Detection of Methane in the Martian Atmosphere: Evidence for Life?

Icarus 172 (2004) 537–547 www.elsevier.com/locate/icarus

Detection of methane in the martian atmosphere: evidence for life?

a, b c Vladimir A. Krasnopolsky ∗,JeanPierreMaillard ,TobiasC.Owen

a Department of Physics, Catholic University of America, Washington, DC 20064, USA b Institute d’Astrophysique de Paris, CNRS, 75014 Paris, France c Institute for Astronomy, University of Hawaii, Honolulu, HI 96822, USA Received 29 March 2004; revised 1 July 2004 Available online 20 August 2004

Abstract Using the Fourier Transform Spectrometer at the Canada–France–Hawaii Telescope, we observed a spectrum of Mars at the P-branch of the strongest CH4 band at 3.3 µm with resolving power of 180,000 for the apodized spectrum. Summing up the spectral intervals at the expected positions of the 15 strongest Doppler-shifted martian lines, we detected the absorption by martian methane at a 3.7 sigma level which is exactly centered in the summed spectrum. The observed CH mixing ratio is 10 3ppb.TotalphotochemicallossofCH in the 4 ± 4 martian atmosphere is equal to 2.2 105 cm 2 s 1,theCH lifetime is 340 years and methane should be uniformly mixed in the atmosphere. × − − 4 Heterogeneous loss of atmospheric methane is probably negligible, while the sink of CH4 during its diffusion through the regolith may be signiﬁcant. There are no processes of CH4 formation in the atmosphere, so the photochemical loss must therefore be balanced by abiogenic and biogenic sources. Outgassing from Mars is weak, the latest volcanism is at least 10 million years old, and thermal emission imaging from the Mars Odyssey orbiter does not reveal any hot spots on Mars. Hydrothermal systems can hardly be warmer than the room temperature at which production of methane is very low in terrestrial waters. Therefore a signiﬁcant production of hydrothermal and magmatic methane is not very likely on Mars. The calculated average production of CH4 by cometary impacts is 2% of the methane loss. Production of methane by meteorites and interplanetary dust does not exceed 4% of the methane loss. Methane cannot originate from an extinct biosphere, as in the case of “natural gas” on Earth, given the exceedingly low limits on organic matter set by the Viking landers and the dry recent history which has been extremely hostile to the macroscopic life needed to generate thegas.Therefore,methanogenesisbylivingsubterraneanorganisms is a plausible explanation for this discovery. Our estimates of the biomass and its production using the measured CH4 abundance show that the martian biota may be extremely scarce and Mars may be generally sterile except for some oases.  2004 Elsevier Inc. All rights reserved.

Keywords: Mars, atmosphere; Exobiology; Atmospheres, composition; Spectroscopy; Infrared observations

1. Introduction recognized subsequently that the observed seasonal variations are caused by windblown dust. However, even after the Half a century ago, seasonal variations of the reﬂectivity careful and detailed but unsuccessful search for life by the of Mars at low and middle latitudes were ascribed to veg- Viking instruments, Mars still represents our greatest if not etation, and attempts were made to detect the chlorophyll only hope to achieve a nonhypothetical subject for astro- or bands and other “bands of life.” G.A. Tikhov, a Russian sci- exobiology. entist, suggested the term astrobotany in 1945 for that new It is currently thought that methanogenesis is a highly ﬁeld and published a book with this title in 1949 and fol- likely metabolic pathway for possible microbial life on Mars lowed it in 1953, with a book entitled “Astrobiology.” It was (McCollom, 1999; Weiss et al., 2000; Jakosky et al., 2003; Varnes et al., 2003).Theprobableexistenceofsubsurface liquid water on Mars (Mellon and Phillips, 2001) would * Corresponding author: 6100 Westchester Park #911, College Park, MD 20740, USA. provide protected habitats for such organisms, perhaps sim- E-mail address: [email protected] (V.A. Krasnopolsky). ilar to the microbial communities thriving in many places

