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So2 So So3 H2so4 Hso S H2s Hs CHEMISTRY RELATED TO POSSIBLE OUTGASSING SOURCES ON MARS. A. S. Wong, S. K. Atreya and N. O. Renno, Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, MI 48109-2143 ([email protected]). Introduction: An earlier paper, “Chemical mark- which the surface mixing ratio of each outgassed spe- ers of possible hot spots on Mars” by A. S. Wong, S. cies is set to 100 ppm (Figure 2). The resulting mixing K. Atreya and Th. Encrenaz [1], explored the modifi- ratios and column abundances of SO2, H2S, and SO are cation of the atmosphere of Mars following an influx summarized in Table 1. All sulfur species scale more of methane, sulfur dioxide and hydrogen sulfide (CH4, or less proportionally with the amount of outgassing SO2, H2S) from any outgassing sources which are re- species. ferred to as hot spots. The feasibility of detection of the new species by Planetary Fourier Spectrometer on O2 Mars Express is reported in a subsequent paper, “At- HSO SO2 mospheric photochemistry above possible martian hot O ,HO hν O ,OH,O spots” by A. S. Wong, S. K. Atreya, V. Formisano, Th. 3 2 2 Encrenaz and N. Ignatiev [2]. This abstract is a fol- HS SO SO3 low-up on the previous two papers. Here we treat the O O3 effect of any outgassed halogens rigorously. We also hν hν O H2O make estimates of dilution factors relative to the source 2 location following convection and meridional trans- H S S H SO port. 2 2 4 The typical terrestrial outgassing species include H2O, CO2, SO2, H2S, CH4, and small amounts of halo- gens. None of the sulfur, hydrocarbon and halogen condense species have yet been detected in large-scale surveys Figure 1. Important reaction pathways for sulfur spe- of the martian atmosphere. Any localized and tran- cies in the martian atmosphere. sient sources, if present, are likely to go undetected in such observations due to dilution following transport away from source and the relatively short photochemi- cal lifetimes of the species. With the above in mind, and due to considerable interest in the question of ex- tinct or extant life on Mars, we have developed a one- dimensional photochemical model, considering the possibility that SO2, H2S, CH4, and HCl from any out- gassing sources may be introduced into the martian atmosphere. From this model we predict the abun- dances of the various new molecules that are expected to be formed in the ensuing atmospheric chemistry. The hot spot photochemical model is based on a glob- ally averaged standard martian atmosphere model which includes all common atmospheric species in- cluding nitrogen species, found between the ground Figure 2. Calculated number density profiles of im- and 220 km. A detailed discussion of the model can portant sulfur species, assuming 100 ppm each of H2S be found in [1]. and SO2 is outgassed at the surface of Mars. Sulfur Chemistry: The presence of SO2 and H2S in the atmosphere of Mars has been speculated previ- Once in the atmosphere, SO2 will be photodissoci- ously [3, 4]. The upper limits of SO2 and H2S are 0.1 ated to produce SO. The reaction of SO with O2 or ppm each for globally averaged observations [5]. In OH recycles some SO2, while the reaction with O3 our model, we have both gases outgassing from the forms SO3, which quickly combines with water vapor surface together, and we vary the concentration of to form H2SO4 and subsequently condenses in the lower atmosphere. Outgassed H2S is rapidly converted each from 0.1 ppm to 100 ppm. The principal reaction pathways are shown in Figure 1. Only for the purpose to SO2 and the reactions follow that of SO2 (see Figure of illustration, we present the result of one case in 1). The photochemical lifetime of SO2 is 160 days, source flux is reasonably large. On the other hand, and that of H2S is 9 days. CH2O has a relatively short lifetime of several hours. Therefore, if the previous tentative detection of CH2O Table 1. Summary of possible detectable species from (0.5 ppm, [6]) is confirmed by future observations, it martian hot spots, their mixing ratios (at 10 km), col- would imply a nearly continuous supply of methane to umn abundances (above 10 km), and photochemical the atmosphere. Moreover, the amount of methane lifetimes, assuming 100 ppm of SO2, H2S and CH4, required would have to be much greater than the and 1 ppm of HCl are outgassed from the surface of “global” upper limit of 20 ppb, since the models pro- Mars. duce only 30 ppt of CH2O with 100 ppm of CH4. On