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Mars methane detection and variability at Gale crater
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How to cite:
Webster, Christopher R.; Mahaffy, Paul R.; Atreya, Sushil K.; Flesh, Gregory J.; Mishna, Michael A.; Meslin,
Pierre-Yves; Farley, Kenneth A.; Conrad, Pamela G.; Christensen, Lance E.; Pavlov, Alexander A.; Martin-Torres,
Javier; Zorzano, María-Paz; McConnochie, Timothy H.; Owen, Tobias; Eigenbrode, Jennifer L.; Glavin, Daniel P.;
Steele, Andrew; Malespin, Charles A.; Archer Jr., P. Douglas; Sutter, Brad; Coll, Patrice; Freissnet, Caroline; McKay,
Christopher P.; Mores, John L.; Schwenzer, Susanne P.; Bridges, John C.; Navarro-Gonzalez, Rafael; Gellert, Ralf and
Lemmon, Mark T. (2015). Mars methane detection and variability at Gale crater. Science, 347(6220) pp. 415–417.
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Revised Manuscript 1261713 November 17
th
2014
1
Mars Methane Detection and Variability at Gale Crater
Christopher R. Webster,
1*
Paul R. Mahaffy,
2
Sushil K. Atreya,
3
Gregory J. Flesch,
1
Michael A. Mischna
1
,
Pierre-Yves Meslin
4
, Kenneth A. Farley
5
, Pamela G. Conrad
2
, Lance E. Christensen
1
, Alexander A. Pavlov
2
,
Javier Martín-Torres
6
, María-Paz Zorzano
7
, Timothy H. McConnochie
8
, Tobias Owen
9
, Jennifer L.
Eigenbrode
2
, Daniel P. Glavin
2
, Andrew Steele
10
, Charles A. Malespin
2
, P. Douglas Archer, Jr.
11
, Brad
Sutter
11
, Patrice Coll
12
, Caroline Freissinet
2
, Christopher P. McKay
13
, John E. Moores
14
, Susanne P.
Schwenzer
15
, John C. Bridges
16
, Rafael Navarro-Gonzalez
17
, Ralf Gellert
18
, Mark T. Lemmon
19
and the MSL
Science Team
†
.
Affiliations:
1
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109.
2
NASA Goddard Space Flight Center (GSFC), Greenbelt, MD 20771.
3
University of Michigan, Ann Arbor, MI 48109.
4
Institut de Recherche en Astrophysique et Planétologie, UPS-OMP, CNRS, 31028 Toulouse, France.
5
California Institute of Technology, Pasadena, CA 91125.
6
Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR), Granada, Spain; and Division of Space Technology, Luleå
University of Technology, Kiruna, Sweden.
7
Centro de Astrobiologia, INTA-CSIC, Madrid, Spain.
8
Department of Astronomy, University of Maryland, College Park, MD 20742
9
University of Hawaii, Honolulu, HI 96822.
10
Carnegie Institution of Washington, Washington DC 20015.
11
Jacobs, NASA Johnson Space Center, Houston, TX 77058.
12
Laboratoire Inter-Universitaires Des Systèmes Atmosphériques (LISA), UMR CNRS 7583, Paris, France
13
NASA Ames Research Center (ARC), Mountain View, CA 94035.
14
York University, Toronto, ON M3J 1P3, Canada.
15
The Open University, Milton Keynes MK7 6AA, England (UK).
16
Space Research Centre, University of Leicester LE1 7RH, England (UK).
17
Instituto de Ciencias Nucleares, Universidad Nacional Autonoma de Mexico, Mexico 04510.
18
University of Guelph, Ontario N1G 2W1, Canada.
19
Texas A&M University, College Station, TX 77843, USA.
*To whom correspondence should be addressed. E-mail: Chris.R.Webster@jpl.nasa.gov
†MSL Science Team authors and affiliations are listed in the supplementary materials.
Abstract: Reports of plumes or patches of methane in the Martian atmosphere that vary over monthly timescales
have defied explanation to date. From in situ measurements made over a 20-month period by the Tunable Laser
Spectrometer (TLS) of the Sample Analysis at Mars (SAM) instrument suite on Curiosity at Gale Crater, we report
detection of background levels of atmospheric methane of mean value 0.69 ±0.25 ppbv at the 95% confidence
interval (CI). This abundance is lower than model estimates of ultraviolet (UV) degradation of accreted
interplanetary dust particles (IDP’s) or carbonaceous chondrite material. Additionally, in four sequential
measurements spanning a 60-sol period, we observed elevated levels of methane of 7.2 ±2.1 (95% CI) ppbv
implying that Mars is episodically producing methane from an additional unknown source.