0019-1035/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2004.07.004 538 V.A. Krasnopolsky et al. / Icarus 172 (2004) 537–547 deep beneath the Earth’s surface (Chapelle et al., 2002).The both the abundance of telluric methane and its line widths, detection of atmospheric methane is a way to deduce the ex- which are proportional to atmospheric pressure. We found istence of this type of life. that the best instrument to search for CH4 on Mars was The strictest upper limit to CH4 in the martian at- the Fourier Transform Spectrometer (FTS) at the Canada– mosphere was obtained by summing up 1747 infrared spec- France–Hawaii Telescope (CFHT). The telescope site at tra from the Mariner 9 orbiter (Maguire, 1977).Noabsorp- Mauna Kea (elevation 4.2 km), size (3.6 m), and the FTS tion of Mars thermal radiation was observed at the CH4 band apodized resolving power ν/δν 180,000 were the main 1 = at 1306 cm− ,andthisresultedinanupperlimitof20parts factors in our choice. per billion (ppb). This limit corresponds to the absorption at The observations were made on 24 and 27 January 1999, aleveloftwicethenoiseequivalentradiance,thatis,0.1%, when Mars’ helio- and geocentric distances were 1.66 and measured with resolving power ν/δν 550 (ν is wavenum- 1.24 AU, diameter 7.6 arcs, solar longitude L 88 ,i.e., = S = ◦ ber). near the summer solstice, and telluric airmass 1.14–1.27. We An indication of a possible presence of methane in the used a diaphragm of 5 arcs and a filter with a band pass of martian atmosphere was obtained by Krasnopolsky et al. 1 2840–3020 cm− at half maximum. Our diaphragm reduced (1997).TosearchforHDO,theyobservedaspectrumof the signal from Mars but also removed a significant uncer- 1 Mars at 2650–2800 cm− with resolving power of 270,000 tainty which could arise from the inclusion of the Mars limb. using the Fourier transform spectrometer at the Kitt Peak This would require some assumptions on the limb bright- National Observatory. Methane was a by-product in that ening of the atmosphere and darkening of the surface and search, and a weak signal from possible methane at 70 ± corrections for light scattering on the limb. The observing 50 ppb was below the two-sigma limit. conditions were very good with relative humidity < 5% and AspectrumofMarsat2.4–45µmwasobservedwith atemperatureof 3to 6 C. However, a strong wind of − − ◦ the short-wavelength spectrometer at the Infrared Space Ob- up to 30 knots blurred the image and reduced the signal-to- servatory with resolving power of 2000 near 3.3 µm. No noise ratio. methane absorption was detected, and the upper limit to Difference between the geocentric velocities for the two methane was 50 ppb (Lellouch et al., 2000). nights was less than 0.03 km s 1,whichcorrespondsto We report here our observations which resulted in a de- − 3 10 4 cm 1,andtheobservedinterferogramsweresim- tection of methane on Mars. We will discuss sources and × − − ply coadded and apodized. The final interferogram was sinks of methane on Mars and its relevance to the problem processed to obtain an oversampled spectrum with sampling of life. An abstract of this work with its basic results was intervals of 0.00171 cm 1.Thetotaleffectiveexposuretime submitted to and published by the European Geosciences − was 4.75 h, and we achieved a signal-to-noise ratio of 110 at Union (Krasnopolsky et al., 2004),andthesubmissiondead- the peak of the filter transmission in the regions without at- line, 11 January 2004, was on the second day of operation of mospheric absorption. A small part ( 3%) of the observed the Planetary Fourier Spectrometer (PFS) on board the Mars ≈ Express orbiter (Formisano et al., 2004).Later,Kerr (2004) spectrum is shown in Fig. 1. reported that the PFS team also announced a detection of The observed spectrum consists mostly of telluric CH4 lines. The spectroscopic database HITRAN 2000 gives methane at a press conference in March 2004. Attempts to 1 detect methane on Mars were made even earlier by Mumma 24,000 lines of CH4 in the range of 2840–3020 cm− .For- et al. (2003).However,noresultsoftheirobservationshave tunately, most of these lines are very weak, and only 1500 been published in that abstract. of them result in telluric absorption exceeding 1%. For ex- 1 ample, a weak telluric line of methane at 2975.74 cm− is seen in Fig. 1.ThenumberofCH4 lines of similar or 2. Observation greater strength is 338 in our spectral range. Four components of the CH4 P4 line are also shown in Fig. 1.Eachof 1 We searched for CH4 on Mars using the P-branch of its these components is stronger than the line at 2975.74 cm− 1 strongest band at 3020 cm− .Thisbandisstrongerthanthe by a factor of 400. Evidently, a wing of only the bluest 1 ≈ band at 1306 cm− ,whichwasusedbyMaguire (1977) to of these four components may be studied to search for establish his upper limit, by a factor of 2.2. Reflection of methane on Mars. 29 strong but unsaturated lines of telluric solar light by Mars with the mean surface albedo of 0.1 methane were used to measure a wavenumber correction, 6 (Erard and Calvin, 1997) dominates the planet’s spectrum which was equal to a factor of 1 (4.75 0.12) 10− , 1 1 + ±1 × near 3020 cm and further doubles the effect of absorption that is, 0.01425 0.0004 cm− at 3000 cm− . − ± because of the two-way path in the atmosphere. However, Figure 1 demonstrates also a few strong lines of telluric our observations were ground-based and possible martian water vapor. The only martian lines immediately seen in the lines of CH4 must therefore be observed against the very spectrum are weak Doppler-shifted satellites to strong tel- strong telluric methane lines. This required the highest spec- luric H2Olines(Fig. 1). We will use these lines to determine tral resolution, maximum Doppler shift from the velocity of the effective spectral resolution and Doppler shift, which Mars relative to the Earth, and a high elevation to reduce may differ from that for Mars’ mass center because of the Detection of methane on Mars 539

