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Home , Acidalia Planitia, Columbia Hills (Mars), Eberswalde (crater), Holden (Martian crater), Mawrth Vallis, Melas Chasma

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2018 Radiolytic H2 Production in Environments Mary Dzaugis university of rhode island, [email protected]

Arthur J. Spivack University of Rhode Island, [email protected]

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Citation/Publisher Attribution Dzaugis, M., Spivack, A.J., D'Hondt, S. Radiolytic H2 production in martian environments (2018) Astrobiology, 18(9), pp. 1137-1146. DOI: 10.1089/ast.2017.1654

This Article is brought to you for free and open access by the Graduate School of Oceanography at DigitalCommons@URI. It has been accepted for inclusion in Graduate School of Oceanography Faculty Publications by an authorized administrator of DigitalCommons@URI. For more information, please contact [email protected]. Authors Mary Dzaugis, Arthur J. Spivack, and Steven D’Hondt

This article is available at DigitalCommons@URI: https://digitalcommons.uri.edu/gsofacpubs/429 ASTROBIOLOGY Volume 18, Number 9, 2018 Mary Ann Liebert, Inc. DOI: 10.1089/ast.2017.1654

Radiolytic H2 Production in Martian Environments

Mary Dzaugis, Arthur J. Spivack, and Steven D’Hondt

Abstract

Hydrogen, produced by water radiolysis, has been suggested to support microbial communities on . We quantitatively assess the potential magnitude of radiolytic H2 production in wet martian environments (the ancient surface and the present subsurface) based on the radionuclide compositions of (1) eight proposed landing sites, and (2) three sites that individually yield the highest or lowest calculated radiolytic H2 production rates on Mars. For the proposed landing sites, calculated H2 production rates vary by a factor of *1.6, while the three comparison sites differ by a factor of *6. Rates in wet martian sediment and micro- fractured rock are comparable with rates in terrestrial environments that harbor low concentrations of microbial (e.g., subseafloor basalt). Calculated H2 production rates for low-porosity (<35%), fine-grained martian sediment (0.12–1.2 nM/year) are mostly higher than rates for Pacific subseafloor basalt (*0.02–0.6 nM/ year). Production rates in martian high-porosity sediment (>35%) and microfractured (1 mm) hard rock (0.03 to <0.71 nM/year) are generally similar to rates in South Pacific basalt, while yields for larger martian fractures (1 and 10 cm) are one to two orders of magnitude lower (<0.01 nM/year). If minerals or brine that amplify radiolytic H2 production rates are present, H2 yields exceed the calculated rates. Key Words: Mars—Water radiolysis—Habitability—Hydrogen—Geochemistry—Mars 2020. Astrobiology 18, 1137–1146.

1. Introduction Previous studies suggested that radiolysis might support (i.e.,Linet al., 2005b; Sherwood Lollar et al., he search for extraterrestrial life has prompted >20 2007; Lefticariu et al., 2010; Tu¨rke et al., 2015). However, Tsurface and orbital missions to Mars (Walter, 2015). few studies quantified radiolysis rates. To better under- Many of these missions have focused on identifying past and stand the origin of the subsurface flux on Mars, present aqueous environments, as water is necessary for life Onstott et al. (2006) investigated the diffusion of He, H2, as we know it. However, it is not the only requirement for life. and CH4 through crust. They calculated ra- Habitable environments must also provide biologically har- diolytic production rates for the deep subsurface crust vestable energy. A variety of geochemical processes produce based on the chemical composition of martian meteorites chemical species that can be metabolized by microorganisms and a 6.6 km-thick crustal profile of estimated porosity. In for energy. Several studies have proposed that some sub- contrast to their global approach, we calculate specific surface microbial communities on Earth rely on molecular rates for a broad range of lithologies, including both water- hydrogen (H2) as their primary donor (i.e., Stevens saturated rock and sediment, based on compositional data and McKinley, 1995; Pedersen, 2000). In terrestrial subsur- at 11 specific sites on Mars. face environments, H2 is abiologically produced by water These 11 sites include the three currently proposed Mars radiolysis (i.e., Pedersen, 2000; Lin et al., 2005a; Blair et al., 2020 Rover landing sites (Witze, 2017), five other sites that 2007; Tu¨rke et al., 2015; Dzaugis et al., 2016), rock weath- were previously under consideration for the Mars 2020 ering (i.e., Stevens and McKinley, 1995; Pedersen, 2000), mission (Ono et al., 2016), and three sites with minimum or and serpentinization (i.e., Kelley et al., 2001, 2005). These maximum mean martian rates of radiolytic H2 production chemolithotrophic communities serve as models for life on (based on radionuclide concentrations). Our purpose of se- other , such as Mars (McCollom and Seewald, 2013). lecting these sites is to quantify (1) the magnitude of ra- Here we quantify the potential for radiolytic production of H2 diolytic H2 production in the wet martian past at potential in water-saturated environments, such as the ancient martian Mars 2020 landing sites, and (2) the range of H2 production surface or the present subsurface. rates in present-day martian subsurface groundwater.

Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island. Mary Dzaugis et al., 2018; Published by Mary Ann Liebert, Inc. This Open Access article is distributed under the terms of the Creative Commons License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.

1137 1138 DZAUGIS ET AL.

Identification of past and present habitable environments eration, as all eight sites may have once harbored liquid water, is one of the objectives of Mars Exploration missions and potentially H2-oxidizing life (Ono et al., 2016). (Mustard et al., 2013). Because oxidative H2 respiration is a Finally, we chose three additional locations, based on source of chemical energy for microbial life, estimates of their radioisotope compositions, to evaluate minimum and past H2 production rates based on the radioisotope compo- maximum radiolytic H2 production rates on the past martian sitions of proposed Mars 2020 sites are important for un- surface and in the present subsurface: derstanding the ancient habitability of the sites. (maximum rate), the northern pole (minimum rate for an Eight sites were originally considered as the possible ice-covered region), and Promethei Terra (minimum rate for landing sites for the Mars 2020 mission (Mustard et al., a region without ice cover). The locations of the 11 sites are 2013). Many of these locations have similar geological and shown in Figure 1. topographic features, in part because the final site must be material ranges from loose, fine-grained safe for rovers to land and travel (Mustard et al., 2013). The to large pieces of fractured rock. To account for this eight candidate sites were narrowed to three based on the heterogeneity, we calculate H2 production rates using two following criteria: an ancient environment likely to have been different radiolysis models. One model is for fractured hard- habitable, accessible rocks with potential for biosignature rock aquifers (Dzaugis et al., 2015, 2016), the other is for preservation, and geological materials that may answer water-saturated fine-grained sediment (Blair et al., 2007). questions about planetary evolution (Witze, 2017). The three sites currently under consideration for the 2. Methods Mars 2020 landing site are as follows: Crater, an In this section, we describe the radiolysis models and ancient that has both deltaic and lacustrine deposits discuss the variables used in our calculations: fractured rock with well-defined stratigraphy (Schon et al., 2012; Witze, composition, sediment porosity, radioactivity, density, and 2017); NE Syrtis Major which was once volcanically ac- H yield per 100 eV absorbed (G-value). Lithologies of the tive, with widespread evidence of past liquid-water activity 2 11 localities are described in Table 1, and calculated H and a variety of interesting environments, including a 2 production rates are given in Table 2. banded -carbonate unit and a crater-retaining cap of mafic rock (Hiesinger and Head, 2004; Ono et al., 2016); 2.1. Fractured hard-rock model and , where provided evidence of ancient hydrothermal activity and/or We previously used this model to calculate production rates hot springs (Squyres et al., 2008). in terrestrial oceanic basaltic aquifers (Dzaugis et al., 2016). In In addition to these three locations, we evaluate radiolytic H2 this study, we use parameter values appropriate for Mars.

