 EXPERIMENTAL EVALUATION OF PHOTOCHEMICAL INFLUENCES ON BROMINE AND CHLORINE GEOCHEMISTRY ON MARS.  Yuyan Zhao and Scott M. McLennan, Department of Geosciences, Stony Brook University, Stony Brook, NY 11794-2100 (zhao_yuyan@yahoo.com; scott.mclennan@sunysb.edu).  Introduction:  The distribution and geochemical behavior of halogen elements bromine (Br) and chlorine (Cl) on the Martian surface are intriguing questions, yet poorly understood. On Earth, halogens are well established tracers for providing important constraints on surficial processes and Br is indeed the most important trace element for evaluating evaporative processes [1]. Hence, when large amounts of Br and Cl data were returned from Mars [2,3], it was expected that we could learn some new insights into surficial processes on Mars. To date, interpretations of Cl and Br variations have focused almost exclusively on their behavior in aqueous fluidmineral systems [4,5,6]. However, recently, several new lines of evidence suggest that photochemical processes may volatilize Br and Cl from fluids and sediments into the atmosphere and subsequently influence their distribution on Mars. On Mars, new hints come from the following: (1) In situ measurements made by the alpha particle X-ray spectrometer (APXS) onboard the Mars Exploration Rovers (MER) reveal that Br/Cl ratios of RATed rock samples are highly variable but primarily controlled by  Br abundances (Figure 1) [7]. In soil samples, Br concentrations of the subsurface soil at both landing sites are consistently higher than the surface (Table 1), which raises the question of whether Br in the topmost soil is lost to the atmosphere.  (2) Mars Odyssey gamma ray spectrometer (GRS) mapping of the equatorial and mid-latitude distribution of near-surface Cl abundances [3] demonstrated that Cl is widely distributed, which could be consistent with an atmospheric influence.  (3) The Phoenix mission detected substantial concentrations of perchlorate in the water-soluble fraction of soils at the north polar landing site [8], which suggests photochemical processes influencing the surface chemistry and distribution of chlorine [9]; processes that could also influence bromine [10]. On Earth, recent direct observations of Br volatilizing into Earth's troposphere by photochemical related heterogeneous reactions of bromide from seawater and sea ice, as well as terrestrial brines and saltpans, greatly reinforce the suggestion that such mechanisms have been previously underappreciated. In addition to well known volatilization of Br from aerosols at the marine boundary layer, new studies also indicate that Br is preferentially transferred into the atmosphere by simple evaporation of brines [11,12] and possibly directly from saltpans in arid environments [13], possibly resulting in significant Cl and Br fractionation from each other.  In the case of the Martian surface, an environment which includes longtime exposure to high solar UV fluxes, possible presence of oxidants, possible saltpans suggesting highly concentrated brines, surface ice deposits, and global scale transfer of dust, it is possible that photochemical processes may play an important role in influencing global halogen element distributions.  Accordingly, we have initiated a series of laboratory experiments under UV light, to examine the behavior of Br and Cl in Martian brine and brinesediment mixtures during evaporation, in order to evaluate whether or not photochemical processes may have played a role in their volatilization and fractionation. And if so, to evaluate what factors may affect these processes.   Figure 1. Rock RAT analyses at Meridiani Planum demonstrate Br/Cl ratios are dominantly controlled by Br variation.   Experiment Design and Analysis Methods:  Two types of photochemical evaporation experiments are being carried out. The first type is evaporation of bromide and chloride-bearing sulfate brine under UV light. The experiment is designed to observe whether bromide and chloride will volatilize during brine evaporation and salt precipitation and whether Br and Cl sequentially fractionate from each other.  During the experiment, the evaporating solution is continuously sampled and monitored until dry. The second type is evaporation of a brine-sediment mixture under UV light. This experiment is aimed to reproduce the work of Wood and Sanford [11],  who observed major atmospheric transfer of Br and fractionation of Br from Cl during evaporation of a brine-sediment mixture, under the conditions more representative of Mars. We evaporate the mix to dryness, and then redissolve and analyze evaporated solids for chloride and bromide concentration change. Condensed vapors from the experiment can also be collected and analyzed for halogen oxide species, if found at significant levels.   For these experiments, the effects of temperature, atmospheric composition, brine composition and pH, sediment grain size as well as wavelength of UV light source are examined. In addition, we also explore the stability of substituting bromide in chloride minerals under UV light, by direct exposure of synthesized Brbearing chloride minerals under the UV source for ~100 hours, to see how bromide and chloride concentrations in the minerals would change after UV exposure.       Figure 2. Experimental apparatus used in this study.   The apparatus used in these evaporation experiments is illustrated in Figure 2. The 1-liter reaction vessel, with a quartz encased low temperature (254 nm) UV light source inserted directly into the chamber, is built to withstand high-vacuum conditions. The temperature during evaporation is controlled by circulating water in the outer jacket of the vessel. During the experiment, temperature and relative humidity are monitored by a hygrometer. The gas phase produced in the experiments are exhausted into a cooling system where it can be condensed and sampled. The starting materials, including brines or brinesediment mixtures, are placed in a quartz dish in the chamber, where they can be easily accessed and monitored.  The whole chamber setting is covered with heavy foil to avoid interference from other light sources as well as to eliminate UV leaks.   Gusev Plains Meridiani Planum  Surface Subsurface Surface Subsurface Br (ppm) 55 ± 37 65 ± 30 51 ± 11 148 ± 69 Cl / Br 28 ± 15 17 ± 7 21 ± 11 7.2 ± 4.5 n 15 15 26 12  Table 1. Comparison of Br and Cl/Br for surface and subsurface soils on Mars. Data compiled as averages and 95% confidence intervals (data from [7,14,15,]).   References: [1] Warren, J. K. (2006) Springer-Verlag (Berlin), pp1035. [2] Rieder, R., et al. (2004) Sci., 306, 1746. [3] Keller, J. M., et al. (2007) JGR, 111, E03S08.[4] Clark, B. C., et al. (2005) EPSL, 240, 7394.[5] Rao, M. N., et al. (2005) JGR, 110, E12S06. [6] Marion, G. M., et al. (2009) Icarus, 200, 436-445. [7] Brückner, J., et al. (2008) Cambridge Univ. Press, pp 628. [8] Hecht, M. H., (2009) Sci., 325, 64-67. [9] Catling, D. C., et al. (2010) JGR, 115, E00E11. [10] Finlayson-Pitts, B. J. (2010) Anal. Chem., 82(3), 770776. [11] Wood, W. W. and Sanford, W. E. (2007) Geophys. Res. Lett., 34, L14405. [12] Smoydzin, L. and von Glasow, R. (2009) Atmos. Chem. Phys., 9, 5057-5072. [13] Hönninger, G., et al. (2004) Geophys. Res. Lett., 31, L04101. [14] Gellert, R., et al. (2006) JGR, 111, E02S05. [15] Ming, D. W., et al. (2008) JGR, 113, E12S39. The Cl-, Br- and SO42- concentration of evaporating solutions are analyzed by Ion Chromatography (IC). Other halogen oxide species, if present in condensed fluids, can be measured by IC-Mass Spectrometry (ICMS). Cation concentrations of the solution are analyzed by Direct Current Argon Plasma Atomic Emission Spectrophotometry (DCP-AES). Synthesized chloride minerals are ground and washed with ethanol or acetone to eliminate possible fluid inclusions. The solid sample phases are identified by X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Microanalysis (EDS). At the time of writing, these experiment are at various stages of completion and we will report preliminary results at the meeting.  
