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Formation of Perchlorate from Chlorine Species Under Simulated Mars Conditions

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Formation of Perchlorate from Chlorine Species Under Simulated Mars Conditions Eighth International Conference on Mars (2014) 1020.pdf FORMATION OF PERCHLORATE FROM CHLORINE SPECIES UNDER SIMULATED MARS CONDITIONS. B. L. Carrier* and S. P. Kounaves, Department of Chemistry, Tufts University, Medford, MA, 02155, USA. ([email protected]). Introduction: an ongoing mechanism for perchlorate formation from - Perchlorate (ClO4 ) was first discovered on Mars at chloride, and likely indicates the probable presence of the Phoenix landing site in 2008 at concentrations of other oxychlorine intermediates and radicals such as - - ● ● 0.4 to 0.6 wt % [1]. More recently results from Mars OCl , ClO2 , ClO2(g), OCl, Cl and others. The pres- Science Laboratory (MSL) pyrolysis experiments per- ence of these intermediates has serious implications for formed by the Sample Analysis at Mars (SAM) in- the survival and detection of organics as most of these strument suite indicate the presence of an oxychlorine intermediates, unlike perchlorate, are strong oxidizing species in samples collected at Rocknest, Cumberland agents at Mars ambient conditions. All of this seems and John Klein sites, with calcium perchlorate being to indicate the presence of a complex chlorine cycle on the most likely candidate [2,3]. Perchlorate has also the martian surface. been identified in martian meteorite EETA79001 [4] Our current reseach aims to investigate factors that and has been proposed as a possible explanation for the may influence the rate of perchlorate formation such as detection of chlorinated hydrocarbons at the Viking UV dosage, exposure time, relative humidity, concen- landing sites [5]. Taken together this seems to indicate tration of atmospheric oxidants and effects of direct a global distribution of perchlorate on the martian sur- exposure to UV photons vs. exposure to atmospheri- face. The presence of perchlorate has wide ranging cally formed oxidants only. implications in terms of the oxidizing nature of the soil, the history of water on mars, the planet’s current Experimental: water cycle, formation of liquid brines, and for the Perchlorate formation experiments have been per- preservation and detection of organics on the surface. formed in a Mars Simulation Chamber (MSC) which is On Earth perchlorate is formed primarily through capable of simulating typical Mars surface conditions the oxidation of atmospheric chlorine by ozone and/or such as temperature, pressure, atmospheric composi- oxygen containing radicals found in the stratosphere. tion and UV flux. Since Mars currently lacks a source of atmospheric The MSC consists of a stainless steel cylindrical chlorine this type of mechanism would only allow for chamber with an internal diameter of 60 cm and a formation of perchlorate in the distant past and would depth of 45 cm. A Mars simulant gas mixture (95.3% not support ongoing perchlorate formation. A recent CO2, 2.7% N2, 1.6% Ar, 0.13% O2) is introduced into photochemical model based on terrestrial reactions the chamber at a constant rate (8.25 SCCM) through a thought to be relevant on Mars over the past 3 byr sug- mass-flow controller. The pressure in the chamber is gests that this atmospheric formation mechanism controlled via a vacuum pump and pressure transducer would be insufficient by several orders of magnitude to and can be regulated to typical Mars atmospheric pres- explain the amount of perchlorate found by Phoenix sure of ~7.5 ±0.1 mbar. and Curiosity [6]. It has also recently been shown that Temperature is maintained using a stainless steel perchlorate is broken down over time by ionizing radi- cold plate coupled to a recirculating water/refrigerant ation to form less oxidized chlorine species such as chiller. Temperature for perchlorate formation exper- - hypochlorite (OCl ) and chlorine dioxide (ClO2) [7]. iments was maintained at -15 °C. UV radiation is de- This seems to indicate that there must be an ongoing livered via a series of filters through a fused silica mechanism for perchlorate production on Mars to ex- window located on top of the MSC by a Newport Oriel plain the high concentrations present on the surface. 