production for five other sites originally under consideration Within hard-rock aquifers, ionizing produced for Mars 2020: Crater, Crater, , during the decay of radioactive elements in the rock decom- , and SW Melas . We include these sites poses water and leads to production of hydrogen as points of comparison for the three sites still under consid- molecules (Le Cae¨r, 2011). To calculate H2 production rates

FIG. 1. Topographic map of Mars. Site names and significant features identified. Topology map based on data from the Mars Orbiter Laser Altimeter ( et al., 2003). RADIOLYTIC H2 IN MARTIAN ENVIRONMENTS 1139

Table 1. Locations, Radioisotope Concentrations, and Lithologic Descriptions of Sites Used to Calculate Potential Martian Radiolytic H2 Production Rates Location Thorium Potassium Site (lat N, long E) (ppm) (wt %) Main lithology/geological feature Acidalia Planitia 48, -32 0.94 0.44 Mottled lava Mawrth Vallis 24.0, -18.9 0.74 0.35 Channel deposits, phyllosilicate minerals Columbia Hills -14.4, 175.6 0.63 0.32 Hydrothermal, hot springs NE Syrtis Major 17.8, 77.1 0.60 0.31 Volcanic and hot spring activity Jezero Crater 18.5, 77.4 0.59 0.31 Ancient delta rich in minerals Nili Fossae 21.0, 74.5 0.57 0.29 Methane plumes, carbonate, and altered clay minerals Holden Crater -26.4, -34.9 0.53 0.28 Ancient lake and flood deposits, alluvial fans Eberswalde Crater -23.0, -33.0 0.51 0.30 Ancient delta, possible lake deposits SW Melas Basin -12.2, -70.0 0.45 0.31 Canyon, cuts through lake deposits Promethei Terra -67, 97 0.25 0.22 Interbedded volcanic rock and impact breccia Northern pole 87, 5 0.17 7.7E-02 Dust and ice

Radionuclide concentrations are determined from the interpolated distribution maps in Figure 1. Uranium concentrations are calculated using a U/Th ratio of 0.28. Bold font marks the current sites under consideration for Mars 2020 rover landing.

within fractured hard-rock aquifers, we first integrate the de- Finally, H2 production rate (nanomolar H2/year) is given cay energy that reaches the water [MeV/(cm2$s)]. This is the as follows: radiant flux, and depends on Production rate (G D G D G D ), Decay power, decay energy per time per solid angle ¼ a a þ b b þ c c [MeV/(sr$s)], determined by the activity decay energy where the absorbed dose rate (D ) from each type of of the radionuclides (U, Th, and K) in the rock. a,b,g radiation is multiplied by its respective G-value (G ), and Irradiance, amount of incident power normalized to a,b,g the number of H molecules created per 100 eV of energy surface area (decay power/cm2), depends on the ge- 2 absorbed (see Section 2.5 for more details on G-values). For ometry of the radiation’s path from the source to the a more detailed description of the parameters and the surface of interest. equations, see Dzaugis et al. (2015).

Attenuation, the decrease of intensity as the radiation As mentioned above, the radiant flux and therefore dose travels through rock and water. rates are affected by the attenuation of radiation energy. At- 3 The water-volume-normalized dose rate [MeV/(cm water$s)], tenuation depends on matrix composition. The martian crust is equal to the radiant flux divergence between rock–water is dominantly basaltic (McSween, 2015). Igneous rocks from interfaces divided by the distance between the interfaces. and craters and SNC (Shergotty, Nakhla, and

Table 2. Radiolytic H2 Production Rates Given the Radionuclide Concentrations of 11 Localities on Mars’ Surface

H2 production rates H2 production rates for water-filled rock for water-saturated fractures in nMH2/year sediment in nMH2/year Width: Width: Width: Porosity: Porosity: Porosity: Site Main lithology/geological feature 1 lm 1cm 10 cm 5% 35% 80% Acidalia Planitia Mottled lava plains 0.35 0.008 0.003 1.2 0.71 0.19 Mawrth Vallis Channel deposits, phyllosilicate minerals 0.27 0.007 0.003 0.93 0.56 0.15 Columbia Hills Hydrothermal, hot springs 0.23 0.006 0.002 0.80 0.48 0.13 NE Syrtis Major Ancient delta rich in clay minerals 0.22 0.006 0.002 0.75 0.45 0.12 Jezero Crater Volcanic and hot spring activity 0.22 0.006 0.002 0.75 0.45 0.12 Nili Fossae Methane plumes, carbonate, and altered 0.21 0.005 0.002 0.72 0.43 0.11 clay minerals Holden Crater Ancient lake and flood deposits, alluvial 0.20 0.005 0.002 0.67 0.40 0.10 fans Eberswalde Crater Ancient delta, possible lake deposits 0.19 0.005 0.002 0.65 0.39 0.10 SW Melas Basin Canyon, cuts through lake deposits 0.17 0.005 0.002 0.58 0.35 0.09 Promethei Terra Interbedded volcanic rock and impact 0.10 0.003 0.001 0.33 0.20 0.05 breccias Northern pole Dust and ice 0.06 0.002 <0.001 0.21 0.12 0.03

Volume-normalized rates are given for three fractured-rock scenarios (varying fracture width) and three sediment scenarios (varying porosity). Bold font marks the current sites under consideration for Mars 2020 rover landing. 1140 DZAUGIS ET AL.

