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49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 2535.pdf

TERRESTRIAL HOT SPRINGS AND THE ORIGIN OF : IMPLICATIONS FOR THE SEARCH FOR LIFE BEYOND EARTH. M. Van Kranendonk1, R. Baumgartner1, E. Boyd2, S. Cady3, K. Campbell4, A. Czaja5, B. Damer6, D. Deamer6, T. Djokic1, M. Fiorentini7, A. Gangidine5, J. Havig8, A. Mulkidjanian9, S. Ruff10, P. Thordarson1, 1Australian Centre for Astrobiology, University of New South Wales Sydney, Kensington, NSW 2052, Australia, [email protected], 2Montana State University, 3Pacific Northwest National Laboratory. 4University of Auckland, 5University of Cincinnati, 6University of California at Santa Cruz, 7University of , 8University of Minnesota, 9Osnabrueck University, Germany, 10Arizona State University.

Introduction: Significant new chemical, geologi- seawater, but similar to freshwater. Saltwater presents cal, and computational evidence increasingly supports a barrier to the formation of protocells[14]. the hypothesis that life originated in hot fields 4) Hot spring pools can, and do, concentrate a vari- on land, rather than at deep-sea hydrothermal vents. ety of prebiotically important elements, including not This has profound implications for Astrobiology and only H, N, O, P, and C, but also Fe, S, and P, as well as the search for life beyond Earth, not only for site selec- B, Zn, and Mn[15,16]. tion on a planet, but also which of the planetary bodies 5) Hydrothermal fields on land receive energy from to investigate. Herein we provide a broad framework three main sources: the hot spring system, dehydration of observations, a conceptual model, and implications energy, and UV light, the latter shown recently to sup- for an origin of life (OoL) on land. port critical prebiotic reactions, including a pathway to Observations: 1) Surface pools would have been activated nucleotides[17,18]. Another source is abiotic able to concentrate in-fall from meteoritic sources and at ZnS and TiO2 crystals[19], both interplanetary dust particles, which were many times found in an ancient Pilbara hot spring analogue more voluminous during earliest Earth history[1] and site[20,21]. contain abundant key building blocks for life including 6) Perhaps most important for an OoL scenario is fatty acids, nucleobases, and amino acids[2-4]. the extreme complexity offered by terrestrial hydro- 2) Terrestrial hot springs have the capacity to un- thermal fields, which can consist of a hundred or more dergo wet-dry cycling – in some cases many times per pools with widely ranging chemistry and temperatures. day (e.g., Yellowstone’s Old Faithful) – both at pool In addition, pools include not only the water-rock in- margins on the surface and in fractures in the near sub- teractions that deep-sea vents have, but also water- surface where prebiotic reactions would be shielded air/, and air/volcanic gas-rock interactions. from harsh UV radiation. Wet-dry cycling has been Pools also have the advantage of being able to ex- shown to be critical in overcoming “The Water Prob- change contents with other pools through flows, lem”, whereby most of the important prebiotic organic splashing, wind, and subterranean plumbing networks reactions require a form of dehydration (condensation that open and close on short geological timescales due reactions, in which water is a leaving group) to form to variable fluid/gas pressure and mineralization. In- long-chain organic polymers (e.g., polysaccharides, deed, hot spring fields constitute a natural system for oligonucleotides, and polypeptides) from their simple combinatorial or “messy” chemistry and support serial building blocks (e.g., amino acids)[5-7]. enrichment capable of creating a continuous supply of 3) Hot spring pools, which contain a mixture of structures, building blocks, and energy sources to drive meteoric water and condensates of magmatic vapors, prebiotic processes through cycles of selection. Terres- can be enriched in phosphorus, ammonia and organic trial pools are concentrating environments – through compounds and produce a range of temperatures and drying and evaporation – that permit many cycles of pH, including mildly acidic pools that have been complex chemical reactions. shown in the laboratory and at field sites to support the 7) The “sweet spot” for supramolecular (e.g., non- formation of membranous compartments or protocells enzymatic RNA duplex formation) assembly is ca. 10– [8-11]. Such protocells are able to encapsulate organic 70°C. This is because formation temperatures need to polymers and subject them to combinatorial selection be high enough for molecules to “search” their con- through wetting-drying cycles that drive ever- formation space (become distorted). Too cold and the increasing complexity and emergence of biological lack of activation energy makes it doubtful that any functions[12,13]. Freshwater is important because mi- “function” would occur between molecules – let alone croorganisms from all three branches of life contain an generate life. Too hot and directional intermolecular internal cytoplasm with K+/Na+ ratios very different forces are weakened and associations are too short for from seawater, or the possible compositions of ancient any chemistry useful for prebiotic selection to take place. 49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 2535.pdf

