Possible Atmospheric Origins of Perchlorate on Mars. Dc
Total Page：16
File Type：pdf, Size：1020Kb
40th Lunar and Planetary Science Conference (2009) 1567.pdf POSSIBLE ATMOSPHERIC ORIGINS OF PERCHLORATE ON MARS. D. C. Catling1,2, M. W. Claire3, R. C. Quinn4, K. J. Zahnle5, B. C. Clark6, S. Kounaves7, M. H. Hecht8. 1Dept. Earth Sciences, University of Bristol, Bristol, United Kingdom. 2Earth and Space Sciences/Astrobiology Program, University of Washington, Box 351310, Seattle WA 98195 ([email protected]). 3Dept. Astronomy/Astrobiology Program, University of Washington, Seattle WA 98195. 4SETI Institute/NASA Ames Research Center, Moffett Field, CA 94035. 5NASA Ames Research Center, Moffet Field, CA 94035. 6Space Exploration Systems, Lockheed Martin, Denver, CO 80201. 7Dept. of Chemistry, Tufts University, Medford, MA 02035. 8JPL/Caltech, Pasadena, CA 91109. Introduction: The wet chemistry experiment on playas [4]. Perchlorate is also found in carbonates of NASA’s Phoenix lander had cells where ~1 cm3 of the upper Eocene Mission Valley Formation of San Martian soil was mixed with 25 cm3 of dilute leaching Diego [5] and sylvinite (KCl) ore in central Canada solution and electrochemical measurements were made and Permo-Triassic New Mexico [4]. to identify a variety of soluble ions. Surprisingly, per- Isotopic analysis allows Atacama perchlorate to be - chlorate (ClO4 ) was measured as a dominant anion [1]. definitively attributed to an atmospheric source. Non- For an assumed soil sample mass ~1g, the soluble per- mass dependent (NMD) fractionation occurs in the - chlorate concentration converts to ~1wt% in the soil oxygen isotopes of ClO4 . This is ascribed to the pho- [1]. tochemical reaction of ozone with volatile chlorine These data suggest that most of the chlorine in the species because perchlorate inherits its distinctive soil at the Phoenix site has been oxidized to perchlo- NMD oxygen isotope signature from O3. Specifically, 17 rate, in which chlorine attains a maximal +7 oxidation the Δ O of perchlorate (4.2-9.6‰) compared to O3 number. From a thermodynamic perspective, perchlo- (30-40‰) suggests that O3 contributes one oxygen - rate is a strong oxidizing agent but kinetically, perchlo- atom in four to perchlorate[6]. Atacama ClO4 also has rate is very inert at typical planetary surface tempera- δ37Cl of -14.5 to -9.2‰, which is the lowest δ37Cl of tures. The tetrahedral coordination of oxygen atoms in any common substance on Earth and consistent with perchlorate is an energetically stable geometry around atmospheric fractionation[7]. The photochemistry to - the central, electrophilic chlorine atom and renders produce atmospheric ClO4 is not fully known. How- perchlorate unreactive. Hence, perchlorate, once ever, once chlorine oxides are generated from chlorine formed, is prone to persist in soil provided that it is in a inputs to the atmosphere, reactions initiated by O3 can dry environment where liquid water cannot wash it produce perchloric acid (HClO4), which can deposit away, given that perchlorate salts are generally highly onto the surface. For example, one possible scheme is soluble. (If an environment is wet, perchlorate is liable OClO + O3 → ClO3 + O2 to accumulate in ground water). Thus, the aridity of OH + ClO3 + M → HClO4 + M Mars, combined with perchlorate being an unreactive, In the Atacama, a variety of Cl sources have been dis- dead-end of oxidation chemistry, probably explains cussed: Volcanic HCl from the Andes, aeolian chlo- why Phoenix discovered relatively abundant perchlo- rides where heterogeneous reactions liberate chlorine rate. Here, we consider the origin of Martian perchlo- volatiles, or methyl chloride from marine sources. rate and suggest that a likely source is atmospheric, Perchloric acid (HClO4) is a stable end-product of associated with the production of oxidants. If this is so, atmospheric chemistry due to its resistance to photoly- perchlorate will act as a tracer of past atmospheric sis [8]. Consequently, HClO4 also occurs in the dry chemistry (and possibly volcanic gas fluxes), which environment of Earth’s stratosphere at abundances of could be important in future missions such as sample 0.5-5 ppt in sulfate aerosols [9]. return or polar ice drilling. Possible origins of perchlorate on Mars: Occurrence and origin of natural perchlorate on Terrestrial perchlorate is a useful analog and suggests Earth: In considering the origin of perchlorate on that perchlorate forms and persists on Mars because of Mars, it is appropriate to start with what we know oxidizing chemistry and a desiccating environment. A about perchlorate on Earth. Most terrestrial perchlorate well known source of oxidizing power on Mars is the is manmade for uses such as fireworks and rocket pro- atmosphere, where photochemistry generates a variety pellants. In contrast, natural perchlorate is rare and is of oxidizing species[10]. Fluxes of H2O2 and O3 are found only in the driest places: unusually arid deserts calculated to dominate the oxidizing fluxes to the sur- and the stratosphere. Natural perchlorate occurs in the face, where the respective fluxes of these two species Atacama at levels of 0.03 to 0.6wt% [2] and is also are roughly equivalent in terms of oxidizing power - reported from deserts of West Texas [3] and Bolivian [11]. Other oxidants include OH, HO2, O and O . 