Chassigny) meteorites, as well as measurements from the where porosity is low. The calculations with 35% porosity are g ray spectrometer (GRS) on Mars Odyssey, all indicate consistent with many previous studies of martian sediment. A *40–55 wt % SiO2 (McSween et al., 2003). We thus value of 35% porosity has been used in many studies that utilize energy–range relationships for each type of radia- calculate timescales for formation of martian depositional tion traveling through basalt or water. For a-radiation, the fans and deltas, such as in channelized fans located in Holden energy–range equation for basalt was derived by Brennan and crater and (Jerolmack et al., 2004; Kleinhans, Lyons (1989). To calculate attenuation of b and g radiation 2005; Metz et al., 2009). Finally, because our model is re- through rock and water, we use energy–range data from the stricted to medium-silt grain size (30 mm), we provide results National Institute of Standards and Technology (NIST) da- for 80% porosity, a value typical of uncompacted pelagic clay tabase (Hubbell and Seltzer, 2004; Berger et al.,2005).Be- on Earth’s seafloor (Hamilton, 1976; Spinelli et al., 2004). cause the NIST database does not include energy–range data We update the values of Blair et al. (2007) for the decay 238 235 232 for basalt, we use data for SiO2 adjusted for the higher density energy of the U, U, and Th series, assuming secular typical of martian basalt. equilibrium, and 40K (Supplementary Table S4). For each ra- The dose rate also depends on the thickness of rock ad- dionuclide, we separately sum the energy of a-, b-, and/or g- jacent to the fracture. We calculate dose rates produced from radiation energy (U and Th: MeV/decay series; K: MeV/decay). the radiation emitted by 1-m-thick rock abutting both sides Our b-energy sums differ greatly from those of Blair et al. of the fracture. One meter of rock accounts for >99% of the (2007), because we calculate an average initial energy for each entire dose rate. a- and b-Radiation generated by the 238U, b-particle, while Blair et al. use the maximum energy value for 235U, and 232Th decay series and b particles by 40K decay each b-particle. b-Particles are emitted with a continuous have ranges which are much <1 m in basalt, and >99% of g- spectrum of energies ranging from near-zero to a maximum radiation is absorbed over this distance. However, since a- value specific to each radionuclide. To account for this distri- radiation is more energetic than b-org-radiation and travels bution, when calculating b-decay energy sums, we use the in rock on average *30 mm, absorbed dose rates in micro- average initial energy, which is approximately one-third of the fractures produced by 30 mm-thick rock are only *10% maximum energy (L’Annunziata, 2007). These updated values lower than rates from 1 m of rock. match the values that we use in our fractured rock calculations. Using the dose rates and G-values, we calculate water- volume-normalized radiolytic H production rates for a range 2 2.3. Radionuclide concentrations of fracture apertures. H2 production rate per volume of water decreases as fracture width increases. The stopping power of Both radiolysis models require radionuclide activities. The water results in loss of particle energy near the rock–water principle radioisotopes that can drive water radiolysis on Mars 238 235 232 40 interface. The decrease in volume-normalized H2 production are in the U, U, and Th decay series and K. The U is due to the limited ranges of a and b radiation (30 mmand andThseriesemita-, b-, and g-radiation, whereas 40Kemits a few mm, respectively) and increased volume of water b-andg-radiation. Radiolysis rate is proportional to radionu- (Dzaugis et al., 2016), which does not produce significant clide activities. For 40Kand232Th abundances, we used data radiation itself. Since we do not know the distribution of collected using the GRS on the Mars Odyssey (Boynton et al., fracture widths in martian basalt, we present three cases to 2007). GRSs determine the abundance and distribution of K illustrate how H2 yields depend on width (1 mm, 1 cm, and andThbymeasuringcharacteristicg rays that are emitted from 10 cm). Supplementary Table S1 (Supplementary Data are the top few tens of centimeters of the ’s surface (Boynton available online at www.liebertonline.com/ast) summa- et al., 2004). The GRS data comprise the most exhaustive rizes factors relevant to H2 calculations. record of radionuclide concentrations for Mars. However, this record is geographically low resolution. The footprint diame- ters over which 50% of the Th and K signals are received are 2.2. Sediment model 240 and 215 km, respectively. Therefore, although microscale We also calculate radiolysis rates for fine-grained water- variability in radionuclide concentrations undoubtedly exists, saturated sediment in which we assume a homogeneous our calculations do not take into account variations in radio- mixture of water and particles <30 mm in diameter (stopping nuclides at distance smaller than the GRS footprints. distance of a-particles, the distance a particle travels before Landers and rovers have also collected K abundance data its kinetic energy is 0) (Blair et al., 2007). (McLennan, 2001). The lander and rover measurements, how- In this protocol, dose rates for a-, b-, and g-radiation are ever, are more spatially restricted,andUandThconcentrations calculated based on activity (Section 2.3), the sum of radia- are not determined. K abundance based on satellite GRS cor- tive energy released by each radionuclide decay series, grain relates to in situ data collected at landing sites by surface in- density (Section 2.4), the ratios of relative stopping power for struments (Boynton et al., 2007; Karunatillake et al., 2007). each radiation type, and porosity of the sediment (Blair et al., Galactic cosmic rays (GCRs) can also contribute to ra- 2007). Once radiation doses are known, we can calculate the diolysis at the surface of Mars, if water is present. GCRs H2 production by multiplying the dose rate from a-, b-, and comprise 85% protons, 14% a particles, and small per- g-radiation by their respective radiation chemical yields centage of heavy ions (Dartnell et al., 2007). However, (G-value, see Section 2.5). For a full summary of the factors *3 m below the surface GCR are no longer the main source used in this protocol, see Supplementary Tables S2 and S3. of radiation; at depths >3 m, radionuclides contained in We include three different porosities (5%, 35%, and 80%) martian sediment or rock provide the main radiation source to parallel the fractured rock model. The calculations with 5% for radiolytic H2 production (Hassler et al., 2014). porosity define a high-H2-production end-member scenario, Th and K are mapped for the globe (Boynton et al., 2007). because H2 production rates per volume of water are highest However, near the poles, ice cover can bias Th and K RADIOLYTIC H2 IN MARTIAN ENVIRONMENTS 1141 abundances toward lower values because some of the top 10 cm element measurements, major element of martian is composed of ice (Taylor et al., 2007; Boynton et al., 2007). meteorites, and chemical analysis of martian igneous rocks by Boynton et al. (2007) summed elemental spectra and binned rovers. They argue that the basaltic component of the martian them in 5·5 bins to improve signal-to-noise ratios. We in- crust has a density >3100 kg/m3, which is higher than previ- clude the reported uncertainty associated with smoothed radio- ously thought (Baratoux et al., 2014; Zharkov and Gudkova, nuclide concentration measurements in our final radiolysis error 2016). In our calculations, we use density of 3350 kg/m3 which calculations reported in Supplementary Tables S5 and S6. U is the peak value calculated using the CIPW (Cross, Iddings, abundances have not been calculated from GRS data due to its Pirsson, and Washington) norm on GRS chemical maps (Bar- weak signal-to-noise ratio (Boynton et al., 2007; Karunatillake atoux et al., 2014). While this density is representative of et al., 2007). In this study, we calculate U concentrations by martianbasalt,itisalsosimilartovaluesusedinsediment using Th concentrations from GRS data and assuming a constant transport models for Mars which use grain density values U/Thratioof0.28(avaluecharacteristic of most SNC mete- of 3400 km/m3 (Kleinhans, 2005; Hoke et al., 2014). orites) (McLennan, 2003; Baratoux et al., 2011). We show global maps of U, Th, and K distributions in Figure 2. 2.5. G-values Radionuclide concentrations vary with lithology. There is We calculate H production rates using pure water evidence on Mars for a global dust unit with relatively 2 G-values (Table 3). These G-values are well constrained for constant basaltic mineralogy (Yen et al., 2005). However, if each type of radiation. For both our models, we assume this global unit is present, it is <10 cm thick since the martian constant G-values for the energy ranges of radiation in this surface does not have a constant radionuclide distribution as study (<5.8 MeV). G-values depend on linear energy transfer measured by GRS (Boynton et al., 2007). (LET), the energy absorbed by the medium normalized to distance (-dE/dx). However, G(H ) appears to plateau at a 2.4. Density 2 LET value of *150 keV/mm, which corresponds to a 5 MeV Density values are based on the range suggested by a-particle (Crumie`re et al., 2013). Therefore, at high LET Baratoux et al. (2014) for martian crust. Baratoux et al. values of a-particles in this study, there is minimal effect on (2014) based their calculations of density on GRS surface the G-value. We did not find any work specifically on the

FIG. 2. Radionuclide concentrations. (A) Thorium (ppm) and (B) Potassium (wt %). Th and K concentrations are based on results from the Mars Odyssey Gamma Ray Spectrometer (Boynton et al., 2007). As described in the text, we calculate U concentrations using a constant U/Th ratio of 0.28. 1142 DZAUGIS ET AL.

