47th Lunar and Planetary Science Conference (2016)
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POTENTIAL FOR MICROBIAL TRACES IN NEAR-SURFACE BASALTIC GLASS ON MARS. M. P. C. Nikitczuk1, M. E. Schmidt1, and R.L. Flemming2. 1Dept. Earth Sci., Brock Univ., (St. Catharines, ON L2S 3A1 Canada (
[email protected]), 2 Dept. Earth Sci., Univ. Western Ontario London, ON N6A 5B7 Canada
Introduction: Considering the evidence of microbially mediated glass dissolution [1,2], subsurface microbes inhabiting ocean basalts [3,4], tubular and granular bioalteration textures in sub-marine volcanic glass [5,6] and the widespread occurrence of basalts on Mars [7-9], there is a high likelihood that Mars presently or in the past hosted endolithic microbes [10-12]. On Earth, bioalteration textures (endolithic microborings) have mainly been identified in sub-marine basalts hundreds of meters into the crust and submerged beneath kilometers of water [5,13,14]. Until recently, Martian endoliths (if present) were thought to be relegated to the deep subsurface where the products of aqueous olivine alteration (e.g., CH4, H2), that are likely useful to anaerobic microbes may be concentrated in porous basalts [10]. Although the Martian surface is cold, dry, has a low organic carbon content, and is subject to intense ultraviolet radiation, it is possible that microbes have adapted to such conditions [15,16]. Recently discovered putative endolithic microborings in continental hydrovolcanic tuffs (Fort Rock Volcanic Field FRVF, Oregon), that are analogous to several locations on Mars demonstrate that such textures can be produced in near-surface deposits on Earth and that subsurface microbes on Mars could exist nearer to the surface than previously thought. We here encapsulate the geologic environmental similarities between FRVF hydrovolcanics and deposits on Mars and the potential for finding a near surface biosphere hosted in Martian volcanic glass. We focus on the occurrence of volcanic glass in similar geologic and environmental contexts with associated palagonitic phases and on the occurrence of other features indicative of hydrothermal/aqueous basalt alteration. Bioalteration Textures in Hydrovolcanic Tuffs: We recently reported finding microbial alteration (tubular and granular) textures in hydrovolcanic basalt tuffs formed in a continental lacustrine setting (Fig.1.) [17]. The FRVF includes >40 hydrovolcanoes (maars, tuff rings/cones) and is the site of a Pliocene-Pleistocene pluvial lake [18]. A secondary alteration mineral assemblage of calcite, zeolites and smectite clays and amorphous to protocrystalline palagonite accompany bioalteration textures formed at temperatures of ~25 to 80°C. Bioalteration textures in these deposits formed in neutral to alkaline water and in the absence of marine of glacial melt water. Geologic Context: Hydrovolcanic deposits identified on the Mars surface include the Home Plate outcr-
A
60 µm
B
20 µm Fig.1. (A) Enhanced depth of focus (EDF) photomicrographs from Reed Rock (A) and Black Hills (B) basaltic tuffs in the FRVF. (A)Tubular micro-tunnels in basaltic glass with mushroom-like terminal enlargements radiating from a matrix-filled vesicle (center) and fracture (top left). Smooth, simple micro-tunnels are visible radiating from another empty vesicle at bottom right. (B) Long, annulated micro-tunnels in basaltic glass rooted on a fracture and terminating in thin branches.
op in Gusev Crater that is interpreted to be variably reworked hydrovolcanic basaltic tephra [9]. Landforms of likely hydrovolcanic origin (tuff rings and tuff cones) have been identified from orbit in various regions such as Tharsis [19]. Such hydrovolcanic landforms on Earth contain abundant basaltic glass, which is a high quality substrate for microbial etching and these are commonly hydrothermally altered. Lacustrine Environments: Sedimentary deposits examined by MSL in Gale Crater, including the Sheepbed mudstone member and Murray Formation are interpreted to have formed in a fluvio-lacustrine setting [20]. The Sheepbed mudstone contains basaltic material and may include an ashfall component [20,21]. Phyllosilicate minerals (smectites) were identified by X-ray diffraction in the Sheepbed, suggesting
47th Lunar and Planetary Science Conference (2016)