2
One Sentence Summary: Mars methane has been detected at background levels of ~0.7 ppbv and at a transient
elevated abundance of ~10 times this value over a 60-sol period.
Main Text:
Because Earth’s atmospheric methane is predominantly biologically-produced (1), determining the abundance and
variability of methane in the current Martian atmosphere is critical to assessing the contribution from a variety of
potential sources or reservoirs that may be biological (such as methanogens (2,1)) or abiotic (1). These latter
processes include: geological production such as serpentinization of olivine (3), UV degradation of meteoritically-
delivered organics (4,5,6), production by impacts of comets (7), release from subsurface clathrates (8) or regolith-
adsorbed gas (9,10), erosion of basalt with methane inclusions (11), or geothermal production (12). Several
detections of Mars methane have been published. Ground-based observations from the Canada-France-Hawaii
Telescope (CFHT) in 1999 found a global average value of 10 ±3 ppbv (2), and those using the NASA IRTF
telescope in 2003 reported (13) methane release in plumes from discrete sources in Terra Sabae, Nili Fossae, and
Syrtis Major that showed seasonal changes with a summer time maximum of ~45 ppbv near the equator. This work
also reported simultaneous detections of methane and carbon dioxide in 2005, and an upper limit of 3 ppbv in 2006,
from which its’ rapid destruction since 2003 was inferred (13). The Planetary Fourier Spectrometer (PFS) on the
Mars Express (MEX) spacecraft reported detection in 2004 (14) with an updated global average abundance of 15 ±5
ppbv (15), with indications of discrete localized sources and a summer time maximum of 45 ppbv in the north polar
region. From the Thermal Emission Spectrometer (TES) of Mars Global Surveyor (MGS), methane abundances
from 5 to 60 ppbv were deduced (16) as intermittently present over locations where favorable geological conditions
such as residual geothermal activity (Tharsis and Elysium) and strong hydration (Arabia Terrae) might be expected.
Using data from NASA-IRTF acquired in February 2006, (17) reported a detection of 10 ppbv at mid-latitudes (42-7
o
N) over Valles Marineris but an upper limit of 3 ppbv outside that region; in December 2009 they obtained an
upper limit of 7-8 ppbv and noted that data from both observations agreed with those of (13). More recent ground-
based observations report methane mixing ratios that have diminished considerably since 2004-2006 to a two-sigma
upper limit of 5 ppbv (17,18,19), suggesting a very short lifetime for atmospheric CH
4
and contradicting the MEX
claim that methane persisted from 2004-2010. At Curiosity’s Gale Crater landing site (4.5°S, 137°E), published
maps of PFS data (15) show an increase from ~15 ppbv in fall to ~30 ppbv in winter, whereas the TES trend (16) is
opposite: ~30 ppbv in fall and ~5 ppbv in winter.
Observational evidence for methane on Mars has been questioned in the published literature (20,21,22) because
photochemical models are unable to reconcile the observed amounts with their reported spatial gradients and
temporal changes over months compared with the expected ~300 year methane lifetime. Contradictions were noted
between the locations of maxima reported from ground-based observations and maps inferred by PFS and TES from
Mars orbit. The plume results (13) were questioned (22) on the basis of a possible misinterpretation from methane
lines whose positions coincided with those of terrestrial isotopic
13
CH
4
lines. Krasnopolsky (7) argued that cometary
and volcanic contributions were not sufficient to explain high methane abundances, noting for the latter possibility
the lack of current volcanism or hot spots in thermal imaging (23), and the extremely low upper limit for Mars SO
2
that in Earth’s volcanic emissions is orders of magnitude more abundant than CH
4
, as predicted for Mars (24).
Model calculations including expected atmospheric transport and circulation (20,25) are to date all unable to
reproduce the spatial and temporal characteristics of the observed high concentration methane plumes, whether
resulting from possible clathrate release (8), surface adsorption by or desorption from the regolith (9) or for
ultraviolet (UV) degradation of surface organics (4,5,6), despite the introduction of a variety of putative loss
mechanisms (26,27,28).
