title: Noachian and more recent phyllosilicates in impact craters on Mars.
abstract: Hundreds of impact craters on Mars contain diverse phyllosilicates, interpreted as excavation products of preexisting subsurface deposits following impact and crater formation. This has been used to argue that the conditions conducive to phyllosilicate synthesis, which require the presence of abundant and long-lasting liquid water, were only met early in the history of the planet, during the Noachian period (> 3.6 Gy ago), and that aqueous environments were widespread then. Here we test this hypothesis by examining the excavation process of hydrated minerals by impact events on Mars and analyzing the stability of phyllosilicates against the impact-induced thermal shock. To do so, we first compare the infrared spectra of thermally altered phyllosilicates with those of hydrated minerals known to occur in craters on Mars and then analyze the postshock temperatures reached during impact crater excavation. Our results show that phyllosilicates can resist the postshock temperatures almost everywhere in the crater, except under particular conditions in a central area in and near the point of impact. We conclude that most phyllosilicates detected inside impact craters on Mars are consistent with excavated preexisting sediments, supporting the hypothesis of a primeval and long-lasting global aqueous environment. When our analyses are applied to specific impact craters on Mars, we are able to identify both pre- and postimpact phyllosilicates, therefore extending the time of local phyllosilicate synthesis to post-Noachian times.
authors.affiliation: Carl Sagan Center for the Study of Life in the Universe, 515 North Whisman Road, Mountain View, CA 94043, USA.alberto.g.fairen@nasa.gov
authors.forename: Alberto G
authors.initials: AG
authors.lastname: Fairén
authors.forename: Vincent
authors.initials: V
authors.lastname: Chevrier
authors.forename: Oleg
authors.initials: O
authors.lastname: Abramov
authors.forename: Giuseppe A
authors.initials: GA
authors.lastname: Marzo
authors.forename: Patricia
authors.initials: P
authors.lastname: Gavin
authors.forename: Alfonso F
authors.initials: AF
authors.lastname: Davila
authors.forename: Livio L
authors.initials: LL
authors.lastname: Tornabene
authors.forename: Janice L
authors.initials: JL
authors.lastname: Bishop
authors.forename: Ted L
authors.initials: TL
authors.lastname: Roush
authors.forename: Christoph
authors.initials: C
authors.lastname: Gross
authors.forename: Thomas
authors.initials: T
authors.lastname: Kneissl
authors.forename: Esther R
authors.initials: ER
authors.lastname: Uceda
authors.forename: James M
authors.initials: JM
authors.lastname: Dohm
authors.forename: Dirk
authors.initials: D
authors.lastname: Schulze-Makuch
authors.forename: J Alexis P
authors.initials: JA
authors.lastname: Rodríguez
authors.forename: Ricardo
authors.initials: R
authors.lastname: Amils
authors.forename: Christopher P
authors.initials: CP
authors.lastname: McKay
mesh_terms: D000538:Aluminum Silicates / Q000032:analysis / Q000737:chemistry; D017632:Asbestos, Serpentine / Q000032:analysis / Q000737:chemistry; D002712:Chlorides / Q000032:analysis / Q000737:chemistry; D005118:Extraterrestrial Environment / Q000737:chemistry*; D005290:Ferric Compounds / Q000032:analysis / Q000737:chemistry; D006358:Hot Temperature; D007616:Kaolin / Q000032:analysis / Q000737:chemistry; D018535:Mars; D008903:Minerals / Q000032:analysis / Q000737:chemistry; D017640:Silicates / Q000032:analysis* / Q000737:chemistry; D013057:Spectrum Analysis / Q000379:methods; D013997:Time Factors
chemical_list: D000538:Aluminum Silicates; D017632:Asbestos, Serpentine; D002712:Chlorides; D005290:Ferric Compounds; D008903:Minerals; D017640:Silicates; C510100:prehnite; D007616:Kaolin; C001599:chlorite