Table 3. G-Values in Molecules H2/100 eV sites in this study, calculated production rates vary by a factor for Pure Water of *6. This range includes the likely low-biased radionuclide concentrations at >75N due to the presence of ice. abcIf ice-covered regions are excluded, the range is *3.5-fold. a b a Acidalia Planitia, in the northern highlands, has the G(H2) 1.30 0.6 0.24 highest production rates due to its high radioisotope abun- aEssehli et al. (2011). dances (Table 2). Fine-grained sediments with 5% porosity b Kohan et al. (2013). have rates of 1.2 nM/year, whereas microfractured rock has aH2 production rate of 0.35 nM/year, assuming the same composition at a depth where water is now present or was present in the past. Between 75N and 75S, the radioiso- effect of b-particle LET, therefore we assume yields to be tope composition of Promethei Terra has the lowest calcu- constant for G (H ) as well. The G(H ) values we use in our b 2 2 lated rates: 0.33 nM/year for 5% porosity sediment and calculations provide a minimum estimate of H production 2 0.10 nM/year for 1 mm-wide fractures. Including polar re- because specific minerals (e.g., Kumagai et al., 2013) and/or gions, H production rates may be as low as 0.21 nM/year in brines (Kelm and Bohnert, 2004; Kumagai et al., 2013) can 2 5% porosity sediment and 0.061 nM/year in 1 mm-wide frac- increase H yields. In Section 3.4, we further discuss po- 2 tures (Table 2). tential effects of minerals and brine on G-values and H2 production rates. As mentioned previously, our calculations 4. Discussion normalize the final radiolytic H2 production values to the total volume of water, presented in units of nanomolar H2 In the following sections, we discuss martian H2 pro- per year (nM/year). duction rates and assess how much life might be supported by radiolysis. We then compare martian and terrestrial rates. 3. Results Because we assume water saturation, the calculated rates do not hold for present surface conditions at these sites. Instead, As we mentioned previously, fracture width and porosity they correspond to rates for ancient (wet-surface) Mars and affect water-volume-normalized H production rate (nM/ 2 present-day wet subsurface environments with the same year), principally because radioactive element concentrations radioisotope compositions and physical properties used in in water are typically much lower than those in rock. We our calculations. assume that concentrations of U, Th, and K in martian water Previous studies identified the potential for chemolitho- are equivalent to those in seawater. These concentrations are trophic life in the martian subsurface due to the likely so low that we do not include them in our calculations (Pil- presence of liquid water and the production of reduced son, 2012). Volcanic rocks have a range of fracture widths. In chemicals (primarily H through serpentinization of ultra- fractured rock, with a constant radioisotope composition, 2 mafic rock) (Mancinelli, 2000; Ehlmann et al., 2010; microfractures have the highest H production rates per vol- 2 McCollom and Seewald, 2013). Most production of H by ume of water (Table 2) (Dzaugis et al., 2016). Production 2 serpentinization is limited to temperatures of 150Cto rates in a 1 mm-wide fracture are more than two orders of 315C (McCollom and Bach, 2009). In contrast, radiolytic magnitude higher than those in a 10 cm-wide fracture H production is more ubiquitous, occurring wherever water (Table 2). H production rates (nM/year) in sediment at 5% 2 2 or ice is in contact with rock or sediment. porosity are *6.4 times higher than those at 80% porosity (Table 2). Production rates in low-porosity, fine-grained 4.1. H production rates at eight sediments are higher than those in fractured rock. At any 2 proposed landing sites given site, H2 production rate in sediment with *60% porosity is similar to the rate in microfractured rock. Calculated production rates for water-saturated litholo- H2 production rates for water-saturated lithologies with gies are all within a factor of 2 for the eight landing sites. radionuclide concentrations equal to our 11 sites are sum- The relative uncertainties in production rates are similar, marized in Table 2. Calculated H2 production rates are *7%, at these sites (Supplementary Tables S5 and S6). within a factor of 2 of each other, given constant physical These uncertainty estimates include the reported uncertainty conditions, and the GRS radioisotope compositions of the of GRS radionuclide data. Of the three sites currently under original eight potential Mars 2020 landing sites. We also consideration for the Mars 2020 mission, radionuclide calculated uncertainties for H2 production, based on re- concentrations characteristic of Columbia Hills lead to H2 ported one-sigma uncertainty, the estimated standard error production rates that are slightly higher than those at the associated with the smoothed GRS radionuclide data other two sites. However, considering the uncertainty of (Boynton et al., 2007). The relative uncertainty for most the GRS measurements at these three sites, the three values sites ranges from 5% to 8%, although Promethei Terra and are indistinguishable from each other (Supplementary the Northern Pole have uncertainties closer to 15% (Sup- Tables S5 and S6). Of the other five sites previously under plementary Tables S5 and S6). consideration for Mars 2020, the radioisotope concentra- Mawrth Vallis, an ancient channel, has the highest rates of tions of Mawrth Vallis yield the highest production rates the landing sites, while SW Melas basin rates are *1.6 times and the concentrations of SW Melas yield the lowest rates lower (Table 2). The production rates for the three sites still (Table 2). For fine-grained sediment with 5% porosity, under consideration for the Mars 2020 mission (Jezero Cra- Mawrth Vallis production approaches 1 nM/year. The ter, NE Syrtis Major, and Columbia Hills), which have the highest rates based on the fractured rock model are three to next highest rates, are within 7% of each other. Across the 11 four times lower. RADIOLYTIC H2 IN MARTIAN ENVIRONMENTS 1143

If radiolytic H2 supported life on the ancient martian The calculated production rates for Mars’ northernmost surface, water-saturated sediment or rock with the radio- latitudes (e.g., 0.21 nM/year for 5% porosity sediment) are isotope content of Mawrth Vallis could have supported 1.6 lower than those associated with Promethei Terra, which has times as many cells in the same volume of water as water- higher radionuclide concentrations (Table 1). As previously saturated sediment or rock with the radioisotope composi- mentioned, the measured concentrations are likely biased by tion of Melas basin (assuming the same physical properties the permanent ice cap. Despite the bias toward low values at Mawrth Vallis and SW Melas). Larger variations in the for this region covered by water ice and dust (Bibring et al., number of cells may have occurred if the dominant sediment 2004), calculated H2 production can reach rates higher than and/or rock at the sites differed in porosity and fracture those derived for ice-free regions of Mars; the maximum width, and/or varied in radionuclide concentration on dis- rate calculated for sediment of the ice-capped area is *0.6 nM/ tance scales shorter than the GRS footprints. The physical year. This comparison suggests that there may be locally high properties of each location, that is, porosity and fracture H2 generation rates within or under the polar ice caps. How- geometry, lead to order-of-magnitude variations in H2 pro- ever, H2 G-values for ice are about half of those for liquid duction. A variety of geological formations likely occur at water used in our study (Table 3). G(H2) values for a-, b-, each of the 11 sites, leading to variations in porosity, frac- and g-radiation in ice at temperatures of 77 K are 0.7, 0.3, ture width, radionuclide concentrations, and consequently, and 0.1 molecules H2/100 eV, respectively (Johnson and radiolytic H2 production rates. Quickenden, 1997).

