A Sensitive Search for SO2 in the Martian Atmosphere: Implications for Seepage and Origin of Methane

Icarus 178 (2005) 487–492 www.elsevier.com/locate/icarus

A sensitive search for SO2 in the martian atmosphere: Implications for seepage and origin of methane

Vladimir A. Krasnopolsky ∗,1

Department of Physics, Catholic University of America, Washington, DC 20064, USA Received 2 December 2004; revised 29 April 2005 Available online 1 July 2005

Abstract −1 Mars was observed near the peak of the strongest SO2 band at 1364–1373 cm with resolving power of 77,000 using the Texas Echelon Cross Echelle Spectrograph on the NASA Infrared Telescope Facility. The observation covered the Tharsis volcano region which may be preferable to search for SO2. The spectrum shows absorption lines of three CO2 isotopomers and three H2O isotopomers. The water vapor abundance derived from the HDO lines assuming D/H = 5.5 times the terrestrial value is 12 ± 1.0 pr. µm, in agreement with the simultaneous ◦ ◦ MGS/TES observations of 14 pr. µm at the latitudes (50 Sto10 N) of our observation. Summing of spectral intervals at the expected positions of sixteen SO2 lines puts a 2σ upper limit on SO2 of 1 ppb. SO2 may be emitted into the martian atmosphere by seepage and is removed by three-body reactions with OH and O. The SO2 lifetime, 2 years, is longer than the global mixing time 0.5 year, so SO2 should be rather uniformly distributed across Mars. Seepage of SO2 is less than 15,000 tons per year on Mars which is smaller than the volcanic −4 −3 production of SO2 on the Earth by a factor of 700. Because CH4/SO2 is typically 10 –10 in volcanic gases on the Earth, our results show seepage is unlikely to be the source of the recently discovered methane on Mars and therefore strengthen its biogenic origin.  2005 Elsevier Inc. All rights reserved.

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

1. Introduction orbiter, has not detected any hot spots (Christensen, 2003). Yet weak seepage of the volcanic and geo- or hydrothermal Other than water vapor and CO2, which are typical of gases is still possible, and photochemical models for those the martian atmospheric environment, sulfur dioxide (SO2) hypothetical sources of SO2,H2S, and CH4 have been cal- is the most abundant species in terrestrial volcanic gases culated (Wong et al., 2003). and constitutes a few per cent of total gas release (Holland, SO2 may also be extracted by the solar UV photons from 1978). However, the latest traces of active martian volcan- sulfates on the martian surface or airborne dust. Sulfates ismare2to∼100 million years old (Hartmann and Berman, have not been detected on Mars by the thermal emission 2000; Sakimoto et al., 2003; Neukum et al., 2004), and the spectrometer on Mars Global Surveyor (McSween et al., thermal emission imaging system which was specially de- 2003; Wyatt et al., 2003). However, both the α-particle X-ray signed to search for hot spots on Mars from Mars Odyssey spectrometer and the miniature thermal emission spectrometer on the Opportunity rover revealed signiﬁcant (15 to * Correspondence address: 6100 Westchester Park #911, College Park, 35% by volume in some locations) amounts of magnesium MD 20740, USA. and calcium sulfates (Rieder et al., 2004; Christensen et al., E-mail address: [email protected]. 2004). 1 Visiting Astronomer at the Infrared Telescope Facility, which is oper- The recent discovery of methane on Mars (Krasnopolsky ated by the University of Hawaii under Cooperative Agreement No. NCC 5-538 with the National Aeronautics and Space Administration, Ofﬁce of et al., 2004; Formisano et al., 2004; Mumma et al., in prepa- Space Science, Planetary Astronomy Program. ration) triggered the discussion of its biogenic and abiogenic

