title: Elemental mapping by Dawn reveals exogenic H in Vesta's regolith.
abstract: Using Dawn's Gamma Ray and Neutron Detector, we tested models of Vesta's evolution based on studies of howardite, eucrite, and diogenite (HED) meteorites. Global Fe/O and Fe/Si ratios are consistent with HED compositions. Neutron measurements confirm that a thick, diogenitic lower crust is exposed in the Rheasilvia basin, which is consistent with global magmatic differentiation. Vesta's regolith contains substantial amounts of hydrogen. The highest hydrogen concentrations coincide with older, low-albedo regions near the equator, where water ice is unstable. The young, Rheasilvia basin contains the lowest concentrations. These observations are consistent with gradual accumulation of hydrogen by infall of carbonaceous chondrites--observed as clasts in some howardites--and subsequent removal or burial of this material by large impacts.
authors.affiliation: Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719-2395, USA. prettyman@psi.edu
authors.forename: Thomas H
authors.initials: TH
authors.lastname: Prettyman
authors.forename: David W
authors.initials: DW
authors.lastname: Mittlefehldt
authors.forename: Naoyuki
authors.initials: N
authors.lastname: Yamashita
authors.forename: David J
authors.initials: DJ
authors.lastname: Lawrence
authors.forename: Andrew W
authors.initials: AW
authors.lastname: Beck
authors.forename: William C
authors.initials: WC
authors.lastname: Feldman
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: McCoy
authors.forename: Harry Y
authors.initials: HY
authors.lastname: McSween
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Toplis
authors.forename: Timothy N
authors.initials: TN
authors.lastname: Titus
authors.forename: Pasquale
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authors.lastname: Tricarico
authors.forename: Robert C
authors.initials: RC
authors.lastname: Reedy
authors.forename: John S
authors.initials: JS
authors.lastname: Hendricks
authors.forename: Olivier
authors.initials: O
authors.lastname: Forni
authors.forename: Lucille
authors.initials: L
authors.lastname: Le Corre
authors.forename: Jian-Yang
authors.initials: JY
authors.lastname: Li
authors.forename: Hugau
authors.initials: H
authors.lastname: Mizzon
authors.forename: Vishnu
authors.initials: V
authors.lastname: Reddy
authors.forename: Carol A
authors.initials: CA
authors.lastname: Raymond
authors.forename: Christopher T
authors.initials: CT
authors.lastname: Russell