4.3. Comparison with terrestrial H2 production rates 4.2. Total range of radiolytic H2 production rates Calculated H production rates for martian microfractured Calculated H production rates based on surface martian 2 2 rock overlap with calculated rates in Earth’s basaltic seafloor. radioisotope concentrations differ by a factor of *6when In the upper 100 m of South Pacific basalt, production rates ice-covered regions are included (Table 2). The distribu- range from *0.02 to 0.6 nM/year (Dzaugis et al., 2016). tionofH production is principally driven by U and Th 2 Calculated martian production rates for water-saturated series nuclide distributions (Fig. 1) since both decay series low-porosity sediment with the radioisotope composition produce a-radiation, which carries the most energy and of the Mars 2020 sites generally exceed those of South Pacific generates the most H per MeV absorbed. U (238U+235U) 2 basement basalt. Rates for water-saturated, high-porosity decay series activity and 232Th decay series activity, re- martian sediment (‡35%) and water-filled martian micro- spectively, are responsible for 49–53% and 39–42% of fractures (1 mm wide) are comparable with South Pacific calculated H production for each site, while 40K decay 2 basalt rates. Rates for larger water-filled martian fractures contributes only 4–11%. (1 cm wide and 10 cm wide) are generally one to two orders The largest production rate, 1.2 nM/year for 5% sediment of magnitude lower than South Pacific basalt rates (com- porosity, is associated with Acidalia Planitia in the northern pare with Dzaugis et al., 2016). highlands due to this area’s high radionuclide abundance Radiolytic H has been suggested to be the dominant (Fig. 2 and Table 2). Acidalia Planitia contains plains of 2 electron donor in Earth’s subseafloor basalt older than 10 eolian deposits, as well as cone and dome structures possibly Ma (Tu¨rke et al., 2015; Dzaugis et al., 2016). On Earth, formed by mud volcanism, hot spring/geysers, or lava flows microbes have been isolated from subseafloor basalt as old (Farrand et al., 2005). If mud volcanism was responsible for as *71 Ma (Sylvan et al., 2015). the cones, relatively unaltered sediment from depth was Although these data from Earth are intriguing in their transported to the surface. This once-fluid-rich material implications for life on Mars, we do not yet have enough could contain biomarkers (Farrand et al., 2005; Oehler and information to definitively calculate how much biomass Allen, 2010). In general, the northern highlands are enriched consumption of radiolytic H might support on Mars. For in K and Th (Fig. 2). This region contains andesite, basaltic 2 example, the Gibbs energy of martian H consumption is not andesite, or weathered basalt (Bandfield, 2000; Wyatt and 2 yet known, because we do not know in situ concentrations McSween, 2002). Basaltic lava flows at Montes of H or relevant oxidants. and shield volcanoes have below average K 2 and Th concentrations. 4.4. Enhanced H production Just as radionuclide-enriched regions yield the maximum 2 calculated H2 production rate, radionuclide-depleted regions The martian rates we calculate are based on bulk GRS yield the minimum rate (Fig. 2). Between 75N and 75S, measurements for the 11 locations. At finer spatial scales, Promethei Terra yields the lowest calculated rate, 0.33 nM/ radiolytic H2 production rates may locally be much higher year for 5% porosity sediment (Table 2). Promethei Terra or lower, as the distribution of radionuclides is not homo- consists of interbedded volcanic rock and impact breccia. geneous throughout all sediment and basalt. In Earth’s There is evidence that both fluvial and glacial processes oceanic crust, Tu¨rke et al. (2015) calculated radiolytic H2 shaped the terrain of Promethei Terra. The landscape of production within palagonite, altered basaltic glass enriched Promethei Terra was also shaped by the impact that formed in radionuclides and containing *14–38 wt % H2O (Pauly the Hellas basin (Ivanov et al., 2010). The radioisotope et al., 2011). This combination of high radionuclides and composition of the large impact basin, , also water content can result in high H2 production rates. Tu¨rke yields a low calculated H2 production rate: 0.44 nM/year for et al. (2015) calculated H2 accumulation rates in palagonite 5% porosity sediment (note there is high uncertainty in GRS basalt samples taken from North Pond Area, ridge flack of measurements in Hellas Planitia, resulting in *17% relative the Mid-Atlantic Ridge, using radiolysis models based on uncertainty of the H2 production rate). Blair et al. (2007) and Lin et al. (2005a). Their calculated 1144 DZAUGIS ET AL. rates are between 0.15 and 1.75 nM/year in the rocks’ in- Biosphere Investigations; Grant NSF-OCE-0939564) for tergranular water. Given fluid flow and estimates of H2 funding this study. This is C-DEBI contribution 388. concentrations at North Pond, Tu¨rke et al. (2015) calculate Author Disclosure Statement that radiolytic H2 can accumulate to levels high enough in the ridge flank to support hydrogentrophic life. No competing financial interests exist. On Mars, there is abundant evidence of the presence of hydrated silicate and sulfate minerals that may provide References habitable environments for microorganisms (Ehlmann et al., 2009). Hygroscopic sulfate salts have also been detected in Al Soudi, A.F., Farhat, O., Chen, F., , B.C., and Schnee- gurt, M.A. (2017) Bacterial growth tolerance to concentra- martian soils (Smith et al., 2014). Through deliquescence, tions of chlorate and perchlorate salts relevant to Mars. Int J these salts can produce saturated brines with low Astrobiol 16:229–235. freezing points (Davila et al., 2010; Al Soudi et al., 2017). If Bandfield, J.L. (2000) A global view of martian surface com- the hygroscopic sulfate salts contain radionuclides, this may positions from MGS-TES. Science 287:1626–1630. produce another habitable microenvironment for microor- Baratoux, D., Toplis, M.J., Monnereau, M., and Gasnault, O. ganisms that obtain energy from sulfate reduction by H2. (2011) Thermal history of Mars inferred from orbital geo- Microscale variation aside, the mean rates in bulk wet martian chemistry of volcanic provinces. Nature 472:338–341. sediment and microfractures are capable of supporting mi- Baratoux, D., Samuel, H., Michaut, C., Toplis, M.J., Monner- crobial life. eau, M., Wieczorek, M., Garcia, R., and Kurita, K. (2014) H2 yields may locally be higher than those we have cal- Petrological constraints on the density of the Martian crust. J culated, because some minerals catalyze radiolytic H2 pro- Geophys Res Planets 119:1707–1727. duction. For example, mordenite, a zeolite mineral, increases Berger, M.J., Coursey, J.S., Zucker, M.A., and Chang, J. (2005) G(H2) values for g-radiation relative to pure water by up to a ESTAR, PSTAR, and ASTAR: Computer Programs for Cal- factor of *3 (Kumagai et al., 2013). There is spectral ev- culating Stopping-Power and Range Tables for , idence of zeolite minerals on Mars’ surface (Ruff, 2004; Protons, and Helium Ions (version 1.2.3). National Institute Ehlmann et al., 2009). of Standards and Technology, Gaithersburg, MD. [Online]. Available online at: http://physics.nist.gov/Star [2016, June]. Bromine-enriched brine also catalyzes radiolytic H2 production, relative to pure water; LaVerne et al. (2009) Bibring, J.P., Langevin, Y., Poulet, F., Gendrin, A., Gondet, B., experimentally showed that Br-saturated solutions have Berthe´, M., Soufflot, A., Drossart, P., Combes, M., Bellucci, G., Moroz, V., Mangold, N., Schmitt, B.; OMEGA Team. higher G(H2) values for g radiation than pure water. The martian surface contains evidence of past and present brines (2004) Perennial water ice identified in the south polar cap of Mars. Nature 428:627–630.