2 sources. SO2 may be an effective tracer of the current out- The instrument slit, 1.5 × 7arcsec , was parallel to the cen- gassing on Mars. tral meridian and between the subsolar point and the cen- An upper limit of 100 parts per billion (ppb) was es- tral meridian. Its position corresponded to local time near tablished for SO2 in the martian atmosphere based on the 13:30 when the thermal contrast between the surface and absence of the absorption band at 7.3 µm in the Mariner 9 the atmosphere is close to maximum. The observation was ◦ infrared spectra (Maguire, 1977). Later this limit was im- at subsolar longitude LS = 205 (southern spring), when proved to 30 ppb using a microwave observation at 1.4 mm the heliocentric distance was near the minimum (1.42 and (Encrenaz et al., 1991). However, even an SO2 abundance 1.38 AU, respectively), and the slit covered latitudes from below this limit may affect martian photochemistry, and 50◦ Sto10◦ N. The longitude changed from 104 to 164◦ W some of the models by Krasnopolsky (1993, hereafter Pa- during four hours of the observation. Our observation cov- per I) were calculated assuming a SO2 mixing ratio of ered the Tharsis volcano region which may be preferable for 10 ppb. a search for SO2. We observed Mars, a flat field (an outer am- bient temperature black body), the Moon, two standard stars, and sky foreground 30 arcsec north of all targets. The total 2. Observation on-Mars exposure was 31 min of four hours of the observing time. Because Mars is such a bright source, we increased the To search for SO on Mars, we also used the strongest 2 amount of time spent observing the blackbody to insure the band at 7.3 µm. The observation was made on 18 June 2003, when Mars had a diameter of 14.7 arcsec, phase (Sun–Mars– final result was not limited by photon noise from the black- Earth) angle 40◦, and geocentric velocity −11.0kms−1. body. We used the NASA Infrared Telescope Facility on Mauna The instrument spectral coverage near the peak of the −1 −1 Kea, Hawaii. A comparatively low atmospheric pressure of SO2 band at 1362 cm is 9 cm . Choosing a spectral range 0.6 bar and dry atmosphere with a typical water abundance should be done carefully to cover the strongest lines and of 2 pr. mm above Mauna Kea (elevation 4.2 km) are essen- to minimize the contamination by telluric lines. We chose −1 tial for the observation near 7.3 µm where both H2O and 1364 to 1373 cm which includes two narrow opaque in- −1 CH4 telluric lines are strong and the atmospheric opacity is tervals at 1368.2–1369.0 and 1372.1–1373.2 cm centered significant even between the telluric lines. The observation at the strong H2O lines. For example, the broad intervals at is essentially impossible at low-altitude observatories. 1360.5–1363.7 and 1373.0–1375.7 adjacent to our spectral We used TEXES, the Texas Echelon Cross Echelle Spec- interval are almost completely opaque. The chosen spectral trograph (Lacy et al., 2002), in its high resolution mode. range covers many strong SO2 lines as can be seen in Fig. 1.

Fig. 1. Spectrum of the martian atmosphere. The spectrum consists of the martian lines of three isotopomers of CO2 and three isotopomers of H2Oaswellas some residuals of the telluric CH4 and H2O lines. The spectrum is shifted at rest position (corrected for the Doppler shift). Positions of seventeen SO2 lines − chosen for extraction are shown by bars whose sizes reﬂect the line strengths. Two lines at 1370.32 cm 1 coincide, and their sum is given. Search for SO2 on Mars 489

