Mineral crusts are strong candidates in the search for evidence of life during planetary exploration, and should be an important target for examination in impact craters. Crusts in the Haughton crater readily yield a biological signature

Bowden, S. A.

Cockell, C. S.

Lee, P.

Osinski, G. R.

Parnell, J.

English

Open Research Online

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsSurface mineral crusts: A priority target in search forlife on MarsConference or Workshop ItemHow to cite:Parnell, J.; Bowden, S. A.; Cockell, C. S.; Osinski, G. R. and Lee, P. (2006). Surface mineral crusts: Apriority target in search for life on Mars. In: 37th Lunar and Planetary Science Conference, 13-17 Mar 2006, Houston,Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1049.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukSURFACE MINERAL CRUSTS: A PRIORITY TARGET IN SEARCH FOR LIFE ON MARS.  J. Parnell1, S.A. Bowden1,  C.S. Cockell2, G.R. Osinski3, P. Lee4, 1Dept. of Geology, University of Aberdeen, Aberdeen AB24 3UE, U.K., (J.Parnell@abdn.ac.uk), 2 Open University, Milton Keynes MK6 7AA, U.K., 3Canadian Space Agency, Saint-Hubert, Quebec J3Y 8Y9, Canada, 4Mars Institute, Moffett Field, CA 94035-1000, U.S.A.   Mineral crusts: Mineral crusts are a widely devel-oped feature at the rock or sediment surface, wherever there is a flux of water. They occur in all climatic zones, from tropical to polar desert, although their chemistry and morphology varies with surface hydrol-ogy, temperature and substrate. Thus the range of de-posits includes duricrusts (calcrete, silcrete, gypcrete, ferricrete), desert varnish, crusts above permafrost, and salt crusts. This list is much greater if mineral precipi-tates from flowing fluids are included, such as speleo-thems, travertine and other hot spring deposits, tufa, and chemosynthetic deposits. We will focus here on precipitates associated with surface moisture. Mineral crusts are strong candidates in the search for evidence of life, both in terrestrial extreme envi-ronments and during planetary exploration. There are several reasons for their suitability: (1) The growth of any mineral precipitate implies an active flow of water to transport the solutes for min-eral growth. Water is a vector for life. (2) The precipitation of mineral from water also implies that the water has a high load of dissolved ions, which should include nutrients for living matter. (3) As the mineral precipitates, the growing crys-tals can entrap, and thereby preserve, organic matter. This can range from different types of biomolecules to whole cells with morphological form, i.e. the evidence entrapped can be both chemical and physical.  (4) Mineral precipitates can also incorporate an inorganic record of life, including microbially medi-ated crystal growths, and isotopic evidence for micro-bial metabolism. (5) In addition to entrapment of ambient life, mineral precipitation may create a new microenviron-ment (microporosity) for an active biota. In the particular case of Mars: (6) The philosophy of searching for evidence of life on Mars has been ‘follow the water’ [1,2]. Any surface crusts on Mars are likely to represent the most recent mineral precipitation from water. (7) We already have evidence of crust formation in Martian soil through precipitation of sulphate salts. (8) Observations of fog/frost in several contexts on Mars [1] suggest that mineral dissolution and repre-cipitation on a micro-scale could be still occurring in some surface crusts. The impact crater environment, widespread on Mars, is an especially favourable case for crust development: (9) Active circulation of water follows the impact event due to hydrothermal activity. (10) Ponding of water may occur temporarily in the crater depression. (11) The impact detritus presents a substantial sur-face area for mineral dissolution, to create saturated ground waters which can reprecipitate as crusts. (12) The anomalous heating in a crater allows chemical reactions (including solution, reprecipita-tion), to proceed at a faster rate than ambient, as well as favouring any organic metabolism. In terms of sampling, additional advantages are: (13) Minerals precipitating at the surface tend to be relatively soluble, and soft, making them amenable to processing by both chemical and physical methods. (14) Wind eflation of crusts, which is likely in dry surface environments, releases abundant crust detritus into the surrounding soil, where it is easy to sample. Haughton Crater crusts: A wide variety of min-eral crusts occurs in and around the Haughton Impact Structure, Devon Island, Canadian High Arctic. These include carbonate, gypsiferous and ferruginous crusts. The carbonate crusts (Fig. 1) are varieties of a phe-nomenon that occurs across the Arctic region [3, 4], but include crusts that occupy fractures relating to the impact event. Pilot studies of the organic geochemistry of the carbonate crusts show that they have a high yield of soluble organic matter.  The organic compounds present in the carbonate crust appear to biological, e.g. squalene, diploptene and the C40 carotenoids (Fig. 2); even the n-alkanes (particularly the C17 n-alkane and other methyl-branched C17 to C21 alkanes) have a distribution that is probably biological. C16–C18 fatty acids dominate the polar fraction and are the most abundant compound that were extracted from the carbonate crust. The rela-tive abundance of C17 n-alkane, C30 diploptene and the limited distribution of fatty acids are all biomarkers characteristics of cyanobacteria, which are abundant across the sample locality. What is unclear at present is whether the biomarkers were biosynthesised by cyano-bacteria living within the crust or whether they carried to the crust by windblown material.  Bacteria are known to precipitate carbonate miner-als from solution. Evidence has now been obtained for both aerobic and anaerobic precipitation [5], suggest-ing that the process may be relevant for surface envi-ronments on rocks, but also subsurface environments. The precipitation of carbonates appears to be carried out by a wide diversity of organisms, including  sul-phate-reducers. As carbonates are precipitated around cell material, the subsequent degradation of the or-Lunar and Planetary Science XXXVII (2006) 1049.pdfganic material may leave biosignatures [6]. Thus, car-bonates can allow the preservation of past life.  Conclusion: Mineral crusts are strong candidates in the search for evidence of life during planetary ex-ploration, and should be an important target for ex-amination in impact craters. The pilot study in the Haughton Impact Structure shows that they can readily yield a biological signature.    Fig. 1. A, Typical hand specimen of carbonate crust growing over the top surface of dolomite bedrock. B, The carbonate crust sits upon an uneven erosion surface above the brown dolomite. C, Plan view of crustal structure shows that tubular structures are interconnected, but where they have broken off they are hollow inside. D, Where tubular structure has not fully formed a white precipitate covers the dolomite. (Photos courtesy of J. Whelan)  30. 40 50 60 70 80sqdip C40 carotenoidsC17methyl-branched and isoprenoids++C16 C18hydrocarbonfatty acid methyl-esters  Fig. 2. Reconstructed ion chromatograms of hydro-carbon and polar fractions extracted from Haughton Crater crust. Hydrocarbons dominated by squalene (sq), diploptene (dip) and a series of n-alkanes de-noted by circles. C40 carotenoids are also present. Polar fraction is dominated by saturated (▲) and mono-unsaturated (∆) fatty acids.  Acknowledgement: Research was under the aus-pices of the NASA Haughton-Mars Project. References: [1] Kargel J.S. (2004) Mars: A warmer, wetter planet. Springer, London, 557pp. [2] Tokano T. (2005) Water on Mars and Life. Springer, Berlin, 331pp. [3] Bunting B.T. & Christensen L. (1980) Geol. Foren. Stock. Forhandl., 100, 361-367.[4] Clark I. et al. (2004) Can. J. Earth Sci., 41, 387-399. [5] Warthman R. et al. (2000) Geology, 28, 1091-94. [6] Bosak T. (2004) Geology, 32, 781-84.   Lunar and Planetary Science XXXVII (2006) 1049.pdf

Surface mineral crusts: A priority target in search for life on Mars

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Surface mineral crusts: A priority target in search for life on Mars

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