J D D Tarnas

J F F Mustard

B Sherwood Lollar

V Stamenković

K M M Cannon

J.-P Lorand

T C C Onstott

J R R Michalski

O Warr

A M M Palumbo

B Sherwood Lollar

A.-C Plesa

Earth-like Habitable Environments in the Subsurface of Mars

whether they are published or not. The documents may come

Introduction

The surface of Mars is an extremely hostile environment characterized by freezing temperatures, desiccating conditions, high levels of ionizing radiation, oxidizing chemicals, low pressures, and a lack of liquid water that preclude any Earth-like organisms from surviving without adaptation that is unprecedented on Earth. Though some extremophiles on Earth have evolved to tolerate certain conditions relevant for Mars, any martian life formed in earlier eras would have taken refuge in the subsurface for survival (National Academies of Sciences, Engineering, and Medicine et al., 2018;[START_REF] Michalski | The Martian subsurface as a potential window into the origin of life[END_REF], perhaps temporarily inhabiting transiently habitable near-surface environments formed by meteorite impacts into ice-rich rock [START_REF] Schwenzer | Puncturing Mars: How impact craters interact with the Martian cryosphere[END_REF]. Within the last three decades, researchers have discovered that fluids preserved within Earth's kilometers deep subsurface contain significant quantities of biomass utilizing a wide diversity of redox reactions to drive their microbial metabolisms [START_REF] Onstott | Paleo-Rock-Hosted Life on Earth and the Search on Mars: A Review and Strategy for Exploration[END_REF]. Similar habitable subsurface environments could potentially have existed on ancient Mars [START_REF] Onstott | Paleo-Rock-Hosted Life on Earth and the Search on Mars: A Review and Strategy for Exploration[END_REF][START_REF] Tarnas | Radiolytic H2 production on Noachian Mars: Implications for habitability and atmospheric warming[END_REF], may still exist on modern Mars [START_REF] Michalski | Groundwater activity on Mars and implications for a deep biosphere[END_REF], and could exist on other planetary objects including ocean worlds, small bodies like Ceres, and exoplanets, making them an exciting frontier of planetary exploration (National Academies of Sciences, Engineering, and Medicine et al., 2018;[START_REF] Stamenković | The next frontier for planetary and human exploration[END_REF].

Planetary objects with lower surface gravity than Earth, including Mars, likely have higher volumes of habitable pore and fracture space that extend deeper into the crust compared to Earth (e.g., [START_REF] Goossens | Evidence for a low bulk crustal density for Mars from gravity and topography[END_REF][START_REF] Lewis | A surface gravity traverse on Mars indicates low bedrock density at Gale crater[END_REF], increasing the volume of feasibly supportable biomass if sufficient liquid water and redox energy is available [START_REF] Sleep | Maintenance of permeable habitable subsurface environments by earthquakes and tidal stresses[END_REF].

In Earth's subsurface, actively metabolizing microbes can be sustained by fluids that have been isolated from surface substrate input for >10 6 -10 9 years [START_REF] Wang | Long-Term Sustainability of a High-Energy, Low-Diversity Crustal Biome[END_REF][START_REF] Holland | Deep fracture fluids isolated in the crust since the Precambrian era[END_REF][START_REF] Li | Sulfur mass-independent fractionation in subsurface fracture waters indicates a long-standing sulfur cycle in Precambrian rocks[END_REF][START_REF] Warr | Tracing ancient hydrogeological fracture network age and compartmentalisation using noble gases[END_REF][START_REF] Lollar | Follow the Water': Hydrogeochemical Constraints on Microbial Investigations 2.4 km Below Surface at the Kidd Creek Deep Fluid and Deep Life Observatory[END_REF]-similar to groundwater isolation timescales expected on Mars [START_REF] Grimm | On the secular retention of ground water and ice on Mars[END_REF]-powered solely by redox energy derived from water-rock reactions including radiolysis and serpentinization [START_REF] Lollar | The contribution of the Precambrian continental lithosphere to global H 2 production[END_REF][START_REF] Warr | Mechanisms and rates of 4He, 40Ar, and H2 production and accumulation in fracture fluids in Precambrian Shield environments[END_REF]. Radiolysis, the breaking of H 2 O molecules in pore spaces by ⍺, β, and ɣ radiation released from decay of radionuclides within host rock, generates both dissolved reductants (e.g. H 2 ; Lin et al., 2005b;[START_REF] Warr | Mechanisms and rates of 4He, 40Ar, and H2 production and accumulation in fracture fluids in Precambrian Shield environments[END_REF] and dissolved oxidants (e.g. H 2 O 2 ; ref. [START_REF] Lefticariu | Anoxic pyrite oxidation by water radiolysis products -A potential source of biosustaining energy[END_REF][START_REF] Li | Sulfur mass-independent fractionation in subsurface fracture waters indicates a long-standing sulfur cycle in Precambrian rocks[END_REF]. The dissolved H 2 generated by radiolysis can be used directly as a reductant by subsurface microbes, while the primary oxidant byproducts of radiolysis can oxidize sulfides within the host rock matrix to form dissolved sulfate (SO 4 ) [START_REF] Lefticariu | Anoxic pyrite oxidation by water radiolysis products -A potential source of biosustaining energy[END_REF][START_REF] Li | Sulfur mass-independent fractionation in subsurface fracture waters indicates a long-standing sulfur cycle in Precambrian rocks[END_REF] that can then be used by microbes as an oxidant (Lin et al., 2005b;[START_REF] Wang | Long-Term Sustainability of a High-Energy, Low-Diversity Crustal Biome[END_REF][START_REF] Chivian | Environmental Genomics Reveals a Single-Species Ecosystem Deep Within Earth[END_REF][START_REF] Li | Sulfur mass-independent fractionation in subsurface fracture waters indicates a long-standing sulfur cycle in Precambrian rocks[END_REF][START_REF] Lollar | Follow the Water': Hydrogeochemical Constraints on Microbial Investigations 2.4 km Below Surface at the Kidd Creek Deep Fluid and Deep Life Observatory[END_REF]. Sulfatereducing microorganisms are found kilometers deep on Earth in many groundwaters that have been isolated for >10 7 years (Lin et al., 2005b;[START_REF] Wang | Long-Term Sustainability of a High-Energy, Low-Diversity Crustal Biome[END_REF][START_REF] Chivian | Environmental Genomics Reveals a Single-Species Ecosystem Deep Within Earth[END_REF][START_REF] Li | Sulfur mass-independent fractionation in subsurface fracture waters indicates a long-standing sulfur cycle in Precambrian rocks[END_REF][START_REF] Lollar | Follow the Water': Hydrogeochemical Constraints on Microbial Investigations 2.4 km Below Surface at the Kidd Creek Deep Fluid and Deep Life Observatory[END_REF], and a significant component of the H 2 and SO 4 in especially ancient groundwaters is typically derived from radiolysis [START_REF] Li | Sulfur mass-independent fractionation in subsurface fracture waters indicates a long-standing sulfur cycle in Precambrian rocks[END_REF][START_REF] Warr | Mechanisms and rates of 4He, 40Ar, and H2 production and accumulation in fracture fluids in Precambrian Shield environments[END_REF]. Radiation doses from decay of rock-matrix-hosted radionuclides are insufficient to sterilize microbial communities, as demonstrated by the presence of microbes in these subsurface settings (e.g., Lin et al., 2005a;Lin et al. 2005b;[START_REF] Wang | Long-Term Sustainability of a High-Energy, Low-Diversity Crustal Biome[END_REF][START_REF] Chivian | Environmental Genomics Reveals a Single-Species Ecosystem Deep Within Earth[END_REF][START_REF] Lollar | Follow the Water': Hydrogeochemical Constraints on Microbial Investigations 2.4 km Below Surface at the Kidd Creek Deep Fluid and Deep Life Observatory[END_REF]. Thus, these Earth systems prove that in any subsurface environment containing radionuclides, liquid H 2 O, and sulfides, radiolysis alone has the potential to provide sufficient biologically-supportive redox energy to sustain a subsurface biome [START_REF] Onstott | Martian CH4: Sources, Flux, and Detection[END_REF][START_REF] Lollar | The contribution of the Precambrian continental lithosphere to global H 2 production[END_REF].