3. Results approximation for the strong collisionally broadened lines (Chamberlain and Hunten, 1987) and provides an uncer- The data were processed using typical TEXES reduction tainty of 4% for martian lines (Krasnopolsky et al., 1997). software. The software differences nod pairs, flatfields, cor- This uncertainty is acceptable for our goals. According to rects for spikes, produces a wavenumber scale based on at- the MGS/TES simultaneous observations (Smith, 2004),the mospheric features, and makes a significant reduction of the mean temperature at the level of half atmospheric pressure telluric absorption lines and emissions from the telescope for latitudes of 50◦ Sto10◦ N was equal to 223 K. and optics by subtraction of the sky foreground. Even better The martian lines are very narrow; therefore we did not compensation for the telluric lines may be achieved using fit the observed lines by a synthetic spectrum and exam- the Moon spectra. However, this results in some increase of ined their equivalent widths instead. Equivalent widths of the noise, and we do not use this possibility. The data were six weak CO2 lines were used to determine an effective 23 −2 resampled and spectral regions with usable data in two or- CO2 abundance of (2.0±0.2)×10 cm . (Effective abun- ders were combined. To search for SO2,wesummedallour dance is the derived abundance before correcting for emis- data over time and spatial position along the slit. The result- sion by the absorbing molecule.) The globally and annually ing spectrum is shown in Fig. 1. This spectrum is a ratio of mean atmospheric pressure is 6.1 mbar on Mars. Using the the Mars minus sky to the blackbody minus sky spectra. This Viking lander measurements (Tillman, 1988), the mean pres- ratio corrects for variations in the pixel sensitivity (flat field). sure is equal to 5.8 mbar at the season of our observation ◦ Telluric absorption lines are significantly reduced in this (LS = 205 ). Using the MOLA data, we find that the mean spectrum. Small variations of the continuum were corrected elevation of the observed region is 3.0 km and the mean using polynomial fitting. The mean continuum brightness is pressure is 4.5 mbar at the observed region. Then the ac- 2 −1 −1 23 −2 25 erg (cm ssrcm ) and corresponds to the surface tem- tual CO2 abundance is 1.7 × 10 cm , and a correction perature of 276 K. This temperature is close to that expected for the mean air mass of 1.15 in the martian atmosphere re- 23 −2 for the season, latitudes, and local time (13:30) of our ob- sults in 2.0 × 10 cm . The weak CO2 lines correspond servation from the Viking IRTM data (Martin, 1981).The to absorption of the surface radiation with T = 276 K and temperature is high because the observation was near the emission with the mean temperature of 223 K. The emit- daytime temperature maximum and Mars was rather close ted radiation is weaker than the absorbed radiation by a to perihelion. factor of e1.439ν(1/223−1/276) = 5.5, where ν is the wavenum- The spectrum shows lines from three isotopes of CO2 ber. Therefore the effective CO2 abundance derived from the and three isotopes of H2O