The Nitrate/Perchlorate Ratio on Mars As an Indicator for Habitability

Wednesday, 17 December 2014
Jennifer C Stern1, Brad Sutter2, Christopher P McKay3, Rafael Navarro-Gonzalez4, Caroline Freissinet5, Pamela Gales Conrad6, Paul R Mahaffy7, Paul Douglas Archer Jr2, Douglas W Ming8, Javier Martín-Torres9 and Maria-Paz Zorzano9, (1)NASA Goddard Space Flight Center, Planetary Environments Laboratory, Greenbelt, MD, United States, (2)Jacobs Technology, NASA Johnson Space Center, Houston, TX, United States, (3)NASA Ames Research Center, Moffett Field, CA, United States, (4)Universidad Nacional Autonoma de Mexico, Mexico City, Mexico, (5)NASA Goddard Space Flight Center, NASA Postdoctoral Program, Greenbelt, MD, United States, (6)NASA Goddard Space Flight Center, Greenbelt, MD, United States, (7)NASA Goddard SFC, Greenbelt, MD, United States, (8)NASA Johnson Space Center, Houston, TX, United States, (9)Centro de Astrobiologia, Madrid, Spain
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 nitrogen, 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. 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 deposition mechanisms [1,2]. Here, low NO3-/ClO4- molar ratios (~1000) 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 (~10,000). The NO3-/ClO4- molar ratio is estimated to be ~ 0.05 on Mars based on SAM measurements at Gale Crater [3]. The possibility exists that perchlorate brines could leach and increase nitrate concentrations at depth, increasing the martian NO3-/ClO4- ratio in the subsurface. However, it is unknown whether terrestrial NO3-/ClO4- molar ratios could be achieved by this mechanism. Nevertheless, the low NO3-/ClO4- the ratio detected by SAM 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.

[1] Kounaves, S.P. et al. "Discovery of natural perchlorate in the Antarctic dry valleys and its global implications." ES&T44, no. 7 (2010): 2360-2364.

[2] Lybrand, R.A., et al. "The geochemical associations of nitrate and naturally formed perchlorate in the Mojave Desert, California, USA." GCA104 (2013): 136-147.

[3] Ming, D. W. et al. "Volatile and organic compositions of sedimentary rocks in Yellowknife Bay, Gale crater, Mars." Science 343, no. 6169 (2014): 1245267.