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

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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 spectrom- eter 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 0019-1035/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2005.05.006 488 V.A. Krasnopolsky / Icarus 178 (2005) 487–492 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.
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