, 1853-1856, doi:10.1126/science.1211997. Nittler, L. R., R. D. Starr, S. Z. Weider, T. J. McCoy, W. V. Boynton, D. S. Ebel, C. M. Ernst, L. G. Evans, J. O. Goldsten, D. K. Hamara, D. J. Lawrence, R. L. McNutt Jr., C. E. Schlemm II, S. C. Solomon, and A. L. Sprague (2011), The major element composition of Mercury's surface from MESSENGER X-ray spectrometry, Science 333, 1847-1850, doi:10.1126/science.1211567. Peplowski, P. N., D. T. Blewett, B. W. Denevi, L. G. Evans, D. J. Lawrence, L. R. Nittler, E. A. Rhodes, C. M. Selby, and S. C. Solomon (2011a), Mapping iron abundances on the surface of Mercury: Predicted spatial resolution of the MESSENGER Gamma-Ray Spectrometer, Planet. Space Sci, 59, 1654-1658, doi:10.1126/science.1211576. Peplowski, P. N., L. G. Evans, S. A. Hauck II, T. J. McCoy, W. V. Boynton, J. J. Gillis-Davis, D. S. Ebel, J. O. Goldsten, D. K. Hamara, D. J. Lawrence, R. L. McNutt Jr., L. R. Nittler, S. C. Solomon, E. A. Rhodes, A. L. Sprague, R. D. Starr, and K. R. Stockstill-Cahill (2011b), Radioactive elements on Mercury's surface from MESSENGER: Implications for the planet's formation and evolution, Science, 333, 1850-1852, doi:10.1126/science.1211576. Peplowski, P. N., D. J. Lawrence, E A. Rhodes, A. L. Sprague, T. J. McCoy, B. W. Denevi, L. G. Evans, J. W. Head, L. R. Nittler, S. C. Solomon, K. R. Stockstill-Cahill, and S. Z. Weider (2012a), Variations in the abundances of potassium and thorium on the surface of Mercury: Results from the MESSENGER Gamma-Ray Spectrometer, Jour. Geophys. Res., 117, E00L04, doi:10.1029/2012JE004141. Peplowski, P. N., E. A. Rhodes, D. K. Hamara, D. J. Lawrence, L. G. Evans, L. R. Nittler, and S. C. Solomon, (2012b), Aluminum abundance on the surface of Mercury: Applications of a new background-reduction technique for the analysis of gamma-ray spectroscopy data, Jour. Geophys. Res., 117, doi:10.1029/2012JE004181. Peplowski, P. N., L. G. Evans, K. R. Stockstill-Cahill, D. J. Lawrence, T. J. McCoy, L. R. Nittler, S. C. Solomon, A. L. Sprague, R. D. Starr, and S. Z. Weider (2013), Enhanced sodium abundances in Mercury's north polar region revealed by the MESSENGER Gamma-Ray Spectrometer, Icarus (in review). Weider, S. Z., L. R. Nittler, R. D. Starr, P. K. Byrne, B. W. Denevi, and S. C. Solomon (2012), Chemical heterogeneity on Mercury's surface revealed by the MESSENGER X-Ray Spec-trometer, J. Geophys. Res. 117, E00L05, doi:10.1029/2012JE004153.