title: MESSENGER observations of the composition of Mercury's ionized exosphere and plasma environment.
abstract: 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.
authors.affiliation: Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, 2455 Hayward Street, Ann Arbor, MI 48109-2143, USA. thomasz@umich.edu
authors.forename: Thomas H
authors.initials: TH
authors.lastname: Zurbuchen
authors.forename: Jim M
authors.initials: JM
authors.lastname: Raines
authors.forename: George
authors.initials: G
authors.lastname: Gloeckler
authors.forename: Stamatios M
authors.initials: SM
authors.lastname: Krimigis
authors.forename: James A
authors.initials: JA
authors.lastname: Slavin
authors.forename: Patrick L
authors.initials: PL
authors.lastname: Koehn
authors.forename: Rosemary M
authors.initials: RM
authors.lastname: Killen
authors.forename: Ann L
authors.initials: AL
authors.lastname: Sprague
authors.forename: Ralph L
authors.initials: RL
authors.lastname: McNutt
authors.forename: Sean C
authors.initials: SC
authors.lastname: Solomon