(Rao et al., 2005; Martı´nez and Renno, 2013). Known Br Blair, C.C., D’Hondt, S., Spivack, A.J., and Kingsley, R.H. concentrations on Mars are generally low (ppm) (Rao et al., (2007) Radiolytic hydrogen and microbial respiration in 2005); however, if Br-rich brines occur or occurred any- subsurface sediments. Astrobiology 7:951–970. where on Mars, they locally enhance (or enhanced) radio- Boynton, W.V., Feldman, W.C., Mitrofanov, I.G., Evans, L.G., lytic H2 production. Reedy, R.C., Squyres, S.W., Starr, R., Trombka, J.I., D’Us- ton, C., Arnold, J.R., Englert, P.A.J., Metzger, A.E., Wanke, 5. Conclusion H., Bruckner, J., Drake, D.M., Shinohara, C., Fellows, C., Hamara, D.K., Harshman, K., Kerry, K., Turner, C., Ward, Calculated radiolytic H2 production rates (nM/year) vary by a factor of 2 for water-saturated sediment or rock with M., Barthie, H., Fuller, K.R., Storms, S.A., Thornton, G.W., the radioisotope concentrations of sites currently or previ- Longmire, J.L., Litvak, M.L., and Ton’Chiev, A.K. (2004) The Mars Odyssey Gamma-Ray Spectrometer instrument ously under consideration as the Mars 2020 landing site. The suite. Space Sci Rev 110:37–83. highest calculated rates are for material with the radioiso- Boynton, W.V., Taylor, G.J., Evans, L.G., Reedy, R.C., Starr, R., tope concentrations of Acidalia Planitia (where surface Janes,D.M.,Kerry,K.E.,Drake,D.M.,Kim,K.J.,Williams, materials are enriched in U and Th). R.M.S., Crombie, M.K., Dohm, J.M., Baker, V., Metzger, A.E., Calculated rates for wet low-porosity martian sediment Karunatillake, S., Keller, J.M., Newsom, H.E., Arnold, J.R., consistently equal or exceed rates previously calculated for Bruckner, J., Englert, P.A.J., Gasnault, O., Sprague, A.L., Mi- South Pacific basement basalt. Rates for microfractured rock trofanov, I., Squryes, S.W., Trombka, J.I., d’Uston, L., Wanke, (with 1 mm-wide fractures) and sediment with >35% po- H.,andHamara,D.K.(2007)ConcentrationofH,Si,Cl,K,Fe, rosity at the Mars 2020 sites match the low-end of rates and Th in the low- and mid- regions of Mars. JGeophys calculated for South Pacific basement basalt. In short, ra- Res E Planets 112:1–15. diolytic H2 production rates in wet martian sediment and Brennan, B.J. and Lyons, R.G. (1989) Ranges of alpha particles microfractured rock are comparable with rates in terrestrial in various media. Ancient TL 7:32–37. regions known to harbor microbial life. Consequently, local Crumie`re, F., Vandenborre, J., Essehli, R., Blain, G., Barbet, J., variation in radiolytic H2 production of the ancient wet and Fattahi, M. (2013) LET effects on the hydrogen pro- martian surface will be fruitful to consider during final se- duction induced by the radiolysis of pure water. Radiat Phys lection and exploration of the Mars 2020 landing site. Chem 82:74–79. Dartnell, L.R., Desorgher, L., Ward, J.M., and Coates, A.J. Acknowledgments (2007) Modelling the surface and subsurface Martian radia- tion environment: implications for astrobiology. Geophys Res We thank the National Aeronautics and Space Adminis- Lett 34, doi:10.1029/2006GL027494. tration (Grant NNX12AD65G) and the U.S. National Sci- Davila, A.F., Duport, L.G., Melchiorri, R., Ja¨nchen, J., Velea, ence Foundation (through the Center for Dark Energy S., de Los Rios, A., Faire´n, A.G., Mo¨hlmann, D., McKay, RADIOLYTIC H2 IN MARTIAN ENVIRONMENTS 1145

