title: Mars' atmospheric history derived from upper-atmosphere measurements of38Ar/36Ar.
abstract: The history of Mars' atmosphere is important for understanding the geological evolution and potential habitability of the planet. We determine the amount of gas lost to space through time using measurements of the upper-atmospheric structure made by the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. We derive the structure of38Ar/36Ar between the homopause and exobase altitudes. Fractionation of argon occurs as a result of loss of gas to space by pickup-ion sputtering, which preferentially removes the lighter atom. The measurements require that 66% of the atmospheric argon has been lost to space. Thus, a large fraction of Mars' atmospheric gas has been lost to space, contributing to the transition in climate from an early, warm, wet environment to today's cold, dry atmosphere.
authors.affiliation: University of Colorado, Boulder, CO, USA. bruce.jakosky@lasp.colorado.edu.
authors.forename: B M
authors.identifier: 0000-0002-0758-9976
authors.initials: BM
authors.lastname: Jakosky
authors.affiliation: University of Colorado, Boulder, CO, USA.
authors.forename: M
authors.identifier: 0000-0002-9045-1168
authors.initials: M
authors.lastname: Slipski
authors.affiliation: NASA/Goddard Spaceflight Center, Greenbelt, MD, USA.
authors.forename: M
authors.identifier: 0000-0002-2770-4820
authors.initials: M
authors.lastname: Benna
authors.affiliation: NASA/Goddard Spaceflight Center, Greenbelt, MD, USA.
authors.forename: P
authors.initials: P
authors.lastname: Mahaffy
authors.affiliation: NASA/Goddard Spaceflight Center, Greenbelt, MD, USA.
authors.forename: M
authors.identifier: 0000-0003-1860-9220
authors.initials: M
authors.lastname: Elrod
authors.affiliation: University of Arizona, Tucson, AZ, USA.
authors.forename: R
authors.identifier: 0000-0002-2587-9539
authors.initials: R
authors.lastname: Yelle
authors.affiliation: University of Arizona, Tucson, AZ, USA.
authors.forename: S
authors.identifier: 0000-0002-7290-2412
authors.initials: S
authors.lastname: Stone
authors.affiliation: University of Colorado, Boulder, CO, USA.|American University of Sharjah, Sharjah, United Arab Emirates.
authors.forename: N
authors.identifier: 0000-0001-6622-4128
authors.initials: N
authors.lastname: Alsaeed