1000W Xe-arc lamp in order to simulate the spectrum One possible pathway is the heterogeneous reaction of UV radiation incident on the martian surface. UV of soil chlorides with atmospherically produced oxi- dosage is measured over the course of the experiment dants or oxidants generated photochemically at the using a Stellarnet UV spectrophotometer. surface. Our current research shows that perchlorate, All samples were analyzed for perchlorate, chlorite - as well as chlorate (ClO3 ), can in fact be formed and chlorate using a Dionex ICS-2000 ion chromatog- through UV-irradiation of chloride salts on mineral raphy system. Perchlorate analysis was performed surfaces under simulated Mars conditions. Further using a Dionex AS-16 analytical column. Chlorite and research has shown that oxychlorine ions such as chlo- chlorate analysis were performed using a Dionex AS- - - rite (ClO2 ) and chlorate (ClO3 ) can also be oxidized to 18 analytical column. perchlorate under the same conditions. This suggests Eighth International Conference on Mars (2014) 1020.pdf - Results and Discussion: All samples were found to contain both ClO3 and - - - ClO4 but not ClO2 . This may indicate that ClO2 is - - Perchlorate formation from chlorite and chlorate not an intermediate in the oxidation of Cl to ClO4 Our results show that perchlorate can be formed on under typical Mars conditions. The amounts of chlo- mineral surfaces under Mars conditions from chloride, rate and perchlorate formed in these experiments were chlorite and chlorate starting materials. Figure 1 de- found to increase with increasing UVC radiation dos- - scribes the percentage of Cl converted from ClO2 to age, as one might expect. The amount of chlorate - ClO4 as a function of exposure time while Figure 2 formed was several orders of magnitude higher than - shows the percentate of Cl converted from ClO3 start- the amount of perchlorate formed in all cases (see Ta- ing material under the same conditions. As expected ble 1). the amount of conversion to perchlorate increases with One sample was run consisting of only 0.4 g NaCl increasing exposure time. The amount of perchlorate in a pyrex dish, and the amount of perchlorate formed formed from chlorite was about an order of magnitude was below the LOQ of the IC, but was approximately of higher than the amount of perchlorate formed from 5x lower than the amount formed when Cl- was depos- chlorate at the same exposure times. ited on a SiO2 substrate. This indicates that a mineral Experiments were conducted with and without the substrate increases the rate of perchlorate formation, presence of water vapor in the MSC and no significant most likely by allowing for adsorption of atmospheri- difference was found, suggesting that water vapor and cally formed oxidants on the surface and increasing hydroxyl radicals formed in situ do not play an im- their interaction with chloride. portant role in the oxidation of chlorite and chlorate to - - perchlorate. Initial ClO3 formed ClO4 formed UVC (J) Cl- (g) (ug) (ng) - - ClO4 formed from ClO2 0.398 3.16 0.137 80 - 4 2.00E-04 0.400 3.21 0.217 84 1.50E-04 0.398 3.50 0.257 96 Table 1. Formation of ClO - and ClO - from simulants 1.00E-04 3 4 consisting of SiO2 and Cl- at appx 0.4 %. 5.00E-05 % Cl convertedCl % toClO 0.00E+00 Conclusion: Our preliminary experiments have 0 6 12 18 24 30 shown that perchlorate can be formed from chloride on Exposure Time (hours) mineral surfaces under current Mars conditions, which may explain the abundance of perchlorate found on the Figure 1. % of Cl (as ClO -) converted toClO - as a function of time 2 4 martian surface by Phoenix and MSL. Perchlorate is in the MSC . formed more quickly from chlorite or chlorate starting - - ClO4 formed from ClO3 materials suggesting an initial slow step in the oxida- - 4.00E-05 tion of chloride to perchlorate. 4 All of this points to a potentially much more com- 3.00E-05 plex chlorine chemistry at the martian surface than as 2.00E-05 is currently understood. This has implications for the formation of liquid brines on Mars, as well as for the 1.00E-05 % % Cl converted to ClO survival and detection of organics on the surface. 0.00E+00 0 6 12 18 24 30 36 42 48 54 References: Exposure Time (hours) [1] Hecht M. H. et al. (2009) Science, 325, 64–67. Figure 2. % of Cl (as ClO3-) converted to ClO4- as a function of [2] Morris R. V. et al. (2014) LPSC XLV, Abstract time in the MSC. #1319. [3] Leshin L. A. et al. (2013) Science, 341, DOI: 10.1126/science.1238937. [4] Kounaves S. P. et Formation of perchlorate from chloride al. (2014) Icarus, 229, 206–213. [5] Navarro-Gonzalez All chloride containing samples were irradiated in R. et al. (2010) J. Geophys. Res., 115, E12010. [6] the MSC for a period of 7 days (168 hours). Total Smith M. L. et al. (2014) Icarus, 231, 51-64. [7] Quinn sample mass was ~10 g and consisted of ground SiO2 R. C. et al. (2013) Astrobiology, 13(6), 515-520. - (50-70 mesh) coated with 0.4% Cl . Samples were - - - then analyzed for ClO2 , ClO3 and ClO4 formation. .
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