47th Lunar and Planetary Science Conference (2016) 2754.pdf

PHOSPHATE RELEASE: THE EFFECT OF PREBIOTIC ORGANIC COMPOUNDS ON DISSOLUTION OF MARS-RELEVANT MINERALS C. L. Bartlett1, E. M. Hausrath1, and C. T. Ad- cock1, 1Department of Geoscience, University of Nevada, Las Vegas, NV 89154, [email protected]

Introduction: Phosphate is essential for life: it synthesized following [12] by heating whitlockite is required to stabilize RNA, DNA as well as phos- for 24-48 hours at 1095 ºC. Chlorapatite was syn- pholipid membranes [1]. Phosphorus, either as thesized using a weight ratio of 6:7.5 hydroxyapatite phosphate or a more reduced form such as phos- to calcium chloride and heating to 1100 ºC for 24 phite, may also have played an important role in hours [13-16]. Before use in dissolution experi- prebiotic reactions [2,3]. Merrillite, ments, merrillite, chlorapatite, and whitlockite were Ca9NaMg(PO4)7, and chlorapatite, Ca5(PO4)3(Cl, F), powdered using an agate mortar and pestle, sieved are the dominant phosphate-bearing minerals found to the 38-75 µm size fraction, washed by sonicating in martian meteorites. Whitlockite, in spectroscopic grade ethanol until the solution was Ca9.1Mg0.9(P1O3OH)(P1O4)6, is the terrestrial, hy- clear, and dried for 24 hours. drated version of merrillite and is potentially im- The organic compounds used in this study were portant on early Mars as well [4]. Phosphate availa- chosen based on the compounds documented in car- bility for prebiotic reactions, or for use by organ- bonaceous chondrites, as well as proposed degrada- isms, largely results from mineral dissolution. tion products in martian soils [17-19]. Five poten- Therefore, the study of how phosphate is released tial prebiotic organic compounds were chosen: ace- and what influences release rates is essential for tic acid, succinic acid, methylsulfonic acid, oxalic interpreting potentially habitable martian environ- acid, and benzenehexacarboxylic (mellitic) acid [17- ments. 19, 20-21]. The sodium salt of each organic com- Previous work has indicated that the availability pound chosen was used for dissolution experiments. of phosphate may have been greater on early Mars Each experiment consisted of dissolution of one than early Earth [5]. However, that study examined mineral in the presence of one organic acid. Each dissolution of merrillite and chlorapatite in inorgan- experiment was performed in duplicate, with an or- ic solutions and the environments relevant to poten- ganic compound-containing blank, as well as a tial early martian life likely contained abundant blank with no organic compound in solution. Or- prebiotic organic compounds delivered to Mars by ganic-containing dissolution experiments were carbonaceous chondrites and interplanetary dust compared to dissolution of the same mineral in or- particles [6, 7]. ganic-free solutions adjusted to the same starting These organic compounds may have aided the pH. Each dissolution experiment contained 0.150 g release, and therefore availability, of phosphate in of merrillite, chlorapatite, or whitlockite placed in early martian environments. Studies have shown 180 mL of solution in 250 mL polypropylene bot- that the presence of organic compounds can enhance tles. Solutions consisted of a 0.01M solution of phosphate release from fluorapatite and hydroxyap- KNO3, with or without the organic compound of atite [8-10]. To the best of our knowledge, however, interest, adjusted to a pH of 5.5 and filter-sterilized. no previous studies have examined the effect of or- Batches were agitated at 100 rpm in a temperature- ganic compounds on whitlockite, merrillite, or controlled shaking water bath at 25 ºC Ten mL of chlorapatite dissolution. This study, therefore, aims solution was removed every hour for six hours the to further knowledge of phosphate availability from first day, and then once per day for the following mineral dissolution in the presence of prebiotic or- two days. Every sample removed was tested for pH ganic compounds, with implications for possible as well as calcium and phosphate concentration. habitability of Mars. Calcium in solution was measured using atomic ab- Methods: Whitlockite was synthesized follow- sorption (AA) spectrometry on a Thermo Scientific ing [11] which modifies the method of [12], using iCE 3000 Series AA after the addition of 0.36 M 0.300 g of magnesium nitrate to 1.000 g of calcium lanthanum chloride solution to 10% v/v to mitigate hydroxyapatite in 90ml of deionized water. Through interference by phosphorus. Phosphorus was meas- the addition of phosphoric acid the solution was ured by the methyl blue/acetic acid method [22] us- acidified to a pH less than 2.8. The solution was ing a Thermo Scientific Genesys 10S UV-Vis. then heated for 7 days at 240 °C. Merrillite was also 47th Lunar and Planetary Science Conference (2016) 2754.pdf

