RADIOLYTIC H2 PRODUCTION, TRANSPORT, AND DISSOLUTION ON NOACHIAN MARS J. D. Tar-nas1, J. F. Mustard1, B. Sherwood Lollar2, M. S. Bramble1, K. M. Cannon3, A. M. Palumbo1, A.-C. Plesa4 1Brown University, Dept. of Earth, Environmental and Planetary Sciences, Providence RI (jesse_tarnas@brown.edu), 2University of Toronto Dept. of Earth Sciences, Toronto ON, 3University of Central Florida Dept. of Physics, Orlan-do FL, 4German Aerospace Center Institute of Planetary Research. Planetary Physics, 12489 Berlin.

Introduction: Subsurface microbial communities

on Earth obtain energy by facilitating energetically favorable redox reactions. Such ecosystems are preva-lent in both the terrestrial [1, 2] and marine [3, 4] crust on Earth, and may have inhabited the subsurface of Noachian Mars in regions with liquid water or brine, temperatures between 0-122 ˚C [5], and available oxi-dants and reductants. H2 is a major reductant in terres-trial subsurface communities [1-4, 6, 7] and a building block for reductants of higher molecular complexity such as CH4, HS-, and NH4 [7]. Thus, characterizing the production, transport, and dissolution of H2 in the Noachian crust is key to assessing habitability. Here we demonstrate that radiolysis, which powers subsur-face microbial communities on Earth [1-3], likely gen-erated sufficient H2 to support subsurface chemo-lithotrophic microbial communities on Noachian Mars. We use models of radiolytic H2 production [6], diffu-sion [8], and dissolution to estimate the availability of dissolved H2 in subsurface fluids within the Noachian crust, which we find to be orders of magnitude more than sufficient to support microbial life (0-349.9 µM). The minimum concentrations of dissolved H2 required for microbially-facilitated SO4 reduction and CH4 for-mation through CO2 reduction are 1 nM and 13 nM, respectively [9]. As such, dissolved H2 availability in the Noachian crust was likely higher than required for sustaining chemolithotrophic microbial communities, regardless of the background climate or groundwater composition.

Methods: We employ a radiolytic H2 production model which has been applied previously to terrestrial environments [6], Europa [10], and modern Mars [8]. Gamma Ray Spectrometer data were used to obtain concentration maps for radionuclides K and Th [11] that were normalized following [12]. U concentrations were estimated by assuming a Th/U ratio of 3.6 [13]. A half-life decay model was used to estimate K, Th, and U abundances during the Noachian.

We apply the porosity model of [14] assuming 5-15% surface porosity and a porosity decay constant of 4.3 km, which we derive by scaling GRAIL measure-ments [15] to martian gravity. Surface porosity as-sumptions agree with values from gravitational analy-sis of Noachian terrain [16]. We generate a tempera-ture profile for the Noachian crust using geothermal heat flux maps [18] and mean annual surface tempera-ture maps for both warm and cold Noachian climates

[17]. This allows us to generate cryosphere depth maps for various climate and heat flux scenarios during the Noachian, assuming different groundwater composi-tions.

We model steady-state H2 diffusion through the Noachian crust and calculate diffusivity through ice and water following [8]. We derive a solution for H2 solubility as a function of temperature and pressure using data compiled by [19] and then calculate dis-solved H2 concentrations in groundwater using the H2 gas concentration profile and solubility function. We compare our model results with required H2 concentra-tions for terrestrial subsurface life to estimate whether radiolysis could be a sufficient mechanism to support sursurface life on Noachian Mars.

Results: Global H2 production estimates range from [0.29-1.16] ´ 1010 moles H2 yr-1 in the cryosphere and [0.91-3.88] ´ 1010 moles H2 yr-1 in the subcry-osphere (Figs. 1 & 2d). H2 gas concentrations range from 0-9.24 mol m-3 in cold Noachian climate and from 0-4.43 mol m-3 in warm Noachian climate (Fig. 2a). For the cold climate scenario, dissolved H2 con-centrations range from 0-349.9 µM. For the warm cli-mate scenario, dissolved H2 concentrations range from 0-282.9 µM (Fig. 2b). Dissolved H2 concentrations are more than sufficient to support a subsurface biosphere in all modeled scenarios, especially when coupled with additional H2 from serpentinization [21].

Discussion: Our results indicate that habitability of the Noachian subsurface was primarily contingent on oxidant availability, as extensive regions with tem-eratures between 0-122 ˚C, protection from radiation, and sufficient quantities of dissolved reductants (H2) likely existed in the Noachian crust. We propose that the region immediately beneath the cryosphere—termed the subcryospheric highly-fractured zone (SHZ)—was habitable for hundreds of millions of years, making it the longest-lived habitat on Mars. The proximity of this region to the surface would also in-crease availability of oxidants from the atmos-phere/surface.