C.P., Ascaso, C., and Wierzchos, J. (2010) Hygroscopic salts Roe, K.K., Lebon, G.T., Rivizzigno, P.; and the AT3-60 and the otential for life on Mars. Astrobiology 10:617–628. Shipboard Party. (2001) An off-axis hydrothermal vent field Dzaugis, M.E., Spivack, A.J., and D’Hondt, S. (2015) A near the Mid-Atlantic Ridge at 30 N. Nature 412:145–149. quantitative model of water radiolysis and chemical produc- Kelley, D.S., Karson, J.A., Fru¨h-, G.L., Yoerger, D.R., tion rates near radionuclide-containing solids. Radiat Phys Shank, T.M., Butterfield, D.A., Hayes, J.M., Schrenk, M.O., Chem 115:127–134. Olson, E.J., Proskurowski, G., Jakuba, M., Bradley, A., Bra- Dzaugis, M.E., Spivack, A.J., Dunlea, A.G., Murray, R.W., and zelton, W.J., Roe, K., Elend, M.J., Delacour, A., Bernasconi, D’Hondt, S. (2016) Radiolytic in the S.M., Lilley, M.D., Baross, J.A., Summons, R.E., and Sylva, subseafloor basaltic aquifer. Front Microbiol 7:76. S.P. (2005) A serpentinite-hosted ecosystem: the Lost City Ehlmann, B.L., Mustard, J.F., Swayze, G.A., Clark, R.N., Bishop, hydrothermal field. Science (New York, N.Y.) 307:1428–1434. J.L., Poulet, F., Des Marais, D.J., Roach, R.N., Milliken, R.E., Kelm, M. and Bohnert, E. (2004) A Kinetic Model for the Wray, J.J., Barnouin-Jha, O., and Murchie, S.L. (2009) Iden- Radiolysis of Chloride Brine, its Sensitivity against Model tification of hydrated silicate minerals on Mars using MRO- Parameters and a Comparison with Experiments (Report CRISM: geologic context near Nili Fossae and implications for FZKA 6977). Institut Fur Nukleare Entsorgung, Karlsruhe. aqueous alteration. J Geophys Res 114:E00D08. Kleinhans, M.G. (2005) Flow discharge and sediment transport Ehlmann, B.L., Mustard, J.F., and Murchie, S.L. (2010) Geo- models for estimating a minimum timescale of hydrological logic setting of serpentine deposits on Mars. Geophys Res activity and channel and delta formation on Mars. J Geophys Lett 37, doi:10.1029/2010GL042596. Res 110, doi.org/10.1029/2005JE002521. Essehli, R., Crumie`re, F., Blain, G., Vandenborre, J., Pottier, F., Kohan, L.M., Sanguanmith, S., Meesungnoen, J., Causey, P., Grambow, B., Fattahi M., and Mostafavi M. (2011) H2 Pro- Stuart, C.R., and Jay-Gerin, J. (2013) Self-radiolysis of tri- duction by g and He ions water radiolysis, effect of presence tiated water. 1. A comparison of the effects of 60Co g-rays TiO2 nanoparticles. Int J Hydrogen Energ 36:14342–14348. and tritium b-particles on water and aqueous solutions at Farrand, W.H., Gaddis, L.R., and Keszthelyi, L. (2005) Pitted room temperature. RSC Adv 3:19282. cones and domes on Mars: observations in Acidalia Planitia Kumagai, Y., Kimura, A., Taguchi, M., Nagaishi, R., Yamagishi, and Mensae using MOC, THEMIS, and TES data. J I., and Kimura, T. (2013) Hydrogen production in gamma ra- Geophys Res 110:E05005. diolysis of the mixture of mordenite and seawater: fukushima Hamilton, E.L. (1976) Variations of density and porosity with NPP accident related. J Nucl Sci Technol 50:130–138. depth in deep-sea sediments. J Sed Pet 46:280–300. L’Annunziata, M.F. (2007) Radioactivity: Introduction and Hassler, D.M., Zeitlin, C., Wimmer-Schweingruber, R.F., Ehres- History,Ist ed., Elsevier, Oxford. mann, B., Rafkin, S., Eigenbrode, J.L., Brinza, D.E., Weigle, G., LaVerne, J.A., Ryan, M.R., and Mu, T. (2009) Hydrogen pro- Bo¨ttcher, S., Bo¨hm,E.,Burmeister,S.,Guo,J.,Ko¨hler, J., Martin, duction in the radiolysis of bromide solutions. Radiat Phys C, Reitz, G., Cucinotta, F.A., Kim, M.-H., Grinspoon, D., Bul- Chem 78:1148–1152. lock, M.A., Posner, A., Go´mez-Elvira, J., Vasavada, A., Grot- Le Cae¨r, S. (2011) Water radiolysis: influence of oxide surfaces zinger, J.P.; MSL Science Team. (2014) Mars’ surface radiation on H2 production under . Water 3:235–253. environment measured with the ’s Lefticariu, L., Pratt, L.A., LaVerne, J.A., and Schimmelmann, Rover. Science 343, doi:10.1126/science.1244797. A. (2010) Anoxic pyrite oxidation by water radiolysis prod- Hiesinger, H. and Head, J.W. (2004) The Syrtis Major volcanic ucts—a potential source of biosustaining energy. Earth Pla- province, Mars: synthesis from data. J net Sci Lett 292, doi.org/10.1016/j.epsl.2010.01.020. Geophys Res 109:E01004. Lin, L.-H., Hall, J., Lippmann-Pipke, J., Ward, J.A., Sherwood Hoke, M., Hynek, B., Di Achille, G., and , E.W.H. Lollar, B., DeFlaun, M., Rothmel, R., Moser, D., Gihring, (2014) The effects of sediment supply and concentrations on T.M., Mislowack, B., and Onstott, T.C. (2005a). Radiolytic the formation timescale of martian deltas. Icarus 228:1–12. H2 in continental crust: nuclear power for deep subsurface Hubbell, J.H. and Seltzer, S.M. (2004) Tables of X-ray mass microbial communities. Geochem Geophys Geosyst 6, doi attenuation coefficients and mass energy-absorption coeffi- .org/10.1029/2004GC000907. cients (version 1.4). National Institute of Standards and Lin, L.-H., Slater, G.F., Sherwood Lollar, B., Lacrampe-Couloume, Technology, Gaithersburg, MD. [Online] Available online at: G., and Onstott, T.C. (2005b). The yield and isotopic compo- http://physics.nist.gov/xaamdi [2016, June]. sition of radiolytic H2, a potential energy source for the deep Ivanov, M.A., Korteniemi, J., Kostama, V.-P., Raitala, J., To¨r- subsurface biosphere. Geochim Cosmochim Acta 69:893–903. ma¨nen, T., and , G. (2010) Major episodes in the Mancinelli, R.L. (2000) Accessng the Martian deep subsurface geologic history of western Promethei Terra, Mars. J Geo- to search for life. Planet Space Sci 48:1035–1042. phys Res 115:E03001. Martı´nez, G.M. and Renno, N.O. (2013) Water and brines on Jerolmack, D.J., Mohrig, D., Zuber, M.T., and Byrne, S. (2004) Mars: current evidence and implications for MSL. Space Sci A minimum time for the formation of Holden Northeast fan, Rev 175:29–51. Mars. Geophys Res Lett 31, doi.org/10.1029/2004GL021326. McCollom, T.M. and Bach, W. (2009) Thermodynamic con- Johnson, R.E. and Quickenden, T.I. (1997) Photolysis and ra- straints on hydrogen generation during serpentinization of diolysis of water ice on outer solar system bodies. J Geophys ultramafic rocks. Geochim Cosmochim Acta 73:856–875. Res Planets 102:10985–10996. McCollom, T.M. and Seewald, J.S. (2013) Serpentinites, hy- Karunatillake, S., Keller, J.M., Squyres, S.W., Boynton, W.V., drogen, and life. Elements 9:129–134. Bruckner, J., Janes, D.M., Gasnault, O., and Newsom, H.E. McLennan, S.M. (2001) Crustal heat production and the ther- (2007) Chemical compositions at sites subject mal evolution of Mars. Geophys Res Lett 28:4019–4022. to Mars Odyssey Gamma Ray Spectrometer constraints. J McLennan, S.M. (2003) Large-ion lithophile element fraction- Geophys Res E Planets 112:1–16. ation during the early differentiation of Mars and the com- Kelley, D.S., Karson, J.A., Blackman, D.K., Fru¨h-Green, G.L., position of the martian primitive mantle. Meteorit Planet Sci Butterfield, D.A., Lilley, M.D., Olson, E.J., Schrenk, M.O., 38:895–904. 1146 DZAUGIS ET AL.