the other hand, CH2O itself could also be outgassed Species Mixing Column Photo- from the interior, or formed in surface reactions. ratio at 10 abundance chemical -2 km (cm ) lifetime CH4 -4 19 SO2 1.7×10 1.8×10 180 days hν hν ν ν -5 17 HCl h h H2S 1.8×10 7.4×10 9 days Cl OH SO 1.7×10-7 7.8×1016 4.6 hours O(1D) -4 19 CH3 H CH4 1.0×10 1.0×10 670 years C2H6 CH3 3CH2 CH -11 13 CH2O 2.8×10 1.6×10 7.5 hours O hν,OH CO2 O2 CH OH 4.3×10-12 8.2×1011 74 days 3 HO C H -12 11 25 days 2 hν 2 6 6.4×10 9.6×10 C H & OH CH2O CHO -5 17 2 n HCl 1.0×10 1.0×10 5.7 years products CH3O O2 O H O,O2 Hydrocarbon Chemistry: If present, methane CH OH hν CO CO could be of geochemical or biological origin. The pre- 3 2 sent upper limit of CH4 from global surveys is 0.02 ppm [5]. Our model calculates the abundances of new Figure 3. Important reaction pathways of hydrocar- species that get formed upon the introduction of CH bon and halogen chemistry in the martian atmosphere. 4 into the martian atmosphere by outgassing, and the important pathways are shown in Figure 3. For the purpose of illustration, the result for a case with 100 ppm CH4 is shown in Figure 4. The most abundant species are CH4, CH2O, CH3OH, and C2H6, and the resulting mixing ratios and column abundances are summarized in Table 1. At altitudes above 80 km, CH4 is photodissociated 3 1 into CH, CH2, CH2 and CH3, and its oxidation by 1 3 O( D) forms CH3. CH2 reacts with CO2 to form CH2O. At the low altitudes, the oxidation of methane by OH forms CH3. CH3 reacts with O to form CH2O, reacts with OH to form CH3OH, and recombines to form C2H6. The abundances of hydrocarbons derived from the methane photochemistry are very low even Figure 4. Calculated number density profiles of im- when the concentration of methane is relatively large portant hydrocarbon species, assuming 100 ppm of CH is outgassed at the surface of Mars. in the atmosphere. The reason is that CH4 does not 4 dissociate efficiently to start the hydrocarbon chemis- try. Photolysis of CH4 is largely shielded by the much Halogen Chemistry: In terrestrial outgassing, more abundant CO2 whose absorption cross section only a very small amount of halogens (HCl, Cl2, etc.) overlaps that of CH4. In the lower atmosphere, the are released from outgassing processes. The total most effective way to break down methane is by reac- amount of halogens comprises a small fraction, about tion with OH, which is quickly removed by CO and O. 1%, of SO2, mostly in the form of HCl [7]. On Mars, -9 The photochemical lifetime of CH4 is about 670 years, the present global upper limit of HCl is 2×10 [8], which is the longest amongst the principal hydrocar- which is presumably the most abundant halogen. Our bon species. Therefore, CH4 may be easier to detect model finds that with up to 1 ppm of HCl outgassing, even far from the source region, provided that its the direct effect of halogens on the “standard” atmos- matter. Methane and its products could serve as possi- phere, as well as on sulfur chemistry, is negligible. ble biomarkers in the martian atmosphere as well. We investigate the effect of halogens on hydrocar- Halogens would probably outgas in very small bon chemistry. When HCl is released into the atmos- amounts, and their only noticeable effect on the at- phere together with methane, it is broken down bythe mosphere and outgassing chemistry is the slight in- solarultraviolet photonsat λ≤230 nm and OH radicals crease of hydrocarbon photoproducts. to form the atomic chlorine. Chlorine reacts in a simi- In the relatively short time-scale of about an hour, lar way to OH, enhancing the breakdown of methane ordinary convective processes alone can reduce the 4 into CH3 (Figure3) and subsequently increasing the mixing ratios of outgassed species by factors of 10 – production of other hydrocarbons. Our model shows, 105, spreading the outgassed species over distances of with 1 ppm of HCl outgassing alongside with 100 ppm up to 50 km away from the source. In addition, the of CH4, the column abundances of CH2O, CH3OH, and time-scale for spreading the outgassed species over the C2H6 are increased by 6, 2 and 3 times, respectively, entire planet (with corresponding dilution factor of compared to the case of no HCl outgassing (Figure 5). ~108) would be approximately one year. In view of The overall effect on CH4 is minimal. The lifetime of these time scales and the relatively short photochemi- HCl is calculated to be 5.7 years.
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