title: Radioactive elements on Mercury's surface from MESSENGER: implications for the planet's formation and evolution.
abstract: The MESSENGER Gamma-Ray Spectrometer measured the average surface abundances of the radioactive elements potassium (K, 1150 ± 220 parts per million), thorium (Th, 220 ± 60 parts per billion), and uranium (U, 90 ± 20 parts per billion) in Mercury's northern hemisphere. The abundance of the moderately volatile element K, relative to Th and U, is inconsistent with physical models for the formation of Mercury requiring extreme heating of the planet or its precursor materials, and supports formation from volatile-containing material comparable to chondritic meteorites. Abundances of K, Th, and U indicate that internal heat production has declined substantially since Mercury's formation, consistent with widespread volcanism shortly after the end of late heavy bombardment 3.8 billion years ago and limited, isolated volcanic activity since.
authors.affiliation: The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA. patrick.peplowski@jhuapl.edu
authors.forename: Patrick N
authors.initials: PN
authors.lastname: Peplowski
authors.forename: Larry G
authors.initials: LG
authors.lastname: Evans
authors.forename: Steven A
authors.initials: SA
authors.lastname: Hauck
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: McCoy
authors.forename: William V
authors.initials: WV
authors.lastname: Boynton
authors.forename: Jeffery J
authors.initials: JJ
authors.lastname: Gillis-Davis
authors.forename: Denton S
authors.initials: DS
authors.lastname: Ebel
authors.forename: John O
authors.initials: JO
authors.lastname: Goldsten
authors.forename: David K
authors.initials: DK
authors.lastname: Hamara
authors.forename: David J
authors.initials: DJ
authors.lastname: Lawrence
authors.forename: Ralph L
authors.initials: RL
authors.lastname: McNutt
authors.forename: Larry R
authors.initials: LR
authors.lastname: Nittler
authors.forename: Sean C
authors.initials: SC
authors.lastname: Solomon
authors.forename: Edgar A
authors.initials: EA
authors.lastname: Rhodes
authors.forename: Ann L
authors.initials: AL
authors.lastname: Sprague
authors.forename: Richard D
authors.initials: RD
authors.lastname: Starr
authors.forename: Karen R
authors.initials: KR
authors.lastname: Stockstill-Cahill