Fig. 1. A small part (3%) of the CFHT/FTS spectrum of Mars. Telluric lines of CH4 and H2Oarethemajorfeaturesinthespectrum.MartianH2Olines 1 (crosses) are weak, narrow, and shifted to the blue by 0.1762 cm− . projection of Mars rotation which is variable within the field of view, and some fine instrumental effects. 1 Fitting of the telluric H2Olineat2973.25cm− by the collisional line shape ηNS#ν t(ν) A exp c = −π (ν ν )2 #ν2 ! [ − 0 + c ]" is shown in Fig. 2.Heret(ν) is the transmission at wavenumber ν, ν0 is the central wavenumber, η is the telluric airmass, N is the column abundance, S is the line strength, #νc is the collisional line width, and A is the scaling factor. The chosen line is the strongest H2Olineinourspectrum,anditsfitting results in #ν 0.058 cm 1 and N 3 1021 cm 2,that c = − = × − is, 0.9 mm of precipitated water. This value confirms the dry conditions on Mauna Kea during the observations. Using the Viking measurements of seasonal variations of 1 the atmospheric pressure (Hess et al., 1980; Tillman, 1988) Fig. 2. Fitting of the telluric H2Olineat2973.25cm− by the collisional line shape. This fitting determines the water abundance of 0.9 pr. mm above and the MGS/MOLA surface elevations, we find that the Mauna Kea during the observation. Ratio of the observed spectrum and mean pressure in our field of view was 6 mbar. Using the fitting near the martian H2OlinemakesitpossibletodeducetheH2Oabun- MGS/TES observations at the same martian season (Smith, dance of 13 pr. µm on Mars. 2004),wefoundthatthemeanatmospherictemperaturewas very close to 200 K in our field of view at a level of 3 mbar 2002).Thedifferencemaybeduetoannualvariationsorto (half total pressure). absorption and scattering by dust, hazes, and clouds, which The fitting in Fig. 2 makes possible an accurate extraction are weak for the far infrared observations of water from the of the martian line using a ratio of the observed spectrum and Mars Global Surveyor, but become stronger at the shorter fitting near the martian line. Its Doppler shift corresponds wavelengths of the Viking and our observations. 1 to a geocentric velocity of 17.79 km s− .Wefindfrom The line full width at half maximum FWHM is equal to − the fitting of 8 lines that the mean value of the geocentric 14 points in Fig. 2.Itsopticaldepthisτ0 2.1intheline 1 = velocity is 17.76 0.06 km s− . center, therefore it is wider than a weak line in the martian − ± The equivalent width of the martian H2OlineinFig. 2 atmosphere. A mean FWHM of the four lines with τ0 < 0.5 1 22 1 is 0.00903 cm− ,andthelinestrengthis1.41 10− cm is 10.5 1.6points 0.0180 0.0027 cm− .Theexpected × ± = ± 1/2 1 at 200 K from HITRAN 2000. We retrieved a water vapor width of a weak martian line is (2π) #νD 0.0076 cm− . 7 1/2 = abundance of 13 µm of precipitated water in the martian Here #νD 3.04 10− ν(T /µ) is the Doppler line atmosphere using the standard curve-of-growth technique width, T =200 K× is the temperature of Mars atmosphere (Chamberlain and Hunten, 1987; Krasnopolsky, 1986).The at a half pressure= level of 3 mbar, and µ 18 is the mole- = measured H2OabundanceonMarsisclosetothatobserved cular mass. The measured FWHM reflects the effects of the for the same season and latitudes from the Viking orbiters line width and the spectral resolution, which appears to be 2 2 0.5 1 (Jakosky and Farmer, 1982) and slightly smaller than the equal to (0.0180 0.0076 ) 0.0163 cm− .Theun- − = 1 17 pr. µm obtained from the Mars Global Surveyor (Smith, apodized resolution is expected to be 0.0134 cm− ,anda 540 V.A. Krasnopolsky et al. / Icarus 172 (2004) 537–547 weak apodization applied resulted in some degradation of the spectral resolution. The resolving power of the apodized spectrum is ν/δν 180,000. = 1 The spectral resolution of 0.0163 cm− corresponds to 9.5 sampling points. Compared to the Nyquist sampling of two points per resolution element, our spectrum is oversampled by a factor of 5.

3. Detection of methane

All possible martian CH4 lines of interest are on the blue wings of the strong telluric CH4 lines. To search for methane, we choose those CH4 lines, which are weakly con- taminated by other telluric lines at the expected positions of the martian lines. We consider all available methane lines with strengths exceeding a given limit, which we choose at 6 10 20 and 1.5 10 20 cm. The numbers of lines ex- × − × − ceeding the two limits are 5 and 15, respectively. The peak opacity from a line with S 1.5 10 20 cm using a methane = × − abundance of 10 ppb is SN Fig. 3. Blue wing of the strongest methane telluric line in our spectrum. 0 The expected positions of the line center and the Doppler-shifted martian 2η0fCH4 0.9%, δν = line are shown. 75 points near the martian line are fitted by a cubic parabola (thin line). Difference between the measured points and the fitting is also where η0 is the airmass in the martian atmosphere which is shown. The greatest difference is exactly centered at the expected position equal to 2.5inourfieldofview,f is the methane mix- ≈ CH4 of the martian line, is of the proper sign, and exceeds the other minima and ing ratio, N 2.2 1023 cm 2 is the column abundance of 0 = × − maxima by more than a factor of 1.8. gas molecules for the pressure of 6 mbar on Mars, and the factor of 2 is because the resolution interval δν refers to half maximum. A signal at the position of the martian line is typically a third of the maximum signal, therefore the expected absorption of 0.009 110/3isathirdofthenoiselevel. × Evidently, inclusion of the weaker lines in the extraction of methane is not helpful. We fit the blue wings of each of the chosen telluric CH4 line with cubic polynomials for the spectral intervals of 75 points centered at the martian lines. The fitting does not include 15 central points where the martian line is expected. This fitting for the strongest martian CH4 line in our spectrum is shown in Fig. 3.Thewingofthetelluriclinehasa wave-like structure, and the greatest wave exactly coincides with the expected position of the martian line. This wave is of the proper sign and exceeds the other waves by more than afactorof1.8.Thelocalnoiseisequaltostandarddeviation (sigma) of the fit and shown with intervals of three points. Excluding the central points, approximately a third of the re- maining points exceed the standard deviation, in accord with the Gauss statistics. Deviations from the fits for the chosen lines are summed up using statistical weights. According to the rules of statistics, these weights should be proportional to the squares of ratios of line intensity to local noise, and the line intensity Fig. 4. Summing of the weighted spectra centered on all lines with should be proportional to the line strength and local con- 20 S>6 10− cm. Weights are shown near each line, and the spectra are tinuum. The summing of the lines with S>6 10 20 cm is × × − displaced vertically. The absorption is exactly centered at the expected posi- shown in Fig. 4.Whilesomespectrashowpeaksandminima tion of the martian line and exceeds the other minima and maxima by more than a factor of 2. d 0.00171 cm 1 is the sampling interval. exceeding that at the position of the martian line, the latter = − Detection of methane on Mars 541