Results and Discussion: Results of this study Garcia, and Mikhail Aquino for laboratory assis- thus far indicate the enhanced release of phosphate tance. from mineral surfaces in the presence of some References: [1] Westheimer, F. H. (1987) Sci- prebiotic organic compounds (Figure 1). Three or- ence 235, 1173–1178. [2] Pasek, M.A. (2008) ganic compounds (acetic acid, succinic acid, and PNAS, 105, 853-858. [3] Powner, M. W., Gerland, methylsulfonic acid) have been run with whitlockite, B., Sutherland, J.D. (2009) Nature, 459, 239-242. merrillite, and chlorapatite. The presence of succinic [4] McCubbin, F.M., Shearer, C.K., Burger, P.V., acid and acetic acid enhanced the release of both Hauri E.H., Wang J., Elardo, S.M., and Papike, J.J. calcium and phosphate from the surfaces of all three (2014) American Mineralogist, 99, 1347-1354. [5] minerals. The presence of methylsulfonic acid, in Adcock, C., Hausrath, E.M., Forster, P.M. (2013) contrast, has decreased calcium and phosphate re- Nature Geoscience 6, 824-827. [6] Flynn, G. J. lease relative to release in organic-free solutions (1996) Earth Moon Planets, 72, 469-474. [7] Ben- adjusted to the same starting pH. Work to interpret ner, S. A., Devine, K.G. Matveeva, L.N., Powell, D. these results, as well as finish the dissolution exper- H. (2000) PNAS, 97, 2425-2430. [8]Goyne, K., iments of the remaining conditions, is ongoing. Brantley, S., Chorover, J. (2006) Chemical Geology, 243, 28-45. [9] Tanaka, H., Miyajima, K., Nakagaki, M. and Shimabayashi, S. (1989) Chem. Pharm. Bull., 37, 2897-2901. [10] Neaman, A., Chorover, J., Brantley, S. (2005) American Journal of Science, 305, 147-185 [11] Adcock, C., Hausrath, E.M., For- ster, P.M., et. al. (2014) American Mineralogist, 99, 1221-1232. . [12] Hughes, J.M., Jolliff, B. I., and Rakovan, J. (2008) American Mineralogist, 93, 1300-1305. [13] Prener, J.S. (1967) J. Electrochem. Soc., 114, 77-83. [14] Prener, J.S. (1971) J. Sol. Sta. Chem., 3, 49-55. [15] Dachs, E., Harlov, D., Benisek, A. (2010) Phys. Chem. Of Min. [16] Tack-

Figure 1. Moles released calcium versus time from er, R.C., Stormer Jr, J. C. (1993) Geochim Cosmo- the calcium phosphate mineral whitlockite show chim. Acta, 57, 4663-4676 (1993). [17] Pizzarello, enhanced release in the presence of solutions con- S., Cooper, G., Flynn, G. (2006) Meteorites and the taining acetic acid and succinic acid relative to or- Early Solar System, 625-650. [18] Huang, Y., ganic-free solutions at the same pH, and decreased Wang, Y., Alexandre, M. et al. (2005) Geochimica release from solutions containing methylsulfonic et Cosmochimica Acta, 69 1073-1084. [19] Piz- acid. Error bars represent analytical uncertainty zarello, S., Huang, Y. (2002) Geochimica et Cos- estimated at ten percent. mochimica Acta, 69, 599-605 (2002). [20] Benner, S.A., Devine, K.G., Matveeva, L.N., Powell, D. H. Conclusions: The results of this study will pro- (2000) PNAS, 97, 2425-2430. . [21] Cooper, G. W., vide further understanding of phosphate availability Onwo, W.M., Cronin, J.R. (1992) Geochimica et from the dissolution of Mars-relevant minerals in Cosmochimica Acta, 56, 4109-4115. [22] Murphy, the presence of prebiotic organic compounds. A J., Riley, J.P (1962) Analytica Chimica Acta, 27, 31- greater understanding of phosphate availability and 36. mobility in the presence of organic compounds will help better interpret the potential habitability of early martian environments. Acknowledgements: This material is based up- on work supported by the National Aeronautics and Space Administration under Grant No. NNX10AN23H issued through the NASA Space Grant Consortium. We would also like to thank the Graduate and Professional Student Association of UNLV for funding, and Renee Schofield, Angela