McSween, H.Y. (2015) Petrology on Mars. Am Mineral 100: Squyres, S.W., Arvidson, R.E., Ruff, S.W., Gellert, R., Morris, 2380–2395. V., Ming, D.W., Crumpler, L., Farmer, J.D., Des Marais, D.J., McSween, H.Y., Grove, T.L., and Wyatt, M.B. (2003) Con- Yen, A., McLennan, S.M., Calvin, W., Bell III, J.F., Clark, straints on the composition and petrogenesis of the Martian B.C., Wang, A., McCoy, T.J., , M.E., and de Souza crust. J Geophys Res 108, doi.org/10.1029/2003JE002175. Jr, P.A. (2008) Detection of Silica-Rich Deposits on Mars. Metz, J.M., Grotzinger, J.P., Mohrig, D., Milliken, R., Prather, Science 320:1063–1067 doi:10.1126/science.1155429. B., Pirmez, C., McEwen, A.S., and Weitz, C.M. (2009) Stevens, T.O. and McKinley, J.P. (1995) Lithoautotrophic micro- Sublacustrine depositional fans in southwest Melas Chasma. bial ecosystems in deep basalt aquifers. Science 270:450–454. J Geophys Res 114, doi.org/10.1029/2009JE003365. Sylvan, J.B., Hoffman, C.L., Momper, L.M., Toner, B.M., Mustard, J.F., Adler, M., Allwood, A., Bass, D.S., Beaty, Amend, J.P., and Edwards, K.J. (2015) Bacillus rigiliprofundi D.W., Bell III, J.F., Brinckerhoff, W.B., Carr, M., Des Marais, sp. nov., an endospore-forming, Mn-oxidizing, moderately D.J., Drake, B., Edgett, K.S., Eigenbrode, J., Elkins-Tanton, halophilic bacterium isolated from deep subseafloor basaltic L.T., Grant, J.A., Milkovich, S.M., Ming, D., Moore, C., crust. Int J Syst Evol Microbiol 65:1992–1998. Murchie, S., Onstott, T.C., Ruff, S.W., Sephton, M.A., Steele, Taylor, G.J., Stopar, J.D., Boynton, W.V., Karunatillake, S., A., and Treiman, A. (2013) Report of the Mars 2020 Science Keller, J.M., Bruckner, J., Wanke, H., Dreibus, G., Kerry, Definition Team, 154 pp., posted July, 2013, by the Mars Ex- K.E., Reedy, R.C., Evans, L.G., Starr, R.D., Martel, L.M.V., ploration Program Analysis Group. Available online at http:// Squyres, S.W., Gasnault, O., Maurice, S., d’Uston, C., Eng- mepag.jpl..gov/reports/MEP/Mars_2020_SDT_Report_ lert, Rp., Dohm, J.M., Baker, V.R., Hamara, D., Janes, D., Final.pdf Sprague, A.L., Kim, K.J., Drake, D.M., McLennan, S.M., and Oehler, D.Z. and Allen, C.C. (2010) Evidence for pervasive mud Hahn, B.C. (2007) Variations in K/Th on Mars. J Geophys volcanism in Acidalia Planitia, Mars. Icarus 208:636–657. Res E Planets 112:1–20. Ono, M., Rothrock, B., Almeida, E., Ansar, A., Otero, R., Tu¨rke, A., Nakamura, K., and Bach, W. (2015) Palagonitization Huertas, A., and Heverly, M. (2016) Data-driven surface of basalt glass in the flanks of mid-ocean ridges: implications traversability analysis for Mars 2020 landing site selection. for the bioenergetics of oceanic intracrustal ecosystems. As- In: 2016 IEEE Aerospace Conference, Big Sky, MT, pp 1–12. trobiology 15:793–803. doi:10.1109/AERO.2016.7500597. Walter, M.R. (2015) The quest for a second origin of life. Onstott, T.C., McGown, D., Kessler, J., Lollar, B.S., Lehmann, Elements 11:14–15. K.K., and Clifford, S.M. (2006) Martian CH4 : sources, flux, Witze, A. (2017) Three sites where NASA might retrieve its and detection. Astrobiology 6:377–395. first Mars rock. Nature 542:279–280. Pauly, B.D., Schiffman, P., Zierenberg, R.A., and Clague, D.A. Wyatt, M.B. and McSween, H.Y. (2002) Spectral evidence for (2011) Environmental and chemical controls on palagonitization. weathered basalt as an alternative to andesite in the northern Geochem Geophys Geosyst 12, doi:10.1029/2011GC003639. lowlands of Mars. Nature 417:263–266. Pedersen, K. (2000) Exploration of deep intraterrestrial microbial Yen, A.S., Gellert, R., Schro¨der, C., Morris, R.V., Bell III, J.F., life: current perspectives. FEMS Microbiol Lett 185:9–16. Knudson, A.T., Clark, B.C., Ming, D.W., Crisp, J.A., Ar- Available online at www.ncbi.nlm.nih.gov/pubmed/10731600 vidson, R.E., Blaney, D., Bruckner, J., Christensen, P.R., Pilson, M.E.Q. (2012) An Introduction to the Chemistry of the DesMarais, D.J., de Souza Jr, P.A., Economou, T.E., Ghosh, Sea,2nd ed. Cambridge University Press, Cambridge. A., Hahn, B., Herkenhoff, K.E., Haskin, L.A., Hurowitz, J.A., Rao, M.N., Sutton, S.R., Mckay, D.S., and Dreibus, G. (2005) Joliff, B.L., Johnson, J.R., Klingelhofer, G., Madsen, M.B., Clues to Martian brines based on halogens in salts from McLennan, S.W., Tosca, N.J., Wang, A., Wyatt, M., and and MER samples. J Geophys Res 110:12–16. Zipfel, J. (2005) An integrated view of the chemistry and Ruff, S.W. (2004) Spectral evidence for zeolite in the dust on mineralogy of Martian soils. Nature 436:49–54. Mars. Icarus 168:131–143. Zharkov, V.N. and Gudkova, T.V. (2016) On the model struc- Schon, S.C., Head, J.W., and Fassett, C.I. (2012) An overfilled ture of the gravity field of Mars. Solar Syst Res 50:235–250. lacustrine system and progradational delta in Jezero crater, Mars: implications for climate. Planet Space Sci Address correspondence to: 67:28–45. Mary Dzaugis Sherwood Lollar, B., Voglesonger, K., Lin, L.-H., Lacrampe- Graduate School of Oceanography Couloume, G., Telling, J., Abrajano, T.A., Onstott, T.C., and University of Rhode Island Pratt, L.M. (2007) Hydrogeologic controls on episodic H2 Narragansett Bay Campus release from precambrian fractured rocks—energy for deep 215 South Ferry Road subsurface life on Earth and Mars. Astrobiology 7, doi.org/ Narragansett, RI 02882 10.1089/ast.2006.0096. Smith, D., Neumann, G., Arvidson, R.E., Guinness, E.A., and E-mail: [email protected] Slavney, S. (2003) Mars Global Surveyor Laser Altimeter Mission Experiment Gridded Data Record. NASA Planetary Submitted 31 January 2017 Data System, MGS-M-MOLA-5-MEGDR-L3-V1.0. Accepted 7 February 2018 Smith, M.L., Claire M.W., Catling, D.C., and Zahnle, K.J. Associate Editor: Christopher McKay (2014) The formation of sulfate, nitrate and perchlorate salts in the martian atmosphere. Icarus 231:51–64. Spinelli, G.A., Giambalvo, E., and Fisher, A.T. (2004) Sedi- Abbreviations Used ment permeability, distribution, and influence on fluxes in GCR ¼ galactic cosmic rays oceanic basement. In Hydrogeology of the Oceanic Litho- GRS ¼ g ray spectrometer sphere, edited by E.E. Davis, and H. Elderfield, Ch. 6, LET ¼ linear energy transfer Cambridge University Press, Cambridge, UK, pp 151–188.