Fig. 5. Weighted-mean spectra of 5 and 15 strongest martian lines of CH4 (upper and lower panels, respectively). The absorption peak is shifted by 10% of the resolution element in the lower spectrum. The absorption peaks correspond to the methane detection at levels of 3.0 and 3.7 sigma, respectively. clearly dominates in the sum. Also, though the absorption is PHOENIX spectrograph (Mumma et al., 2003),whichis significantly shifted in the individual spectra, it exactly co- now at the South Gemini Observatory. However, its resolv- incides with the expected position in the sum. ing power is lower by a factor of 2.5 and its range is smaller The sums for 5 lines with S>6 10 20 cm and 15 lines by a factor of 10 than those of the FTS/CFHT. × − ≈ with S>1.5 10 20 cm are shown in Fig. 5.Theabsorption × − in the lower panel is shifted by one point which is 10% of the resolution element. Deviations of approximately a quar- 4. Photochemical loss of methane ter of the points in both spectra exceed one sigma, which is within the uncertainty of the Gauss statistics. Deviations The observed abundance of methane is too small to af- of three of 60 points exceed two sigma in the lower spec- fect the abundances of major photochemical species. We trum, again in accord with the statistics. (Here we exclude found four reactions for the photochemical loss of methane 15 central points.) Oversampling does not affect a percent of on Mars (Table 1). The main process is direct photol- points exceeding, say, one or two sigma. The oversampling ysis by solar Lyman-alpha radiation. The mean Lyman- is a factor of 5 in our spectra, the number of points for the alpha photon flux is I 3.7 1011 cm 2 s 1 at 1 AU 1216 = × − − Nyquist sampling would be 12 instead of 60, and one point (Woods et al., 1996),andtheCH4 and CO2 cross sections at in the lower spectrum would exceed two sigma. The Gauss 1216 Å are σ 1.85 10 17 cm2 and σ 7.4 CH4 = × − CO2 = × statistics predicts 0.05 12 0.6 point with uncertainty of 10 20 cm2 (Huebner, http://espsun.space.swri.edu/amop). × = − 0.61/2 0.8point. Absorption of Lyman-alpha photons occurs near 80 km in = The peak values are equal to 3.0 sigma in the upper spec- the martian atmosphere, and the mixing time is much shorter trum and 3.7 sigma in the lower spectrum. They exceed the than the CH4 lifetime relative to photolysis at these heights. largest deviations in the spectra by factors of 2.02 and 1.65, Therefore, the CH4 mixing ratio is constant with height near respectively. Both of the summed spectra show a detection 80 km, and the loss of CH4 by photolysis is of martian CH4 absorption near the standard detection crite- I1216 σCH4 5 2 1 rion of 3 sigma. Summing 15 central points, the CH4 mix- L f 1.0 10 cm− s− . 1216 = r2 σ CH4 = × ing ratio is 12.6 4.2and9.2 2.5ppbfromtheupper 4 CO2 ± ± and lower spectra, respectively. The weighted-mean value is Here r 1.52 AU is the mean Mars heliocentric distance, = 10.05 2.9ppb.Theuncertaintiesofthetwovaluesarenot 4istheratioofthespheretodiskareas,andf 10 8 ± CH4 = − statistically independent and therefore are averaged linearly, is the CH4 mixing ratio. The three other loss processes are not as squares. The recommended value for the methane the reactions with OH, O, and O(1D). Column rates for mixing ratio in the martian atmosphere is 10 3ppb. these reactions are calculated using the data of photochem- ± Since these data were obtained, the Fourier transform ical models by Nair et al. (1994) and Krasnopolsky (1995). spectrometer has been decommissioned at the CFHT, and it Different approaches were used in these models to fit the ex- is impossible to repeat our observation. Probably, currently perimental data, and the models are significantly different. the best instrument for detection of methane on Mars is the The total lifetime of CH4 on Mars is 250 and 430 years, 542 V.A. Krasnopolsky et al. / Icarus 172 (2004) 537–547