relatively neutral pH waters [22]. Additionally, the Sheepbed lake likely experienced periodic drying [20], similar to the evaporative waxing and waning of pluvial lakes in Western U.S. (e.g., Fort Rock lake) caused by Pleistocene glacial fluctuations [23]. Amorphous Material, Aqueous Alteration Minerals: Widespread across the Martian surface and in Martian dust is a significant amorphous component [9,22,24,25]. The amorphous material may include basaltic glass or its devitrification products. It also includes nano-phase Fe-oxides and an amorphous palagonite similar to phases frequently found in terrestrial basalts containing aqueously altered glass [25-27]. In the FRVF basalts, the main alteration products include amorphous to protocrystalline palagonite, zeolites such as chabazite, and smectite clays (nontronite and saponite) formed during low temperature aqueous/hydrothermal alteration. Zeolites and smectite clays (including saponite) have also been identified on the Martian surface [22,28]. Geological and mineralogical compositional variations across Home Plate are consistent with low temperature aqueous/hydrothermal alteration as well [29]. Liquid Water: Several lines of evidence suggest Mars had ancient aqueous environments [20,22,29-33]. Aqueous environments may also presently exist just below the Martian surface. Recurring slope lineae (RSL) for example, are thought to be formed by the intermittent flow of shallow subsurface fresh or briny water [34,35]. If RSL were formed by the flow of shallow subsurface water, then abundant liquid water may exist in some near-surface regions on Mars [34,35]. Alteration Conditions and Microbes: On Earth, hydrovolcanics contain liquid water and a source of biogenic elements and energy (basaltic glass) for endolithic microbes. In oceanic and continental basalts, alteration temperatures of 60-70°C [13] and 25-80°C [17] respectively, are conducive to the production of microbial biosignatures in circum-neutral to alkaline environments. The Sheepbed mudstone and Cumberland sample in Gale Crater lack highly ordered illite and chlorite minerals that typically require high alteration temperatures, suggesting temperatures below 6080°C [22]. Also, the absence of sulfate veins, the presence of phyllosilicates and lack of evidence indicating Al-mobility in the Sheepbed suggest an environment with neutral pH [20]. Hydrovolcanics/basaltic glassbearing deposits that have altered in circum-neutral to alkaline conditions at temperatures within the range that microbial life is known to exist [36] may thus be associated with evidence of microbial activity on Mars. Conclusion: A key objective in future Mars missions (e.g., Mars 2020 Project) is to search for potential biosignatures [37]. Hydrovolcanics not only repre-
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sent deposits in which near-surface microbial activity or evidence thereof may be found, but they also fulfill other threshold and qualifying geological criteria for evaluating potential landing sites. Threshold criteria include sub-aqueous sediments/hydrothermally altered or low-T fluid altered rocks, minerals indicative of aqueous alteration (e.g., phyllosilicates) and unaltered igneous float rocks [38]. Potential qualifying geological criteria such as evidence of water, secondary mineral assemblages, igneous rocks (radiometrically datable) and probability of samples of opportunity (e.g., accidental and cognate clasts) are also likely. Therefore, in view of the science/geologic criteria, hydrovolcanic deposits may provide suitable targets for future rover/sample-return missions. References: [1] Thorseth IH et al. (1995) CG, 126, 137. [2] Staudigel H et al. (1995) CG, 126, 147. [3] Furnes H et al. (1996) PODP-SR, 148, 191. [4] Giovannoni S and Fisk MR (1996) PODP-SR, 148, 207. [5] Fisk MR et al. (1998) Science, 281, 978. [6] Furnes H and Staudigel H (1999) EPSL, 166(3-4), 97. [7] Mustard JF et al. (2005) Science, 307, 5715, 1594. [8] McEwan AS et al. (2010) Icarus, 205, 2. [9] Squyres SW et al (2007) Science, 316, 738. [10] Fisk MR and Giovannoni SJ (1999) JGR, 104(E5), 11805. [11] Banerjee NR et al. (2004) LPSC 35, Abstract #1197. [12] McLoughlin N et al. (2007) Astrobiology, 7(1), 10. [13] Furnes H et al. (2007) PR, 158, 156. [14] Torsvik T et al. (1998) EPSL, 162, 165. [15] Klein HP et al (1998) JGR, 103, 28463. [16] Clark BD (1998) JGR, 103, 28545. [17] Nikitczuk et al. (in revision) GSA Bull. [18] Heiken GH (1971) JGR, 76(23), 5615. [19] Brož P and Hauber E (2012) Icarus, 218, 88. [20] Grotzinger JP et al. (2014) Science, 343. [21] McLennan SM et al. (2014) Science, 343, 1244734. [22] Vaniman DT et al. (2014) Science, 343, 6169, 1243480. [23] Martin JE et al. (2005) SCO-JG, 113, 139. [24] Yen AS et al (2008) JGR, 113-E06S10. [25] Bish DL et al. (2013) Science, 341, 1238932. [26] Morris RV et al. (2001) JGR-P, 106(E3), 5057. [27] Morris RV et al. (2006) JGR-P, 111(E12). [28] Ehlmann BL et al. (2009) JGR-P, 114(E2). [29] Schmidt ME et al. (2009) EPSL, 281(3), 258. [30] Squyres SW et al. (2008) Science, 320, 1063. [31] Williams RME et al. (2013) Science, 340, 1068. [32] Malin MC and Edgett KS (2000) Science, 288, 2330. [33] Baker VR (2006) Elements, 2(3), 139. [34] McEwan AS et al. (2014) NG, 7(1), 53. [35] Stillman DE et al. (2014) Icarus, 233, 328. [36] Stetter KO et al. (1990) FEMS MR, 75, 117. [37] Mustard JF et al. (2013) RM 2020 SDT, 154. [38] NASA-JPL (2015) 2020 LSMRM Acknowledgements: Funding provided by NSERC Discovery to (Schmidt) and NSERC-CGS-M, OGS and CSA-ASTRO to Nikitczuk.