Material from the SHZ can be excavated and ex-posed at the surface in ejecta from craters that pene-trate beneath the depth of the Noachian cryosphere on Noachian-aged terrain. Such deposits are thus compelling targets for astrobiological exploration of ancient subsurface Mars. Martian megabreccia blocks and ejecta contain phyllosilicates likely formed by hydrothermal alteration [22-24], either in the

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subsurface pre-impact [25], in a primordial supercritical atmosphere [24], or from impact-generated hydrothermal systems [23]. Some megabreccias also contain unaltered megaclasts with their original ultramafic lithologies [22,23], revealing H2 production potential from serpentinization of this material. These units may contain organic, isotopic, and morphological biosignatures from the SHZ and transient, impact-induced hydrothermal habitats [23].

In the presence of equal amounts of CH4, the pre-dominantly CO2 reducing greenhouse atmosphere pro-posed by [20] to warm the Noachian surface to above 273 K requires [0.7-1.2] ´ 1020 moles H2 for 2 and 1 bar atmospheres, respectively. Even if 100% of H2 produced through radiolysis and serpentinization [21] were locked in clathrate hydrates within the cry-osphere, ~109 years would be required for H2 to build up in sufficient quantities to generate this reducing greenhouse atmosphere upon major cryospheric per-turbation. Another mechanism must account for the missing atmospheric H2, if the transient reducing greenhouse atmosphere did exist.

Conclusions: Based on models of H2 production [6,7], diffusion, and dissolution, we conclude that con-

centrations of dissolved H2 in Noachian crustal groundwaters were likely more than sufficient to sup-port a subsurface biosphere. Based on temperature gradients and liquid water and nutrient availability, the region immediately beneath the cryosphere—the SHZ—was the longest-lived habitable environment on Mars. Material from this zone can be excavated and exposed at the surface in impact ejecta and central up-lifts. This should be considered when selecting future targets for astrobiological exploration of Mars.

References: [1] Sherwood Lollar B. et al. (2014) Nat., 516, 379-382. [2] Lin L.-H. et al. (2006) Sci., 314, 479-482. [3] D’Hondt, S. et al. (2009) PNAS 106: 11651-11656. [4] Schrenk, M. O. et al. (2013) Rev. in Min. & Geo. 75: 575-606. [5] McKay, C.P. (2014) PNAS, 111, 12628-12633. [6] Lin L.-H. et al. (2005) G3, 6, Q07003. [7] Osburn, M. R. et al. (2014) Front. in Micro. 5: 610 [8] Onstott T.C. et al. (2006) Astrobio., 6, 377-395. [9] Lovley, D. R. & Goodwin, S. (1988) GCA 52:2993-3003. [10] Bouquet, A. et al. (2017) ApJ 840:1-7. [11] Boynton W.V. et al. (2007) JGR, 112, E12S99. [12] Hahn, B. C. et al. (2011) GRL 38: L14203 [13] Meyer, C. (2016) Astromat. Res. & Exp. Sci. [14] Clifford S.M. et al. (2010) JGR, 115, E07001. [15] Besserer F.B. et al. (2014), GRL, 41, 5771-5777. [16] Goossens, S. et al. GRL 44: 7686-7694. [17] Palumbo, A. M. et al. (2018) Icarus 300: 261-286. [18] Plesa, A.-C. et al. (2016) JGR 121:2386-2403. [19] Young, C. L. et al. (1981) Int. Un. Of Pur. & App. Chem., ISBN 0-08-023927-7: 305-313. [20] ] Wordsworth et al. (2016) GRL, 44, 665-671. [21] Vance, S. D. et al. (2016) GRL 43:4871-4879. [22] Osinski G.R. et al. (2013) Icar., 224, 347-363. [23] Tornabene L.L. et al. (2013) JGR, 118, 994-1012. [24] Cannon et al. (2017) Nature 552: 88-91. [25] Ehlmann, B.L. et al. (2011) Nat., 479, 53-60. [26] Michalski J.R. et al. (2013) Nat. Geo., 6, 133-138.

Figure 1 | Latitudinal cross section of H2 production with topography. Surface H2 production variations with latitude and longitude displayed on Noachian-age terrain (right) and H2 production variations with latitude and depth (left) for H2O groundwater. In the latitudinal cross sections (left), the cryosphere base is indicated as a black line. Topography data for the latitudinal cross sections are from MOLA. H2 production is max-imized at shallower depths where porosity is highest.

Figure 2 | Noachian subsurface habitability model. Modified after [26]. For cold Noachian climate with H2O groundwater a) H2 gas concentra-tion within the top 10 km of crust. b) Dissolved H2 concentrations in crust based on solubility model. c) Cross-section of Noachian crust display-ing the cryosphere, H2 from radiolysis and serpentinization, CH4 associated with serpentinization, CH4 clathrates, groundwater circulation (which contains dissolved H2), and a subsurface aquifer-based habitat within the subcryospheric highly fractured zone (SHZ). d) Temperature increase with depth given cold climate [17] and two possible heat fluxes [18] from initial mantle temperatures of 1650 K (60.16 mW/m2) and 1850 K (67.83 mW/m2). e) Radiolytic H2 production esimates for various assumed surface porosities.

2073.pdf49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)