 MODELING AQUEOUS PERCHLORATE CHEMISTRIES WITH APPLICATIONS TO MARS.  G. M. Marion1 D. C. Catling2, M. Claire3, and K. J. Zahnle4.  1Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512, giles.marion@dri.edu. 2University of Washington, dcatling@u.washington.edu.  3University of Washington, mclaire@astro.washington.edu. 4NASA-Ames, Kevin.J.Zahnle@nasa.gov.           Introduction:  Among the most interesting findings of the Phoenix Mission to Mars was the presence of a perchlorate salt, probably magnesium, in the North Polar region [1-4].  Naturally occurring perchlorate salts are rare on Earth; so rare that no perchlorate salt has yet been given an official "mineral" name [5] despite its known occurrence as a salt in the Atacama Desert in Chile [5-6].  The high solubility of most perchlorates would lead to significantly low eutectic temperatures {e.g., -68.6°C [Mg(ClO4)2.8H2O] and -74.6°C [Ca(ClO4)2.6H2O]} [7] that may play an important role in cold geochemical processes on Mars.  The objectives of this paper were to (1) describe integrating perchlorate chemistries into the FREZCHEM model, and (2) discuss the significance of these salts to Martian biogeochemistry.        FREZCHEM Model:  FREZCHEM is an equilibrium chemical thermodynamic model parameterized for concentrated electrolyte solutions (to ionic strengths > 20 molality) using the Pitzer approach [89] for the temperature range from <-70 to 25°C and the pressure range from 1 to 1000 bars [10].  The current version of the model is parameterized for the NaK-Mg-Ca-Fe(II)-Fe(III)-Al-H-Cl-Br-SO4-NO3-OHHCO3-CO3-CO2-O2-CH4-Si-H2O system and includes 95 solid phases including ice, chloride minerals, sulfate minerals, carbonate minerals, solid-phase acids, nitrate minerals, acid-salts, iron oxides and hydroxides, aluminum hydroxides, silicon minerals, gas hydrates, and bromide sinks.  What is currently lacking in this model for Mars applications are perchlorate salts.        Perchlorate Chemistry:  Pitzer parameters and solubility products for NaClO4.H2O, KClO4, HClO4, Ca(ClO4)2.6H2O (Fig. 1), and Mg(ClO4)2.6H2O and .8H2O (Fig. 2) were integrated into the FREZCHEM model from 25°C to their respective eutectic temperatures, which for the above salts are -32°C, -0.18°C, 60°C, -74.6°C(Fig.1), and -68.6°C(Fig. 2), respectively.  KClO4 is an anomaly for perchlorate salts because of its low solubility, and as a consequence, high eutectic temperature (-0.18°C).        The degree of hydration for Mg(ClO4)2.nH2O is an important question for Mars because it would tie up water to different degrees in the soil.  The "old data" for salts in Fig. 2 [11,12] designated the hydration number as 6, while the "new data" from Pestova et al. [7] designated the hydration number as 8.  If we treat these two datasets as independent and estimate and integrate solubility products for both Mg(ClO4)2.6H2O and Mg(ClO4)2.8H2O into the FREZCHEM model, then the model calculated peritectic (transition between .8H2O and .6H2O) occurs at 33°C, which is in excellent agreement with an earlier estimate of 32°C [13].  It appears that .8H2O is likely to be the dominant Mg(ClO4)2 hydrate for cold aqueous processes on Mars.        Mars Applications:  The newly perchlorateparameterized FREZCHEM model will allow us to simulate natural environments on Mars where perchlorate salts are forming along with associated alkaline carbonates.  But these simulations will ultimately require detailed analysis of released Phoenix Mission reduced datasets that are still in process.  But meanwhile, we can still address broad questions related to perchlorate salts on Mars.        What are the forms of the perchlorate salts that are forming on Mars?  Most indirect evidence to date suggest Mg(ClO4)2 because of substantial amounts of magnesium and perchlorate found by the Phoenix Mission [1,3-4].  These soils also have pH values in the 8-9 pH range suggesting CaCO3 precipitates [2-3].  Because of the relative insolubility of CaCO3 and CaSO4 minerals that are likely present at the Phoenix Mission site, it seems unlikely that perchlorates have precipitated as the highly soluble Ca(ClO4)2 salt.  However, on Earth, it is precisely in cold polar regions that the similar CaCl2.6H2O (antarcticite) mineral naturally forms [14-16].  Therefore, Ca(ClO4)2.6H2O could possibly form on a cold planet such as Mars.  On the other hand, the insolubility of KClO4 argues in favor of this salt as a possible sink for perchlorate in solutions that dry out.  In support of this argument are the observations that perchlorates are often found associated with K-rich materials such as potash (K2CO3), playa crusts, and plant materials [5-6].        On Earth in arid environments, perchlorates are often found associated with nitrate salts where these nitrates, such as those in the Atacama, are deposited from atmospheric sources [17]. This analog suggests that perchlorate may be a useful tracer of atmospheric processes. But solution chemistry will be necessary to understand any subsequent aqueous or thermodynamic transformation of the perchlorate. Given that perchlorate is a useful metabolite for certain microbes while it also is a contaminant for drinking water [18], modeling its behavior is important for future Mars missions both to look for life and safely explore Mars with humans.      The bottom line is that the latest version of FREZCHEM, which now contains perchlorate chemistries, provides  a means to address these  biogeochemical  issues..         Acknowledgments: We thank Lisa Wable for help preparing this abstract.  Funding was provided by a     NASA DDF project on "Volcanic SO2, atmospheric photochemistry, and climate on early Mars."       References:  [1] Fisher D.A. et al. (2008) AGU Fall Meet., Abstract U11B-0019.  [2] Hecht M.H. et al. (2008) AGU Fall Meet., Abstract U14A-04.  [3] Smith P.H. et al. (2008) AGU Fall Meet., Abstract P13F-01.  [4] Stoker C. et al. (2008) AGU Fall Meet. Abstract U11B-0018.  [5] Orris G.J. et al. (2003) USGS Open-File Report 03-314.  [6] Trumpolt C.W. et al. (2005) Remediation, 16, 65-89.  [7]Pestova O.N. et al. (2005) Russ. J. Appl. Chem. 78, 409-413.  [8] Pitzer K.S. (1991) Activity Coefficients in Electrolyte Solutions, CRC Press.  [9] Pitzer K.S. (1995) Thermodynamics, McGraw-Hill.  [10] Marion G.M. and Kargel J.S. (2008) Cold Aqueous Planetary Geochemistry with FREZCHEM, Springer.  [11] Nicholson D.E. and Felsing W.A. (1950) J. Am. Chem. Soc. 72, 4469-4471.  [12] Linke W.F. (1965) Inorganic and Metal Organic Compounds, Vol II, Am. Chem. Soc.  [13]  Besley L.M. and Bottomley G.A. (1969) J. Chem. Thermodynamics, 1, 13-19.  [14] Herut B. et al. (1990) Geochim. Cosmochim. Acta, 54, 13-21.  [15] Marion G.M. (1997) Antarctic Sci. 9, 92-99.  [16] Starinsky A. and Katz A. (2003) Geochim. Cosmochim. Acta, 67, 1475-1484. [17] Michalski G. et al. (2004) Geochim. Cosmochim. Acta 68, 4023-4038.  [18] Coates J.D. and Achenbach L.A. (2004) Nature Reviews/Microbiology 2, 569-580.            	LPSC09FINAL