Table 1 Photochemical loss of CH4 in the martian atmosphere 3 1 2 1 Reaction Rate coefficient (cm s− )Columnrate(cm− s− ) CH hν (1216 Å) products – 1.0 105 1.0 105 4 + → × × CH OH CH H O2.45 10 12 exp( 1775/T)a 1.0 105 1.2 104 4 + → 3 + 2 × − − × × CH O CH OH 8.3 10 12(T /300)1.56 exp( 4270/T)b 560 120 4 + → 3 + × − − CH O(1D) CH OH 1.5 10 10 a 7.8 104 5 104 4 + → 3 + × − × × Total 2.8 105 1.6 105 × × The second column does not refer to photolysis of CH4,whichisdiscussedinthetext,andtothetotalloss.Columnratesforthethreereactionsarecalculated using the photochemical models of Nair et al. (1994, Fig. 15) and Krasnopolsky (1995, Fig. 2a) in the left and right columns, respectively. a Sander et al. (2003). b Baulch et al. (1992). and the total loss is 2.8 105 and 1.6 105 cm 2 s 1,re- Table 2 × × − − spectively. We will use the mean values of 340 years and Times of methane diffusion 2.2 105 cm 2 s 1,thatis,270ty 1 (tons per year). Depth 10 m 100 m 1 km 10 km × − − − We have not found any data on the heterogeneous loss of Thin layer 0.33 7.549046,000 methane in the literature. The heterogeneous reaction prob- Thick layer 0.16 3 170 15,000 9 ability must be equal to γ 6 10 to double the loss of Times are in years, and thick layer extends from the surface to a given depth. = × − methane calculated above. (We assumed a uniformly mixed dust with opacity of 0.5 to calculate this value.) It may be The equation of diffusion from a thin layer at depth d to compared to the gas reaction probability of 2 10 11 in × − the surface may be integrated to give the diffusion time the collisions between CH4 and O. Comparison with some n d d2 other heterogeneous reactions (Sander et al., 2003) results t 0 . in a conclusion that the heterogeneous loss of atmospheric = Φ + 2D 5 2 1 methane is probably negligible on Mars. Here Φ 2.2 10 cm− s− is the CH4 flow and n0 2 2 6 = × = × Time of vertical mixing is H /K 10 s 10 days in 109 cm 3 is the CH surface density. Diffusion time from ≈ ≈ − 4 the martian lower atmosphere. Time of horizontal mixing alayerwhichextendsfromthesurfacetodepthd may be is 5R/V 0.5y.HereH 106 cm is the scale height, calculated by a further integration: ≈ 6 ≈2 1 ≈ 8 K 10 cm s− is the eddy diffusion, R 3.4 10 cm is 2 ≈ 1 = × n0d d Mars’ radius, V 100 cm s− is the meridional wind speed, τ . ≈ and V/5isthechaoticcomponentofthewindspeed.Both = 2Φ + 6D times of vertical and horizontal mixing are much shorter than The calculated diffusion times (Table 2)arelong,sothe the CH4 lifetime of 340 y. Therefore methane should be well existence of a heterogeneoussink for methane during its dif- mixed with the constant mixing ratio of 10 3ppbthrough- fusion through the regolith to the atmosphere is a distinct ± out the whole atmosphere on Mars. possibility. Therefore, the production of methane may sig- 5 2 1 Methane is more abundant than other hydrocarbons in the nificantly exceed the very weak flow of 2.2 10 cm− s− × atmospheres of the outer planets and satellites by three or- from the regolith to the atmosphere. This flow is thus a lower ders of magnitude and more, and the expected abundances limit to the production of methane in the regolith. of the other hydrocarbons are negligible on Mars. Wong et al. (2003) calculated that formaldehyde H2CO, which is the most abundant product of methane chemistry on Mars, is less 6. Abiogenic sources of methane abundant than methane in the martian atmosphere by six orders of magnitude, and the products of methane chemistry 6.1. Hydrothermal and magmatic methane are not helpful as tracers of methane. There are no processes of methane formation in the at- 5 2 1 mosphere, so the loss of 2.2 10 cm− s− must therefore be balanced by abiogenic and× biogenic sources. Welhan 5. Diffusion of methane through regolith (1988) considered sources of methane in the hydrothermal systems on the Earth. He discussed two abiogenic sources of Diffusion of gas through the martian regolith (Weiss et methane, from magmatic outgassing and high-temperature al., 2000) is of the Knudsen type, when molecular collisions chemical reactions within a hydrothermal fluid. For exam- with the walls of the pores dominate in the transport process. ple, methane may form in the following equilibria: The calculated diffusion coefficient (Weiss et al., 2000) may 1/2 2 1 CO2 2H2O CH4 2O2, be approximated as D 1.37µ− cm s− ,andµ is the + = + = molecular mass in atomic units. 8FeO 2H O CO 4Fe O CH , + 2 + 2 = 2 3 + 4 Detection of methane on Mars 543