The Tunable Laser Spectrometer (TLS) of the Sample Analysis at Mars (SAM) (29) instrument suite on Curiosity
rover has a spectral resolution (0.0002 cm
-1
) that offers unambiguous identification of methane in a unique
fingerprint spectral pattern of 3 well-resolved adjacent
12
CH
4
lines in the 3.3-µm band (30). The in situ technique of
tunable laser absorption in a closed sample cell is simple, non-invasive and sensitive. TLS is a two-channel tunable
laser spectrometer that uses both direct and second harmonic detection of IR laser light. One channel uses a near-IR
tunable diode laser at 2.78 µm that has yielded robust data on carbon, oxygen and hydrogen isotopic ratios on Mars
(31). The second channel uses an interband cascade (IC) tunable laser at 3.27 µm for methane detection alone,
scanning across seven rotational lines that includes the R(3) triplet used in this study. This laser makes 81 passes of
a 20-cm long sample cell of the Herriott design fitted with high-vacuum microvalves that allow evacuation with a
3
turbomolecular pump for “empty cell” scans, or filled to Mars ambient pressure (~7 mbar) for “full cell” runs. Our
methane determination is made by differencing the measured methane abundances in our sample cell when filled
with Mars atmosphere from measurements of the same cell evacuated, as detailed in the Supplementary Material
(SM) (32).
From our first six observations spanning a 234-sol period (1 sol= 1 Mars day = 24 hrs 37.3 mins), we previously
reported (33) a mean value of 0.18 ± 0.67 ppbv that was not precise enough to claim detection of Mars methane, but
instead set an upper limit of 1.3 ppbv (95% CI) that was significantly lower than those reported (5 ppbv, 95% CI)
from recent ground-based observations (17,18,19).
We have now reprocessed our entire data set (with a small modification explained in (32)). Our data set now extends
the measurement period over 605 sols, including 11 direct ingest measurements and two recent measurements using
a “methane enrichment” experiment run on sols 573 and 684. In this latter procedure, the atmospheric methane is
effectively enriched by 23 ±1 times by flowing the ingested gas slowly over a carbon dioxide scrubber material.
Prior to running on Mars, the instrument script was optimized using the test-bed SAM suite (32). Results from the
complete data set are given in Table 1 and plotted in Fig. 1. We partition our data points of Fig. 1 into 3 groups for
independent analysis: (i) the “low methane” direct ingest results of sols 79, 81, 106, 292, 313 and 684; (ii) the “low
methane” enrichment results for sols of 573 and 684; and (iii) and the four sequential “high methane” runs of sols
466, 474, 504, and 526, as there is no statistically-significant variation within each grouping. Mean values for these
grouped data sets (final three lines, Table 1) form the basis for our analysis and conclusions. We note the good
agreement between the direct and enriched experiments that were run back to back on sol 684. The daytime run of
sol 306 is not included in group (iii) because it was not part of the high methane sequence, nor is it included in the
low methane group (i) since it is clearly higher than the background average.
Our “low methane” enrichment experiments produce a mean value for atmospheric methane of 0.69 ± 0.25 (95% CI)
ppbv, as described in (32). The direct-ingest (non-enrichment) group yields a mean methane value of 0.89 ± 1.96
(95% CI) ppbv, agreeing with the higher precision enrichment value within error. For the “high methane” abundance
seen in direct-ingest we measure a mean value of 7.19 ± 2.06 (95% CI) ppbv for the four sols 466, 474, 504, and
526. In the SM (32), we provide arguments to rule out the possibility of terrestrial contamination, and therefore
conclude that the enrichment result and the “high methane” result independently produce detection of methane at
two levels of abundance. Although TLS samples only the very lowest part (~1 m) of the Mars atmosphere in the
Gale Crater region, the atmospheric mixing time of a few months suggest that our measured value of 0.69 ± 0.25
(95% CI) ppbv is likely representative of the mean background level for Mars atmospheric methane abundance,
which is only expected to vary significantly and seasonally over the winter poles (20).
The principal sources of organics delivered exogenously to Mars are isotropically-accreted interplanetary dust
particles (IDP’s) and low-mass carbonaceous chondrites containing up to 10% organics by weight (1,4,5,6). Recent
observations by SAM on Curiosity have detected the presence of chlorobenzene and simple chlorinated alkanes (34)
in a drilled martian mudstone in Gale Crater. Laboratory studies of meteoritic materials have shown that UV
irradiation of organic molecules can produce methane either directly (5) or through secondary photochemical
reaction (35), and that certain molecules can form a photoresistant layer leading to methane over extended time
periods (6). Constrained by laboratory production rates, models have assessed the rate and size of infall of meteoritic
material such as IDP’s, carbonaceous chondrites and other sources of organic carbon to the martian surface that
might reproduce methane observations under Mars-like UV conditions. The UV/CH
4
model of isotropically accreted
IDP organics of Schuerger et al. (4) predicts that the UV-induced production of methane is carbon-limited, and over
geological time can produce a globally-averaged methane abundance of 2.2 ppbv methane for a 20% conversion rate
of organic carbon to methane. No significant diurnal or seasonal changes are predicted by this model, which cannot
explain the variability of methane over relatively short timescales observed in earlier studies (12,14). Even with
consideration of single large bolide impacts or multiple airburst events, the models struggle to emplace sufficient
carbon over the large surface areas of the plume observations, and more importantly cannot supply methane fast
enough to create plumes over the observed timeframe (5).
