Evidence for perchlorates and the origin of chlorinated hydrocarbons detected by SAM at the Rocknest aeolian deposit in Gale Crater

Abstract

[1] A single scoop of the Rocknest aeolian deposit was sieved (< 150 μm), and four separate sample portions, each with a mass of~50 mg, were delivered to individual cups inside the Sample Analysis at Mars (SAM) instrument by the Mars Science Laboratory rover's sample acquisition system. The samples were analyzed separately by the SAM pyrolysis evolved gas and gas chromatograph mass spectrometer analysis modes. Several chlorinated hydrocarbons including chloromethane, dichloromethane, trichloromethane, a chloromethylpropene, and chlorobenzene were identified by SAM above background levels with abundances of~0.01 to 2.3 nmol. The evolution of the chloromethanes observed during pyrolysis is coincident with the increase in O 2 released from the Rocknest sample and the decomposition of a product of N-methyl-N-(tert-butyldimethylsilyl)-trifluoroacetamide (MTBSTFA), a chemical whose vapors were released from a derivatization cup inside SAM. The best candidate for the oxychlorine compounds in Rocknest is a hydrated calcium perchlorate (Ca(ClO 4 ) 2 ·nH 2 O), based on the temperature release of O 2 that correlates with the release of the chlorinated hydrocarbons measured by SAM, although other chlorine-bearing phases are being considered. Laboratory analog experiments suggest that the reaction of Martian chlorine from perchlorate decomposition with terrestrial organic carbon from MTBSTFA during pyrolysis can explain the presence of three chloromethanes and a chloromethylpropene detected by SAM. Chlorobenzene may be attributed to reactions of Martian chlorine released during pyrolysis with terrestrial benzene or toluene derived from 2,6-diphenylphenylene oxide (Tenax) on the SAM hydrocarbon trap. At this time we do not have definitive evidence to support a nonterrestrial carbon source for these chlorinated hydrocarbons, nor do we exclude the possibility that future SAM analyses will reveal the presence of organic compounds native to the Martian regolith.Citation: Glavin, D. P., et al. (2013), Evidence for perchlorates and the origin of chlorinated hydrocarbons detected by SAM at the Rocknest aeolian deposit in Gale Crater,

The Modern Near-Surface Martian Climate: A Review of In-situ Meteorological Data from Viking to Curiosity

“…However, the presence of water ice in the shallow subsurface midlatitude and polar regions Mitrofanov et al 2002;Byrne et al 2009), along with the detection of perchlorates in polar and equatorial soil Glavin et al 2013;Ming et al 2014) and of chloride-bearing deposits in the southern highlands at low and midlatitudes (Osterloo et al 2008), is important because they can melt this ice at Mars' present-day environmental conditions and produce liquid saline water (brine) (Clark 1978;Brass 1980;Clark and Van Hart 1981;Haberle et al 2001;Chevrier and Altheide 2008;Rennó et al 2009;McEwen et al 2011;Ojha et al 2015).…”

“…Perchlorate salts, in particular Ca(ClO 4 ) 2 , Mg(ClO 4 ) 2 and Na(ClO 4 ) salts, are extremely relevant for brine formation via deliquescence and melting because they have low eutectic temperatures and were found in polar and equatorial regions Glavin et al 2013), suggesting that they are distributed globally.…”

Large sulfur isotope fractionations in Martian sediments at Gale crater

“…This is particularly interesting in light of the abundance of oxychlorine compounds at the Martian surface 89 , as well as the detection of nitrate in samples of Gale crater 90 . Reduction of perchlorate by terrestrial microbes produces chloride that is isotopically light (by ~15‰) compared to the original perchlorate 91 .…”

“…Reactions of the derivatization reagent N-methyl-N-(tert-butyldimethylsilyl)-trifluoroacetamide (MTBSTFA) 89 with Martian samples produce three compounds observed in SAM data with ionization fragments at m/z 50 (Fig. S5).…”

“…The oxychlorine compounds, interpreted as predominantly perchlorate 89 37 . To test effects of perchlorate, small aliquots of the WRL1 sample were mixed with the JSC Mars-1 simulant described above, then spiked with 1 wt% of either Ca-perchlorate or Mg-perchlorate.…”