Conceptual Model: Testing of some of the above loss of atmosphere, desiccation and irradiation. Life, if properties both in the laboratory and in the field has it emerged on , would have had to retreat to ref- led to the publication of a new model for biogenesis in uges in the saline, deeper . The plumbing of a anoxic hydrothermal fields[11,13,15,22] (Fig. 1). hydrothermal system could access that refuge and might carry such life with it through a temporary effu- sion of water up to a surface hot spring, where it may have temporary viability at this last surface outpost. Robust evidence for hot spring deposits has already been identified on Mars by the rover adjacent to “” in the Columbia Hills[28,29], including evidence for potential biosignatures[26]. Other candi- date hot spring deposits have been observed from orbit, including mounds on the flanks of a volcanic cone in Nili Patera[30]. The combination of high potential for habitability and biosignature preservation of silica- depositing hot spring systems make them attractive astrobiology targets for future missions to Mars. References: [1] Pearce BKD et al., 2017. PNAS 114: 11327–11332. [2] Kvenvolden K et al., 1970. Figure 1. The Hot Spring Hypothesis for an origin of Nature 228: 923-926. [3] Deamer DW, Pashley R, life, illustrating how organic compounds synthesize in 1989. Orig Life Evol Biosphere 19:21-38. [4] Martins space (1) and accumulate (2) within interconnected Z et al., 2008. Earth Planet Sci Lett 270: 130-136. [5] hydrothermal field pools (3). These organics are then Cairns-Smith AG, 1982. Genetic takeover and the delivered to a cycling pool where protocells undergo origins of life. Cambridge University Press. [6] selection toward an origin of life (4). This earliest life De Guzman V et al., 2014. J Mol Evol 78: 251-262. [7] is then distributed along an adaptation pathway into Hud N. et al., 2013. Chem & Biol 20 doi:10.1016/ ever more extreme environments such as lacustrine (5), j.chembiol.2013.03.012. [8] Hargreaves WW et al. salty estuarine (6), and tidal marine (7) settings[13]. 1977. Nature 266: 78-80. [9] Deamer D et al., 2002. Implications: From an astrobiological perspective, Astrobiology 2: 371 - 382. [10] Deamer D et al., 2006. the consideration of an OoL in terrestrial hot springs is Philos Trans R Soc Lond B 361: 1809-18. [11] Deamer important for two reasons. First, it can provide focused DW, Georgiou CD, 2015. Astrobiology 15: 1091- exploration strategies for planetary bodies where this 1095. [12] Ross D, Deamer D, 2016. Life (Basel) doi: 10.3390/life6030028. [13] Van Kranendonk MJ et al., combination of ingredients is known to have, or may 2017. Scientific American 317(2): 28-35. [14] Mon- have, occurred. Second, hot springs typically deposit nard PA et al., 2002. Astrobiology 2: 139 - 152. [15] opaline silica, providing an easily recognizable target Mulkidjanian AY et al. 2012. PNAS 109: E821–E830. in the search for evidence for life. Critically, hot [16] Van Kranendonk MJ et al., 2015. AbSciCon, Ab- springs are important not only as hosts of life, but their stract #7136. [17] Mulkidjanian AY et al., 2003. BMC deposits can be preservers of biosignatures over bil- Evolution Biol 2003, 3: 12. [18] Powner MW et al., lions of years[21,23-25]. Entombment of microbial 2009. Nature 459: 239-242. [19] Mulkidjanian AY mats and living on opaline silica deposits in 2009. Biol. Direct 4: 26. [20] Van Kranendonk MJ et and around hot springs results in the formation and al. 2008. Precamb Res 167: 93-124. [21] Djokic T et preservation of numerous microbial biosignatures that al., 2017. Nat Comm 8:15263. [22] Damer B, Deamer include macro-to-microscale fabrics and structures, as D, 2015. Life (Basel) 5: 872-87. [23] Walter MR, Des well as organic and inorganic chemical traces of Marais D, 1993. Icarus 101: 129-143. [24] Walter MR life[26,27]. Indeed, opaline silica is the most important et al., 1996. Palaios 11: 497-518. [25] Berelson WM et primary mineraloid responsible for preserving morpho- al., 2011. Geobiol 9: 411-424. [26] Ruff SW, Farmer logically and chemically identifiable traces of life on JD, 2016. Nat Comm 7: 13554. [27] Cady SL et al., early Earth and is the most common host lithology – 2003. Astrobiology 3(2): 351-368. [28] Van by a factor of 10:1 - of the most ancient traces of life in Kranendonk MJ, 2006. Earth-Sci Rev 74: 197-240. both the Pilbara (Australia) and Kaapvaal (South Afri- [29] Westall F et al., 2001. Precamb Res 106: 93–116. ca) cratons[28,29]. [28] Squyres SW et al., 2007. Science 316: 738-742. Importantly, hot springs could truly be the "first [29] Ruff SW et al., 2011. J. Geophys. Res. 116: and last outpost" for , or any habitable E00F23. [30] Skok JR et al., 2010. Nat. Geosci. 3: world that becomes uninhabitable at its surface through 838-841.