in the martian atmosphere and weak CO2 lines should be equal to the true slant CO2 abun- −1 some residuals of telluric CH4 and H2O lines. Compar- dance times a factor of 1 − 5.5 = 0.82, that is, 1.64 × 18 23 −2 ing wavenumbers of ten CO O lines of the strong 10001– 10 cm . The measured effective CO2 abundance slightly 00001 band in our spectrum with their wavenumbers in exceeds the calculated value. the HITRAN 2004 spectroscopic database, we found a sys- The HDO lines at 1365.301, 1367.574, and 1372.015 tematic difference of (56.9 ± 0.5) × 10−3 cm−1, of which cm−1 give a water vapor abundance of 12 ± 1pr.µm.D/H 0.050 cm−1 is the Doppler shift and the remaining (6.9 ± on Mars is 5.5 times the terrestrial value (Owen et al., 1988; 0.5) × 10−3 cm−1 represents a systematic correction to the Krasnopolsky et al., 1997), and this ratio has been applied. wavenumber. Therefore the wavenumber scale in Fig. 1 has Saturation of water vapor occurs at h 30 km at the season been corrected by 0.057 cm−1, and the spectrum is at rest and latitudes of our observation and water vapor may be con- position. sidered as uniformly mixed in the lower atmosphere. The de- The martian weak lines are very narrow, and their shape rived water vapor abundance is close to that observed simul- in the observed spectrum reflects the instrument response taneously with the MGS/TES, which is equal to 14 pr. µm function. We fitted some weak lines by the Gaussian and for the latitudes of our observation (Smith, 2004). Lorentzian profiles and found that the Gaussian fit is much To search for SO2 in the spectrum, we examine the posi- better. A mean dispersion of this fit is equal to 3.3 ± 0.1 sub- tions of seventeen lines which are strong, not contaminated pixel intervals of d = 0.00228 cm−1. (The instrument pixel by other lines, and have low noise. (Noise is variable in our size is twice the subpixel value. The spectral pixel size was spectrum.) We compared high-precision heterodyne mea- reduced by a factor of 2 to avoid blurring when data from surements of frequencies for some of the SO2 lines of our two diffraction orders were added together. The wavenum- interest (Vanek et al., 1990; Flaud et al., 1993) with those ber sampling slightly varies from the beginning to the end given in HITRAN 2004 and concluded that an uncertainty −1 of the spectrum which is typical of grating spectrographs.) of the chosen SO2 line positions is 0.0003 cm . This un- This dispersion corresponds to a spectral resolution element certainty is much smaller than our resolution element, and δν = 0.0177 cm−1 (full width at half maximum) and the in- the line positions from HITRAN 2004 may be used with a strument spectral resolving power ν/δν = 77,000. good confidence. The chosen seventeen SO2 lines and their For our analysis of the martian lines we adopt their strengths are shown in Fig. 1. strengths at a temperature at a level of half atmospheric Thirteen subpixels centered at the expected position of pressure. This approach is similar to the Curtis–Godson a chosen SO2 line are used for the extraction of SO2. Four 490 V.A. Krasnopolsky / Icarus 178 (2005) 487–492