40th Lunar and Planetary Science Conference (2009) 1567.pdf Meanwhile ultraviolet action on minerals can produce rate to far exceed chloride. Either more efficient oxida- free radicals, further providing an environment condu- tion pathways (such as gas phase reactions) are implied cive to oxidation. or multiple events over geologic time with high cumu- Table 1 summarizes broad classes of oxida- lative efficiency would be required. tive processes that could generate perchlorate on Mars. Flux Constraints: An atmospheric origin of We also identify two possibilities for chlorine sources perchorate sets an upper limit on the perchlorate pro- that lead to perchlorates: chlorine-containing gases duction rate. Mars’ overall oxidation flux is set by the released into the atmosphere that take part in gas phase rate that hydrogen escapes to space. Theory and evi- reactions or chlorides in minerals (aerosol or surface dence (discussed in [11]) suggests that hydrogen es- soil particles) that participate in heterogeneous reac- capes from Mars close to the diffusion-limited rate of tions. Volatile chlorine must have occurred in the past ~3.7×108 H atoms cm-2 s-1. This translates to a net oxi- on Mars as volcanic HCl and also could have been -9 -2 -1 dation flux of 4.8×10 mol O2 cm (Earth yr) . If we generated by acid displacement reactions with salts in assume that all the perchlorate is in the form of acidic, near-surface aqueous environments (e.g., CaCl 2 Mg(ClO4)2, and all oxidation is used to make perchlo- + H2SO4 = 2HCl + CaSO4). If the Cl derives from vol- rate, we obtain an upper limit on perchlorate produc- canism, the required flux turns out to be quite modest. -9 -2 -1 8 -1 tion as <1.2×10 mol cm yr . This upper limit sets a A source of ~10 mol HCl yr over ~1 Myr could pro- minimum age constraint on the perchlorate at the duce ~1wt% Mg(ClO4)2 in 10 cm of soil (with density Phoenix site. If we assume 1%wt Mg-perchlorate ~1g cm-3) over ~10% of Mars’ surface. On Earth, the mixed to ~5 cm depth of soil above ice, the perchlorate volcanic HCl/SO ratio is 1/20 and the SO flux is 2 2 at the Phoenix site cannot be younger than ~0.2 Myr. ~5×1011 mol yr-1, so Earth releases ~2×1010 mol HCl -1 yr . Hence Mars needs to generate only 0.5% of the Table 1: Possible mechanisms to produce perchlorate terrestrial volcanic HCl flux for 1 Myr to generate our on Mars. In all cases, the ultimate source of oxidation distribution scenario above. Chlorine gas may also be is atmospheric. sourced from aerosols. On Earth, OH radicals react at Mechanism Chlorine source the deliquesced water-gas interface of seasalt particles - Gas phase pho- - Volcanic HCl to release chlorine (OH⋅(g) + Cl (aerosol) → 0.5Cl2(g) tochemical reac- - HCl liberated from acid dis- - + OH (aerosol)) but the liquid phase seems essential tions with oxi- placement reactions [12]. If any similar reactions took place on Mars, Cl 2 dants such as O3 - Cl atoms liberated from aerosol would rapidly photolyze to reactive chlorine atoms. chloride If ozone is specifically required to create Mar- Electrostatic - Aerosol or surface soil chlorides tian perchlorate, as it is on Earth, perchlorate formation discharge - Volatile species as above might be enhanced in polar environments due to the meridional distribution of ozone. Observations by ul- Gas-solid reac- - Aerosol or surface soil chlorides traviolet spectrometers on Mariner 9 and Mars Express tions with oxi- show that ozone is most abundant in the spring-winter dants circumpolar regions but non-detectable or barely de- References: [1]. Hecht, M. H. et al. (2009) Sci- tectable in the tropics during all seasons[13, 14]. ence, submitted. [2]. Ericksen, G. E. (1981) USGS Dry, gas-solid reactions of UV-activated chlo- Prof. Paper No. 1188. [3]. Dasgupta, P. K. et al. rides with oxidizing atmospheric species at the ground (2005) Environ. Sci. Tech. 39, 1569. [4]. Orris, G. J. et or on aerosol surfaces may be alternative ways to pro- duce perchlorate without volatile chlorine species but al. (2003) USGS Open File Report 03-314. [5]. labwork under Mars-like conditions is required to con- Duncan, P. B. et al. (2005) Environ. Forensics 6, 205. firm that such pathways exist and to assess their effi- [6]. Bao, H. M., Gu, B. H. (2004) Environ. Sci. Tech. ciency. A role for ice as a reaction surface for adsorbed 38, 5073. [7]. Bohlke, J. K. et al. (2005) Anal. Chem. species cannot be excluded since it is an effective ad- 77, 7838.