2C 2H2O CO2 CH4, orbiter (Christensen, 2003) does not reveal any hot spots + = + on Mars. Hypothetical hydrothermal systems can hardly CO 4H CH 2H O. 2 + 2 = 4 + 2 be warmer than the room temperature at which methane If the first equilibrium dominates (Holland, 1984),thenabio- is very low abundant in terrestrial waters. Based on these genic methane is negligible on Mars. facts, a significant production of hydrothermal and mag- Gas-phase reactions between stable molecules at room matic methane is not very likely on Mars. temperatures are extremely slow. For example, a rate coef- 10 ficient of the reaction between H2 and O2 is 2.4 10− 6.2. Cometary methane on Mars 3 1 × × exp( 28,500/T) cm s− (Tsang and Hampson, 1986),that is, 1.−3 10 51 cm3 s 1 at 300 K, and a mixture of H and Methane may be brought to Mars by cometary impacts. × − − 2 O could exist for a period exceeding the age of the Solar The methane abundance in the cometary ice is 1% (Gibb 2 ≈ System by a factor of 5 1014.Therefore,thereactionsoc- et al., 2003).TheimpactfrequencyontheEarthforcomets curring in hot fluids are× only of interest here. with sizes exceeding 1 km is 2.2 10 6 y 1 (Shoemaker × − − Welhan (1988) considered the outgassing from the hy- et al., 1994).Impactsofbodieswithmassexceeding4 × drothermal systems at mid-oceanic ridges where pressure 1016 gremovetheatmosphereabovetheplanetangential is hundreds bars, temperature reaches 350 ◦C(620K),and to the impact (Rahe et al., 1994).Weassumethatthegases concentrations of methane are higher than those in the mean from the comet ice are removed by the blast if the comet ocean water by a factor of up to a million. The outgassing mass exceeds 1016 g. is traced using the rare isotope 3He, which represents juve- There are three populations of comets, which contribute nile helium trapped during the Earth’s accretion. The mantle almost equally to the impacts on the Earth (Shoemaker et al., helium is enriched in 3He by a factor of 8 (Welhan, 1988) 1994):theJupiterfamily,theHalleyfamily,andlong-period relative to the atmospheric helium, which has 3He/4He and new comets. We included extinct comets in our calcu- 6 = 1.4 10− .ConcentrationsofCH4 in the mid-oceanic ridge lation, though those comets may lose some of their ice. We × 3 fluids are smaller than those of He by a factor of 90 adopted the mean density of nuclei of 1 g cm− and the mass ≈ (Welhan, 1988).Wecombinethesedatawiththeknown of ice as half the nucleus mass. The mass of a comet with 3 2 1 14 He global-mean outgassing rate of 4 cm− s− (Prather diameter of 1 km is 5 10 g, and the number of comets ex- × 0.58 17 and McElroy, 1983),basedonthemeasurementsofgas ceeding mass m is proportional to m− for m<3 10 g × dissolved in the ocean. Then the abiogenic outgassing of (Rahe et al., 1994).Thenthefrequencyofimpactsonthe 6 methane on the Earth is as low as 4/(8 90 1.4 10− ) Earth is 2 1 1 × × × = 4000 cm− s− 17 t y− . 14 0.58 = 6 5 10 1 H OandCO are the most abundant volcanic gases on f 2.2 10− × y− . 2 2 = × m the Earth. Thermochemical equilibrium in the first of the ! " above reactions at high temperature of magma ( 1400– The impact cross-section of Mars is 0.2 of the Earth ≈ 1500 K on Earth) results in a release of methane by the cross-section (Chyba et al., 1994) because of the smaller volcanic eruptions. Its mixing ratio is typically a few ppm geometric cross section and the gravitational potential. The (Etiope and Klusman, 2002) in the erupted gases. The annual number of comets crossing Mars orbit is greater than those eruption rate of magma is 4 km3 on Earth, the erupted gases for the Earth, however, this number refers to the area, which 2 are 5% of this mass (Geist, 1992),andthiscorrespondsto is greater by a factor of 1.52 2.3. (Mars orbit is at 1.52 ≈ 5 2 1 1 = amethaneproductionof 3 10 cm− s− 1300 t y− . astronomic units.) Therefore, the densities of the cometary Geothermal emission≈ of methane× is typically= ignored in flux are approximately equal near Mars and Earth. 16 the budget of terrestrial methane which is accurately doc- Using the limiting mass is 10 g, the mass influx from umented (Yavitt, 1992; Yung and DeMore, 1999).Itisvery the impacts on Mars is much weaker than the biogenic production of methane on the 1016 11 2 1 Earth (511 megatons per year 1.2 10 cm− s− )and df 9 1 = × Φm 0.2 m dm 10 gy− . therefore very uncertain. Etiope and Klusman (2002) esti- = dm = 1 # mated a global CH4 flux from gas vents as 0.01–1 Mty− 0 6 8 2 1 = 2.4 10 2.4 10 cm− s− .Thisestimateexceedsthe The calculated mean delivery of methane by comets is × − × 1 two values given above, and our discussion with G. Etiope 5ty− ,thatis,2%ofthetotalloss.Thisalsomeansthat has not revealed any error in our values and methods. the observed methane may be mostly due to a recent impact Outgassing from Mars is weaker than that from the Earth of comet, but probability of this event is 2%. by an order of magnitude (Krasnopolsky et al., 1994).The ≈ youngest lava flows which represent the last case of martian 6.3. Methane from meteorites volcanism are at least 10 million years old (Hartmann and Berman, 2000),fargreaterthanthemethanelifetime,andthe Delivery of meteoritic carbon to Mars was considered 9 1 released methane disappeared since that time. The thermal by Flynn (1996).Thetotalmassfluxis12 10 gy− de- emission imaging system (THEMIS) at the Mars Odyssey livered mostly by interplanetary dust particles× smaller than 544 V.A. Krasnopolsky et al. / Icarus 172 (2004) 537–547