Fig. 3. Sum of spectral intervals centered at the expected positions of sixteen SO2 lines and corrected for their continua (see text). Error bars show stan- − dard deviations of the summed points. Each subpixel is d = 0.00228 cm 1. − S = 1.0 × 10 18 cm is the sum of the sixteen line strengths. The Gaussian Fig. 2. Top: 21 points of the spectrum centered at two coinciding SO2 lines − −1 at 1370.3209 and 1370.3214 cm 1. Thin line is a third-order polynomial has a width of the instrument spectral resolution (0.0177 cm ) and corre- fitting to four left and four right points. This fit is a continuum model. Bot- sponds to the SO2 mixing ratio of 1 ppb in the martian atmosphere. tom: difference between the spectrum and the continuum is compared with the SO2 absorption line (thin line) calculated for a mixing ratio of 1 ppb. 0.0005 cm−1 in our spectrum. Therefore, the spectrum in Fig. 2 does not show the SO2 absorption. To improve the subpixels to the left and to the right of these 13 subpixels confidence of our search, differences between the spectrum are approximated by a third degree polynomial to model a and the modeled continuum are summed up for all chosen continuum near the line. (Spectra around some lines have lines and shown in Fig. 3. The line strengths are equal to − inflection points and therefore need third or higher degree (4–8) × 10 20 cm at 223 K, and the summed spectrum was polynomials for approximation. We use the third degree multiplied by the subpixel size and divided by the sum of the − polynomials, and just two parameters, intensity and slope, line strengths, S = 1.0×10 18 cm, to give an effective abun- are actually taken from the four points at the each side of the dance of SO2. Evidently no absorption has been detected. line.) To obtain an upper limit to the SO2 abundance, we need −1 Two strong SO2 lines at 1370.3209 and 1370.3214 cm to estimate the uncertainty in the summed spectrum in Fig. 3. blend together within our subpixel size, and a sum of their Two methods have been applied. The first method is to esti- strengths significantly exceeds the strengths of the other mate the noise in the spectrum and multiply it by square root lines (Fig. 1). 13 central and 8 outer points for this line and of the number of the lines. Four spectrally clean regions at the modeled continuum are shown in Fig. 2 (top). A differ- 1365.32–1365.41, 1366.70–1366.90, 1367.36–1367.52, and − ence between the spectrum and the continuum is shown in 1371.06–1371.29 cm 1 were taken to estimate the noise: Fig. 2 (bottom) where it is compared with a SO2 line shape 2 1/2 (yi − y0i) calculated for a SO2 mixing ratio of 1 ppb. The SO2 line N = . n/2 − k equivalent width is equal to the CO2 effective abundance 23 −2 (2 × 10 cm ) times the adopted SO2 mixing ratio times Here N is the noise, yi is the spectral point, y0i is the point the line strength. Then this equivalent width is scaled to the from linear (k = 2) or quadratic (k = 3) fit to the spectrum, instrument line shape which is a Gaussian with FWHM of and n/2 is the true pixel number in the chosen interval. Then the instrument resolution. This approach works if both the the mean noise from these four regions is N = 0.0012, the 1/2 13 −2 CO2 and SO2 lines are weak and SO2/CO2 is constant with noise in Fig. 3 is 16 Nd/S = 1.1 × 10 cm per sub- height. (This will be discussed below.) This analysis avoids pixel, and a two-sigma upper limit for SO2 is 2 × 3.3 × radiative transfer modeling which involves thermal proper- (2π)1/2 × 1.1 × 1013 = 1.8 × 1014 cm−2.(3.3 × (2π)1/2 is ties of the atmosphere and the surface. the effective number of subpixels in the Gaussian with the The spectrum in Fig. 2 is on the wing of the strong CO18O dispersion of 3.3 subpixels.) line, and the continuum fitting by the third degree polyno- Another method is to use a standard deviation for the mial may be not perfect for this spectrum. Therefore the points in the 16 summed spectra. It is similar to the de- difference between the spectrum and the continuum in the rived noise and also results in the two-sigma upper limit lower panel of Fig. 2 is actually a sum of the noise, a possible of 1.8 × 1014 cm−2. 5% of the signal is beyond of the error in the continuum fitting, and a possible SO2 absorption. thirteen central subpixels, and the corrected upper limit is −1 14 −2 The minimum seen in the figure is displaced by 0.009 cm 1.9 × 10 cm . This limit divided by the effective CO2 which significantly exceeds the wavenumber uncertainty of abundance of 2×1023 cm−2 corresponds to a two-sigma up- Search for SO2 on Mars 491 per limit to the SO2 mixing ratio of 1 ppb. This limit is much mixing (0.5 year, see Krasnopolsky et al., 2004), and SO2 more restrictive than the currently existing limit of 30 ppb should generally be uniformly mixed in altitude up to 30 km (Encrenaz et al., 1991). and across the planet. (SO2 cannot condense anywhere on Mars including the polar caps where the temperature cannot fall below the CO2 condensation temperature of 145 K 4. Photochemical loss of SO2 at 6 mbar.) However, the difference in the times is not large, and some low variations in SO2 are not ruled out. Loss and A possible photochemical impact of SO2 was considered production of SO2 are balanced, and the calculated upper −1 in Paper I where the calculated indirect photolysis of O2 in limit to these processes is 17,000 t y (tons per year). the reactions

