sign in sign up
<<
Home , Noachian

49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 2073.pdf

RADIOLYTIC H2 PRODUCTION, TRANSPORT, AND DISSOLUTION ON NOACHIAN 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 ([email protected]), 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 [17]. This allows us to generate cryosphere depth maps on Earth obtain energy by facilitating energetically for various climate and heat flux scenarios during the favorable redox reactions. Such ecosystems are preva- Noachian, assuming different composi- lent in both the terrestrial [1, 2] and marine [3, 4] tions. on Earth, and may have inhabited the subsurface of We model steady-state H2 diffusion through the Noachian Mars in regions with liquid water or brine, Noachian crust and calculate diffusivity through ice temperatures between 0-122 ˚C [5], and available oxi- and water following [8]. We derive a solution for H2 dants and reductants. H2 is a major reductant in terres- solubility as a function of temperature and pressure trial subsurface communities [1-4, 6, 7] and a building using data compiled by [19] and then calculate dis- block for reductants of higher molecular complexity solved H2 concentrations in groundwater using the H2 - gas concentration profile and solubility function. We such as CH4, HS , and NH4 [7]. Thus, characterizing compare our model results with required H2 concentra- the production, transport, and dissolution of H2 in the Noachian crust is key to assessing habitability. Here tions for terrestrial subsurface life to estimate whether radiolysis could be a sufficient mechanism to support we demonstrate that radiolysis, which powers subsur- sursurface life on Noachian Mars. face microbial communities on Earth [1-3], likely gen- Results: Global H production estimates range erated sufficient H to support subsurface chemo- 2 2 from [0.29-1.16] ´ 1010 moles H yr-1 in the cryosphere lithotrophic microbial communities on Noachian Mars. 2 and [0.91-3.88] ´ 1010 moles H yr-1 in the subcry- We use models of radiolytic H production [6], diffu- 2 2 osphere (Figs. 1 & 2d). H gas concentrations range sion [8], and dissolution to estimate the availability of 2 from 0-9.24 mol m-3 in cold Noachian climate and dissolved H in subsurface fluids within the Noachian 2 from 0-4.43 mol m-3 in warm Noachian climate (Fig. crust, which we find to be orders of magnitude more 2a). For the cold climate scenario, dissolved H con- than sufficient to support microbial life (0-349.9 µM). 2 centrations range from 0-349.9 µM. For the warm cli- The minimum concentrations of dissolved H required 2 mate scenario, dissolved H concentrations range from for microbially-facilitated SO reduction and CH for- 2 4 4 0-282.9 µM (Fig. 2b). Dissolved H concentrations are mation through CO reduction are 1 nM and 13 nM, 2 2 more than sufficient to support a subsurface biosphere respectively [9]. As such, dissolved H availability in 2 in all modeled scenarios, especially when coupled with the Noachian crust was likely higher than required for additional H from serpentinization [21]. sustaining chemolithotrophic microbial communities, 2 Discussion: Our results indicate that habitability regardless of the background climate or groundwater of the Noachian subsurface was primarily contingent composition. on oxidant availability, as extensive regions with tem- Methods: We employ a radiolytic H production 2 eratures between 0-122 ˚C, protection from radiation, model which has been applied previously to terrestrial and sufficient quantities of dissolved reductants (H ) environments [6], Europa [10], and modern Mars [8]. 2 likely existed in the Noachian crust. We propose that Gamma Ray Spectrometer data were used to obtain the region immediately beneath the cryosphere— concentration maps for radionuclides K and Th [11] termed the subcryospheric highly-fractured zone that were normalized following [12]. U concentrations (SHZ)—was habitable for hundreds of millions of were estimated by assuming a Th/U ratio of 3.6 [13]. A , making it the longest-lived habitat on Mars. The half-life decay model was used to estimate K, Th, and proximity of this region to the surface would also in- U abundances during the Noachian. crease availability of oxidants from the atmos- We apply the model of [14] assuming 5- phere/surface. 15% surface porosity and a porosity decay constant of Material from the SHZ can be excavated and ex- 4.3 km, which we derive by scaling GRAIL measure- posed at the surface in from craters that pene- ments [15] to gravity. Surface porosity as- trate beneath the depth of the Noachian cryosphere on sumptions agree with values from gravitational analy- Noachian-aged terrain. Such deposits are thus sis of Noachian terrain [16]. We generate a tempera- compelling targets for astrobiological exploration of ture profile for the Noachian crust using geothermal ancient subsurface Mars. Martian megabreccia blocks heat flux maps [18] and mean annual surface tempera- and ejecta contain phyllosilicates likely formed by ture maps for both warm and cold Noachian climates hydrothermal alteration [22-24], either in the 49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 2073.pdf

subsurface pre-impact [25], in a primordial centrations of dissolved H2 in Noachian crustal supercritical [24], or from impact- were likely more than sufficient to sup- generated hydrothermal systems [23]. Some port a subsurface biosphere. Based on temperature megabreccias also contain unaltered megaclasts with gradients and liquid water and nutrient availability, the their original ultramafic lithologies [22,23], revealing region immediately beneath the cryosphere—the H2 production potential from serpentinization of this SHZ—was the longest-lived habitable environment on material. These units may contain organic, isotopic, Mars. Material from this zone can be excavated and and morphological biosignatures from the SHZ and exposed at the surface in impact ejecta and central up- transient, impact-induced hydrothermal habitats [23]. lifts. This should be considered when selecting future In the presence of equal amounts of CH4, the pre- targets for astrobiological . dominantly CO reducing greenhouse atmosphere pr o- References: [1] Sherwood Lollar B. et al. (2014) Nat., 516, 379- 2 382. [2] Lin L.-H. et al. (2006) Sci., 314, 479-482. [3] D’Hondt, S. et posed by [20] to warm the Noachian surface to above al. (2009) PNAS 106: 11651-11656. [4] Schrenk, M. O. et al. (2013) 20 Rev. in Min. & Geo. 75: 575-606. [5] McKay, C.P. (2014) PNAS, 273 K requires [0.7-1.2] ´ 10 moles H2 for 2 and 1 111, 12628-12633. [6] Lin L.-H. et al. (2005) G3, 6, Q07003. [7] bar , respectively. Even if 100% of H Osburn, M. R. et al. (2014) Front. in Micro. 5: 610 [8] Onstott T.C. 2 et al. (2006) Astrobio., 6, 377-395. [9] Lovley, D. R. & Goodwin, S. produced through radiolysis and serpentinization [21] (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] were locked in clathrate hydrates within the cry- Hahn, B. C. et al. (2011) GRL 38: L14203 [13] Meyer, C. (2016) 9 osphere, ~10 years would be required for H2 to build 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] up in sufficient quantities to generate this reducing Goossens, S. et al. GRL 44: 7686-7694. [17] Palumbo, A. M. et al. greenhouse atmosphere upon major cryospheric per- (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. & turbation. Another mechanism must account for the 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 missing atmospheric H2, if the transient reducing 43:4871-4879. [22] Osinski G.R. et al. (2013) Icar., 224, 347-363. greenhouse atmosphere did exist. [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) Conclusions: Based on models of H2 production Nat., 479, 53-60. [26] Michalski J.R. et al. (2013) Nat. Geo., 6, 133- [6,7], diffusion, and dissolution, we conclude that con- 138.

Figure 1 | Latitudinal cross section of H2 production with topography. Surface H2 production variations with latitude and longitude displayed on Noachian- 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 2 (67.83 mW/m ). e) Radiolytic H2 production esimates for various assumed surface .