2 10− g. Some part of this flux is melted and vaporized, some Clifford do not see “how the life once established in this is subject to pyrolysis, and 2.4 109 gy 1 is the unaltered niche could become extinct.” They consider formation of × − fraction of the delivered meteoritic matter. Flynn (1996) as- methane hydrate CH 6.1 H Oasaformofunderground 4· 2 sumed that the average carbon content of interplanetary dust methane. However, diffusionthroughtheregolithtotheat- is 10%. Then the total carbon delivery is 1.2 108 gy 1 mosphere (Section 5)mayinhibittheformationofhydrate. × − and the delivery of polymeric organic matter known as kero- Summers et al. (2002) adopted the maximum potential 1 9 2 1 gen (Benner et al., 2000) is at a rate of 240 t y− of reduced fluxes ΦCO 8.8 10 cm− s− and ΦH2 8.6 8 2 =1 − × = − × carbon (Benner et al., 2000).Thisrateiscomparabletothe 10 cm− s− from Weiss et al. (2000) (though they con- photochemical loss of methane. fused them and gave without explanation and reference) and 4 Gerasimov (2002) simulated the high-temperature de- aconversionefficiencyof10− for these species to CH4 by composition of meteorites impacting a planet using a laser methanogenic bacteria. Then they found that a flow of CH4 beam with a power density of 105–107 Wcm 2.Themea- of 10 4 8.8 109 8.8 105 cm 2 s 1 results in the CH − − × × = × − − 4 sured total yield of gases varied with a mean value of mixing ratio of 15 ppb using the photochemical model by 7 1 (8 4) 10− gJ− .Forameanvelocityoftheimpact- Nair et al. (1994).ActuallyabundanceofCH4 is just a prod- ± × 1 5 1 ing bodies of 15 km s− their energy is equal to 10 Jg− . uct of its flow and lifetime, and their values correspond to Aproductofthesevaluesis0.08andexceedstheabundance∼ the lifetime of 120 years, though they wrote twice in the pa- of volatiles in meteorites. Therefore most of the meteorite per that the lifetime is 300 years and our calculations for the volatiles are released during the impact, in accord with the model by Nair et al. (1994) resulted in 250 years (Section 4). estimate of Flynn (1996).CarbonisreleasedmostlyasCO According to their paper, if all CH4 molecules hitting the and CO2 with a yield of 0.9% for CH4 (Gerasimov, 2002). surface are lost, then the CH4 mixing ratio would decrease No organic matter was detected by the Viking gas by 30%. Our calculations similar to that in Section 4 show chromatograph—mass spectrometer (Biemann et al., 1977). that the CH4 mixing ratio would be halved if the probability 8 This is explained by its decomposition due to solar ultra- of heterogeneous loss on the surface is as low as 10− . violet radiation or oxidation by the regolith (Flynn, 1996; Benner et al., 2000).Methanecannotforminanyofthese 7.1. Biomass production ways. Therefore the yield of 0.9% from Gerasimov (2002) corresponds to an upper limit to the production of methane In our discussion of methanogenesis on Mars, we will from the meteoritic carbon on Mars. This upper limit is equal rely on what is known about such organisms on the Earth, 1 to 11 t y− ,thatis,4%oftherequiredproduction. keeping in mind that martian microbes may employ very different biochemistry to achieve the same result. Methanogens consume metabolic energy from the sum- 7. Methanogenesis on Mars mary reactions 1 CO2 4H2 CH4 2H2O 1.71 eV (39.35 kcal mol− ), We are therefore led to consider a biogenic origin of + → + + 2H2O 3CO CH4 2CO2 0.48 eV methane on Mars. “Natural gas” on Earth is related to the + → 1 + + extinct macroscopic life forms, which were and are impos- (11.04 kcal mol− ). sible on Mars, given the exceedingly low limits on organic The H2-based methanogenesis is more effective energeti- matter in the soil set by the Viking landers (Biemann et al., cally and met on the Earth. However, CO is more abundant 1977) and the dry recent history of the planet. The major than H2 in the martian atmosphere where their mixing ratios source of atmospheric methane on the Earth (Yavitt, 1992) are equal to 800 and 15 ppm (Krasnopolsky and Feldman, is the microbial methanation of dead macroscopic biota: 2001),respectively,andtheCO-basedmethanogenesismay be important on Mars. 2CH2O CH4 CO2. 5 2 1 → + The required fluxes of 9 10 cm− s− for H2 and There are presumably no macroscopic biota to decompose, 7 105 cm 2 s 1 for CO are× much smaller than the pho- × − − so this process cannot occur on Mars; therefore we will dis- tochemical production of these species (5.7 108 and 8 2 1 12 × cuss production of methane on Mars by methanogenic or- 3.3 10 cm− s− for H2 and 1.1 10 and 8.3 11× 2 1 × × ganisms in the soil. 10 cm− s− for CO from the models by Nair et al. (1994) Max and Clifford (2000) consider a possible accumula- and Krasnopolsky (1995)). They are also much smaller 8 2 1 tion of significant abundances of methane by methanogenic than the potential fluxes of 8.6 10 cm− s− for H2 and 9 2 1 × bacteria in the liquid saline groundwater layer which they 8.8 10 cm− s− for CO calculated for a biotic layer at suppose to exist near a depth of 6 km on Mars. Though they adepthof10min× Weiss et al. (2000).However,thereisa do not rule out abiogenic methane from aqueous alteration significant uncertainty in thefluxesconsumedandreleased of basalt, they conclude that “the detection of substantial by the bacteria on Mars because of the lack of the data on quantities of subsurface methane, as either gas or hydrate, heterogeneous loss of CH4,H2,andCOinthemartiansoil. 5 would by itself provide persuasive evidence that an Earth- The lower limit for the CH4 production of 2.2 10 like subsurface biosphere evolved on early Mars.” Max and cm 2 s 1 corresponds to the metabolic energy of× 3.8 − − × Detection of methane on Mars 545