SO + HO2 → SO2 + OH, 5. Discussion SO + O2 → SO2 + O, S + O2 → SO + O, Our upper limit of 1 ppb to SO2 in the martian atmosphere equaled the direct photolysis of O2 for the SO2 mixing ratio and the comparatively long lifetime of SO2 do not support of 10 ppb. With the upper limit established here of 1 ppb, the existence of extended regions with a well-developed pho- SO2 cannot significantly affect martian photochemistry and tochemistry where the local densities of SO2 may exceed the the photochemical effect of SO2 may be neglected. measured limit by a few orders of magnitude (Wong et al., The derived upper limit may be converted to a limit on 2003). Furthermore, if some vents exist then their products production of SO2 if we calculate its photochemical loss. are quickly blown off, restricted to a few hundred meters in Many of the reactions with SO2 considered in Paper I (see altitude, and do not form a well-developed photochemistry also Wong et al., 2003) result in recycling of SO2 with no up to 100 km. net loss. For example, SO reacts with O, O2,O3, and HO2 Our upper limit of 17 ktons per year to the SO2 produc- and returns SO2; therefore photolysis is not a sink of SO2. tion may be compared with the production of 10 megatons Photochemical loss of SO2 is provided by the reactions of SO2 annually by volcanoes on Earth (Yung and DeMore, SO2 + O + M → SO3 + M; 1999). Currently there is no active volcanism on Mars, and −33 3.6 6 −1 k1 = 2.6 × 10 (T /300) cm s traces of the latest active volcanism are 2 to 100 million years old (Hartmann and Berman, 2000; Sakimoto et al., SO2 + OH + M → HSO3 + M; −31 3.3 6 −1 2003; Neukum et al., 2004). Delivery of SO2 by cometary k2 = 6 × 10 (300/T) cm s . impacts may be calculated using a total mass influx from Reaction 2 is followed by comets to Mars, 1000 t y−1 (Krasnopolsky et al., 2004), and + → + the abundance of sulfur in comet Halley (Jessberger and HSO3 O2 SO3 HO2, −1 Kissel, 1991). The calculated rate is 70 t y of SO2.De- and SO3 quickly absorbs water to form sulfuric acid H2SO4. livery of SO2 by meteorites and interplanetary dust may Sulfuric acid either reacts with dust or precipitates to the sur- be calculated by scaling the delivery of carbon, 1200 t y−1 face and reacts with the rocks. The reaction rate coefficients (Flynn, 1996), to the S/C ratio in meteorites, 0.05 (Anders −1 are taken from Sander et al. (2003) where they are given for and Grevesse, 1989). This results in 330 t y of SO2. air and are doubled to account for the higher efficiency of Therefore, the external sources are weak and negligible. CO2 as a third body. However, delivery of volcanic and other gases from the in- SO2 is uniformly mixed in the atmosphere up to 30 km in terior into the martian atmosphere may be done by seepage Paper I and up to 60 km in Fig. 2 from Wong et al. (2003). with no eruption of lava. Evidence for recent (less than 106 The difference is mainly due to the extremely large abun- years old) groundwater seepage comes from MGS images of dance of SO2, 100 ppm, in Wong et al. (2003) which screens Mars that show features such as gullies (Mellon and Phillips, SO2 from photolysis in the lower atmosphere. The model in 2001). Therefore, our limit refers to seepage of SO2 and the Paper I corresponds to the optically thin conditions which solar UV weathering of sulfates on Mars. Our observation are applicable to our upper limit. Therefore we reduce the shows that the production of SO2 from both groundwater SO2 vertical profile from Paper I by a factor of 10 to fit our and volcanic seepage of SO2 on Mars is weaker than the upper limit. production of SO2 by the terrestrial volcanoes by a factor of Using densities of O, OH, and CO2 and temperature pro- more than 600. Mars may be sulfur-rich but much of the sul- files from model 3 in Krasnopolsky (1995) and the adjusted fur may be locked up in the core as FeS. There are no plate model of Nair et al. (1994), we calculate the total loss of tectonics on Mars, and this also restricts the delivery of sul- SO2 in both models. The reaction with OH is more effective fur and other gases into the atmosphere. in removal of SO2 than the reaction with O by a factor of Our upper limit is also relevant to the origin of methane 6 in both models. The models result in a lifetime of SO2 on on Mars which was recently detected by three independent Mars of 2.4 and 1.5 years, respectively. We adopt the lifetime teams (Krasnopolsky et al., 2004; Formisano et al., 2004; of 2 years. It is longer than the time for global atmospheric Mumma et al., in preparation). The abundance of SO2 in 492 V.A. Krasnopolsky / Icarus 178 (2005) 487–492 terrestrial volcanic gases is a few percent (see above) while Hartmann, W.K., Berman, D.C., 2000. Elysium Planitia lava flows: Crater the abundance of CH4 is a few ppm (Etiope and Klusman, count chronology and geological implications. J. Geophys. Res. 105, 15011–15025. 2002). 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