The Nitrate/Perchlorate Ratio on Mars as an - USRA

46th Lunar and Planetary Science Conference (2015)
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 [3].
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 [4] during heating of all samples,
and is consistent with 0.6 wt. % ClO4- detected by
Phoenix [5]. 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
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. [10]
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‰[11], lighter than present day Martian
atmospheric N2 at δ15N ≈ 572‰ [12], 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
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
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.
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1955-1973. [5] Hecht, M. H. et al. (2009) Science 325(5936)
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Figure 3. Mars in situ measurements reveal deviations from nitrate/perchlorate concentrations in terrestrial analogs due to large wt. % of perchlorate in Mars surface materials.