5 5 2 1 10 and 1.1 10 eV cm− s− for the H2-andCO-based size comparable to the uncertainty of landing on Mars (say, methanogenesis,× respectively. It may be readily converted to 1 ), and locations of the oases could be properly deter- ± ◦ biomass production if a mass per energy yield of methano- mined, then the derived limits are higher for the oases by genesis is known. However, we have not found this yield in three orders of magnitude, and the oases may have a de- the literature and therefore assume that it is similar to that of tectable biota. photosynthesis on Earth, The very long lifetime of methane in the martian atmosphere and short times of vertical and horizontal mixing CO H O 2hν CH O O . 2 + 2 + → 2 + 2 are not encouraging for detection of the oases by observing Two visible photons have total energy of 5 eV, and the yield methane above them. Let us assume that the total production 1 is 6 u eV− where u is the atomic mass unit. Then the bio- of methane is concentrated in n vents each with radius r. mass production rate by methanogenesis on Mars exceeds Then the CH4 flux in a vent is equal to 10 11 2 1 1.2 10− and 3 10− gcm− y− ,thatis,170and 2 × 1 × 4Φ R 50 t y− ,respectively. ΦV , = n r Agreatervalueofthebiomassyieldmaybeobtainedus- ! " 5 2 1 ing another scheme (Jakosky and Shock, 1998) with a mole- where Φ 2.2 10 cm− s− is the global-mean produc- cule such as adenosine triphosphate (ATP) with a yield of tion rate= and R×is Mars radius. The accumulation time is 1 ATP of 0.1 mol kcal − and production of 1 g of cells from t 2r/V where V 1ms 1 is the mean wind velocity. An ≈ ≈ − 0.02 mol ATP.Converting units to eV and u, this corresponds excess in the CH4 abundance above the vent over the mean 1 to a biomass yield of 115 u eV− ,whichisgreaterthanthat atmospheric abundance is by a factor of of photosynthesis by a factor of 20. Accordingly, the lower Φ t 8R2 8600 limits to the biomass production rate by methanogenesis are V , 9 10 2 1 1 2.3 10− and 6 10− gcm− y− (3300 and 940 t y− ) Φτa = nV rτa = nr × × 10 for the two reactions, respectively, by this scheme. where τa 10 sisthemethanelifetimeintheatmosphere Aminimalenergyrequirementtomaintainabiomassis and r is in= meters. The expected methane abundance above 5 1 1 7 1 1 10− kcal(gdryweight)− s− 4.4 10− eV u− s− the vents is 86/nr ppm. If the vent number is 10 and their ≈ = × ≈ (Weiss et al., 2000).Thenmethanogenesiscansupport1.5 radii are 10 m, then the methane abundance is 1ppm 10 12 and 4 10 13 gcm 2 (2 and 0.6 t) of dry biomass,× ≈ ≈ − × − − above them. The calculated excess refers to the densities respectively. Total biomass exceeds the dry biomass by an measured in situ. Remote observations involve column abun- order of magnitude. dances where the excess above the vents becomes extremely The long diffusion time calculated in Section 5 (Table 2) low, and detection of the oases and vents by measurements is also applicable to the diffusion of atmospheric H2 and of methane will present a difficult problem. CO into the soil to feed the methanogens. These species are reducing and may be lost in the long contact with oxidiz- ing minerals in the soil. Therefore, the martian biotic layer 8. Conclusions may be comparatively thin, for example, 100 m. Then ≈ the lower limits to the biomass production by methanogen- We observed a spectrum of Mars at the P-branch of 14 13 3 1 esis are 1.2 10 –2.3 10 gcm yr for the H2 the strongest CH band at 3.3 µm with resolving power of × − × − − − 4 methanogenesis and 3 10 15–6 10 14 gcm 3 yr 1 for 180,000 for the apodized spectrum. Summing up the spectral × − × − − − the CO methanogenesis. The limits to the dry biomass are intervals at the expected positions of the strongest Doppler- 1.5 10 16 and 4 10 17 gcm 3,respectively.Theselimits shifted martian lines, we detected methane at a level which × − × − − are lower by a few orders of magnitude than those from the is slightly above the standard detection criterion of 3 sigma. Viking gas chromatographwith mass spectrometer (GCMS), The observed CH4 abundance is 10 3ppb.Photochemical 11 3 5 2 ±1 whose maximum sensitivity was 10− gcm− for some or- loss of methane is 2.2 10 cm− s− and corresponds to its ganic species (Biemann et al., 1977).Themassofasmall lifetime of 340 years. × terrestrial microbe is 10 15 g (Brock and Madigan, 1991), Outgassing from Mars is low, the latest martian vol- ≈ − and media with a microbe population less than 10 cm 3 canism is at least 10 million years old, and thermal emis- ≈ − are considered as sterile. Therefore, the above limits show sion imaging from the Mars Odyssey orbiter does not re- that the martian biota may be exceptionally scarce and Mars veal hot spots on Mars. Hypothetical hydrothermal systems may be almost sterile relative to methanogens if the hetero- can hardly be warmer than the room temperature at which geneous sink of methane is ineffective. methane production is very low. Based on these facts, a significant production of hydrothermal and magmatic methane 7.2. Search for possible oases is not very likely on Mars. Delivery of methane by comets is near 2% of its loss. The biomass distribution is highly irregular on the Earth Methane from meteorites and interplanetary dust is smaller and may be even more irregular on Mars. If life is con- than 4% of its loss. Therefore, we have not found any sig- centrated in a few ( 10) oases on Mars, each oasis has a nificant abiogenic source of methane and methanogenesis ≈ 546 V.A. 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