Our background methane abundance reported here of ~0.7 ppbv from the low methane enrichment is significantly
lower than the 2.2 ppbv obtained from the Schuerger UV/CH
4
model estimate (5) described above, despite the fact
4
that measured surface UV levels from Curiosity’s REMS instrument (32,36) agree with the model values (5). This
implies that either the quantity of delivered carbon or its conversion efficiency to methane is one-third the model
estimates or that an indigenous source may be having an effect. It is also likely that the fresh analog material used by
Schuerger et al. (5) is not completely representative of the bulk of material (UV-processed IDP’s) being delivered to
Mars.
As detailed in (32), our high methane result of 7.19 ± 2.06 ppbv (95% CI) shows no significant quantitative
correlation with relative humidity, atmospheric pressure (carbon dioxide abundance), ground or air temperature,
inlet pointing, or radiation levels measured by other Curiosity instruments: the Rover Environmental Monitoring
Station (REMS (36), the Chemistry and Camera complex (ChemCam (37)), and the Radiation Assessment Detector
(RAD (38)). The REMS observations suggest a plausible anti-correlation with water abundance, air and ground
temperatures and both REMS and the Curiosity mast camera (MastCam (39)) show a possible anticorrelation with
atmospheric opacity (32), but our first enrichment measurement on sol 573 spoils this. However, all methane
measurements (including sol 573) support an anti-correlation of methane abundance with column measurements of
oxygen abundance and water vapor as measured by the ChemCam instrument (see Fig. S9), the latter contrasting
with the weak positive correlation observed by the Mars Express PFS (8, 40). However, the lack of O
2
and H
2
O data
for the range Ls = 160-220 spoils this comparison, and we must await future measurements to assess this fully.
Concerning the possibility of spatially-variable methane abundance, although the high methane measurements were
observed within 200-300 m of each other (Fig. S10), the rover had not traveled far (~ 1 km) since the lower value of
sol 466, and the high methane disappeared after traveling only a further ~1km away. Typical ground winds of ~7
m/sec (25) would cover that distance in only 2 minutes, and given rotation of diurnal wind, it’s impossible to isolate
one location from another. This suggests a short-duration event that is either local and weak, or more distant and
stronger. The persistence of the high methane values over 60 sols and their sudden drop 47 sols later is not
consistent with a well-mixed event, but rather with a local production or venting that, once terminated, disperses
quickly. Most of our data is taken at night, when prevailing winds are likely from the south. The marginally higher
daytime values suggested in sols 306 and 526 indicate a source to the rover’s north, because prevailing daytime
winds would advect toward the rover location. The change in rover location is therefore unimportant as this is a
temporal, not locational variation. With a concern that Curiosity transit over varying surface materials (identified by
the Alpha Particle X-ray Spectrometer (APXS (41)) measurements) could be associated with the high methane
observations, we studied rover stand-time and local terrain composition (32) and rule out such potential
contributions. While we cannot rule out possible clathrate release (8) or surface adsorption into the regolith with
subsequent release (9), both these mechanisms do not support the local, short timescale variation we observe.
Our measurements of a background methane abundance of ~0.7 ppbv can be reconciled with photochemical models
that include an exogenous source such as UV degradation of organics (5) because model results likely represent
upper limits with extensive UV processing in space prior to delivery to Mars. Like the earlier plume measurements,
our higher transient methane amounts of ~7 ppbv require an additional source of methane, in our case suggesting
advection to the rover location from a local unidentified source. If that source were a recent bolide impacting Gale
Crater and producing 1% methane, we estimate that it would have to be several meters in size and leave a crater of
tens of meters in diameter, but no new impact craters have been observed within Gale Crater from Mars orbit time-
series imaging (42) since landing. Our measurements spanning a full Mars year indicate that trace quantities of
methane are being generated on Mars by more than one mechanism or a combination of proposed mechanisms --
including methanogenesis either today or released from past reservoirs, or both.
References and Notes
1. S. K. Atreya, P. R. Mahaffy, and A. S. Wong, Methane and related trace species on Mars: Origin, loss,
implications for life, and habitability. Planet. Space Sci. 55, 358-369 (2007).
2. V. A. Krasnopolsky, J.P. Maillard, and T.C. Owen, Detection of methane in the martian atmosphere:
evidence for life? Icarus 172, 537-547 (2004).
3. C. Oze and M. Sharma, Have olivine, will gas: Serpentinization and the abiogenetic production of methane
on Mars, Geophys. Res. Lett. 32: doi10.1029/2005GL022691 (2005).
