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Science 4 July 2008:
Vol. 321. no. 5885, pp. 90 - 92
DOI: 10.1126/science.1159314
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Reports

MESSENGER Observations of the Composition of Mercury's Ionized Exosphere and Plasma Environment

Thomas H. Zurbuchen,1* Jim M. Raines,1 George Gloeckler,1 Stamatios M. Krimigis,2,3 James A. Slavin,4 Patrick L. Koehn,5 Rosemary M. Killen,6 Ann L. Sprague,7 Ralph L. McNutt, Jr.,2 Sean C. Solomon8

The region around Mercury is filled with ions that originate from interactions of the solar wind with Mercury's space environment and through ionization of its exosphere. The MESSENGER spacecraft's observations of Mercury's ionized exosphere during its first flyby yielded Na+, O+, and K+ abundances, consistent with expectations from observations of neutral species. There are increases in ions at a mass per charge (m/q) = 32 to 35, which we interpret to be S+ and H2S+, with (S+ + H2S+)/(Na+ + Mg+) = 0.67 ± 0.06, and from water-group ions around m/q = 18, at an abundance of 0.20 ± 0.03 relative to Na+ plus Mg+. The fluxes of Na+, O+, and heavier ions are largest near the planet, but these Mercury-derived ions fill the magnetosphere. Doubly ionized ions originating from Mercury imply that electrons with energies less than 1 kiloelectron volt are substantially energized in Mercury's magnetosphere.

1 Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, 2455 Hayward Street, Ann Arbor, MI 48109–2143, USA.
2 Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA.
3 Academy of Athens, Research Center for Astronomy and Applied Mathematics, Soranou Efessiou 4, Athens 11527, Greece.
4 Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA.
5 Department of Physics and Astronomy, Eastern Michigan University, 303 Strong Hall, Ypsilanti, MI 48197, USA.
6 Department of Astronomy, University of Maryland, College Park, MD 20742, USA.
7 Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA.
8 Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA.

* To whom correspondence should be addressed. E-mail: thomasz{at}umich.edu

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THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
MESSENGER Observations of Mercury's Exosphere: Detection of Magnesium and Distribution of Constituents.
W. E. McClintock, R. J. Vervack Jr, E. T. Bradley, R. M. Killen, N. Mouawad, A. L. Sprague, M. H. Burger, S. C. Solomon, and N. R. Izenberg (2009)
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Return to Mercury: A Global Perspective on MESSENGER's First Mercury Flyby.
S. C. Solomon, R. L. McNutt Jr., T. R. Watters, D. J. Lawrence, W. C. Feldman, J. W. Head, S. M. Krimigis, S. L. Murchie, R. J. Phillips, J. A. Slavin, et al. (2008)
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The Structure of Mercury's Magnetic Field from MESSENGER's First Flyby.
B. J. Anderson, M. H. Acuna, H. Korth, M. E. Purucker, C. L. Johnson, J. A. Slavin, S. C. Solomon, and R. L. McNutt Jr. (2008)
Science 321, 82-85
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Mercury's Magnetosphere After MESSENGER's First Flyby.
J. A. Slavin, M. H. Acuna, B. J. Anderson, D. N. Baker, M. Benna, G. Gloeckler, R. E. Gold, G. C. Ho, R. M. Killen, H. Korth, et al. (2008)
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