46th Lunar and Planetary Science Conference (2015) 2590.pdf THE NITRATE/PERCHLORATE RATIO ON MARS AS AN INDICATOR FOR HABITABILITY J. C. Stern1, B. Sutter2, C. P. McKay3, R. Navarro-González4, C. Freissinet5, P. G. Conrad1, P. R. Mahaffy1, P. D. Archer, Jr.2, D. W. Ming6, P. B. Niles6, M.-P. Zorzano7, F. J. Martin-Torres8 and the MSL Science Team. 1NASA Goddard Space Flight Center, Greenbelt, MD, 20771, [email protected], 2Jacobs, NASA Johnson Space Center, Houston, TX 77058, 4NASA Ames Research Center, Moffett Field, CA 94035, 4Universidad Nacional Autónoma de México, México, D.F. 04510, Mexico, 5NASA Postdoctoral Program, NASA Goddard Space Flight Center, Greenbelt, MD 20771, 6NASA Johnson Space Center, Houston TX 77058, 7Centro de Astrobiología, Torrejón de Ardoz, Madrid, Spain, 8Instituto Andaluz de Ciencias de la Tierra, Granada, Spain & Division of Space Technology, Luleå University of Technology, Kiruna, Sweden Introduction: Discovery of indigenous martian nitrogen in Mars surface materials has important implications for habitability and the potential development of a nitrogen cycle at some point in martian history. The Sample Analysis at Mars (SAM) instrument suite on the Mars Science Laboratory (MSL) Curiosity Rover detected evolved nitric oxide (NO) gas during pyrolysis of scooped aeolian sediments and drilled mudstone acquired in Gale Crater. The detection of NO suggests an indigenous source of fixed N, and may indicate a mineralogical sink for atmospheric N2 in the form of nitrate. The ratio of nitrate to oxychlorine species (e.g. perchlorate) may provide insight into the extent of development of a nitrogen cycle on Mars. Background: Nitrate and perchlorate on Earth are geochemically related in arid environments such as the Atacama Desert and the Dry Valleys of Antarctica due to their similar mobilities and the fact that both accumulate via dry deposition mechanisms [1,2]. Here, low NO3-/ClO4- molar ratios (~1000 in the Atacama, ~10,000 in the Dry Valleys) dominate, in comparison to other places on Earth, where the main nitrate source is biological fixation of N2 to NO3-, and there is no corresponding biological source of perchlorate, resulting in much higher NO3-/ClO4- molar ratios (~50,000). Biological input of NO3- also results in a less significant correlation between the two species, such as in the Mojave Desert . In situ Mars measurements: During SAM’s nominal solid sample analysis mode, evolved gas analysis (EGA) is performed, in which the quadrupole mass spectrometer (QMS) directly analyzes the gases released from samples heated to ~870 ºC at 35ºC/min under a ~0.8 sccm He flow and a pressure of ~25 mbar in the pyrolysis oven. NO3- abundances are calculated from m/z 30 (NO) abundances. ClO4- was not directly detected but the presence of chlorinated oxidants is inferred from the evolution O2, HCl, and chlorinated organic compounds  during heating of all samples, and is consistent with 0.6 wt. % ClO4- detected by Phoenix . ClO4- equivalents are calculated from O2 abundance [e.g. 6,7]. Results: NO3- and ClO4- are strongly correlated (Figure 1) in results from solid sample EGA performed by SAM at four sample sites in Gale Crater (Rocknest aeolian deposits, John Klein, Cumberland, and Wind- Figure 1. Linear regression of SAM measurements yield an r2 of 0.854, indicating strong correlation between NO3- and ClO4-. jana drill fines), similar to the Atacama and the Dry Valleys of Antarctica, hyperarid environments dominated by dry deposition of photochemically produced nitrates. It is unclear as to whether the correlation of NO3- and ClO4- in Gale Crater sediments is due to formation processes, or reflects similarities in mobility and reconcentration of both species after their initial deposition. SAM measurements give NO3-/ClO4- molar ratios of ~ 0.1 for Gale Crater sites (Figure 2). These very low ratios are vastly different than even the Ata- Figure 2. NO3-/ClO4- molar ratios of ~ 0.1 for Gale Crater sites are low in comparison to terrestrial Mars analogs. 46th Lunar and Planetary Science Conference (2015) cama due to not only lower NO3- abundances on Mars than on Earth, but primarily due to the presence of several weight percent of perchlorate as inferred from SAM data (Figure 3). The low NO3-/ClO4- ratio at all sites in Gale Crater suggests that N fixation to nitrate on Mars, whether biologically mediated or abiotic, was extremely limited compared to the potentially ongoing abiotic formation and deposition of oxychlorine species on the martian surface [e.g., 8]. It is possible that the nitrate seen in both aeolian deposits at Rocknest and in drilled sediments at John Klein, Cumberland, and Windjana represents N2 fixation from impacts long ago [e.g., 9,10] and that NO3- is no longer being produced in the atmosphere. Both the NO3- abundances and the NO3-/ClO4- ratios <1 measured by SAM in Gale Crater sediments are consistent with predictions for impact generated nitrates and the notion that nitrates have not been recycled back into atmospheric N2. Manning et al.  calculated an impact generated nitrate reservoir of ~5 x 1015 moles on Mars that is unlikely to have experienced significant decomposition. This translates into ~0.11 wt. % NO3, using a 950 m impact veneer depth [e.g., 10] and a soil density of 2 g/cm3, and is consistent with the upper limit of our detection of ~0.11 wt % NO3 in Cumberland drill fines. Little data for nitrate and perchlorate in Mars meteorites has been published, but recent δ15N for Elephant Moraine 79001 (EETA 79001) NO3- was reported as δ15N ≈ -10.5‰, lighter than present day Martian atmospheric N2 at δ15N ≈ 572‰ , suggesting nitrate in the meteorite is older than the meteorite itself (~173 Mya) and formed prior to atmospheric loss. The presence of reduced nitrogen species, which have not been definitively detected, would lend evidence to development of more complex N chemistry 2590.pdf than is evident from the presence of nitrates alone. However, reduced N species such as HCN and NH3 are highly volatile and can be difficult to detect by mass spectrometry due to mass interferences with other species. Detection of reduced N could provide evidence for development of either an abiotic or a biological mechanism to both fix N and recycle N back into the atmosphere. Summary: Nitrates detected on Mars are proposed to represent the establishment of the first half of an abiotic N cycle to fix atmospheric N to a biologically available substrate [9,10]. Currently there is no data to support any abiotic or biological means of recycling N2 back into the atmosphere. Instead, NO3-/ClO4- molar ratios <1 suggest that nitrate formation may have been limited to an earlier period in Mars history, prior to atmospheric loss, while ClO4- continues to be produced by current atmospheric processes [eg., 8]. This is in contrast to terrestrial Mars analogs, where accumulation of NO3- even in areas thought to be mostly devoid of biology, such as the Atacama Desert, outstrips production and accumulation of ClO4-, resulting in NO3/ClO4- ratios >1000. References:  Kounaves, S.P. et al. 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