There are many alternative potential pathways to generate additional redox couples for life in subsurface martian environments, including in fluid mixing zones that create chemical disequilibria [START_REF] Lollar | Hydrogeologic Controls on Episodic H2 Release from Precambrian Fractured Rocks-Energy for Deep Subsurface Life on Earth and Mars[END_REF], which will not be considered here. Accordingly, the gas production rates and supportable cell density ranges presented in this work should be considered conservative estimates.

Methods

Summary

The model used to estimate H 2 production rates, sulfate production rates, dissolved H 2 concentrations, dissolved sulfate concentrations, and supportable sulfate-reducing cell densities is similar to models employed in [START_REF] Lefticariu | Anoxic pyrite oxidation by water radiolysis products -A potential source of biosustaining energy[END_REF]; [START_REF] Li | Sulfur mass-independent fractionation in subsurface fracture waters indicates a long-standing sulfur cycle in Precambrian rocks[END_REF][START_REF] Altair | Microbial habitability of Europa sustained by radioactive sources[END_REF] (Section 2.2). Porosity (Section 2.3), sulfide concentrations (Section 2.4), average sulfide grain sizes (Section 2.5), radionuclide concentrations (Section 2.6), and sulfate-reducing metabolic rates are the key parameters necessary to constrain the model. H 2 production rates are dependent on porosity and radionuclide concentrations, increasing as porosity and radionuclide concentrations increase. Sulfate production rates are dependent on porosity, radionuclide concentrations, sulfide concentrations, and average sulfide grain size. They increase as porosity, radionuclide concentrations, and sulfide concentrations increase and decrease as sulfide grain size increases, as less reactive surface area is exposed when sulfide grains are larger. Dissolved H 2 and sulfate concentrations increase as porosity decreases, as there is less fluid in which to dissolve the H 2 and sulfate that is produced. Supportable sulfate-reducing cell densities in units of cells (kg rock) -1 increase as porosity and sulfate production rates increase, since sulfate is the limiting redox nutrient in all model results presented here. Supportable sulfate-reducing cell densities in units of cells L -1 decrease as porosity increases because there is less fluid per volume rock for the cells to reside in. Supportable sulfate-reducing cell densities are higher when the assumed sulfate-reducing metabolic rate is lower. Porosity is constrained using density estimates for the martian crust from orbital gravimetry [START_REF] Goossens | Evidence for a low bulk crustal density for Mars from gravity and topography[END_REF] that are supported by rover gravimetry [START_REF] Lewis | A surface gravity traverse on Mars indicates low bedrock density at Gale crater[END_REF] (Section 2.3). Sulfide concentrations (Section 2.4), average sulfide grain sizes (Section 2.5), and radionuclide concentrations (Section 2.6) are constrained by their measured values in martian meteorites. The maximum and minimum assumed sulfate-reducing metabolic rates are based on the fastest and slowest sulfate-reducing metabolic rates estimated for fracture fluids in the gold mines of the Witwatersrand Basin [START_REF] Wang | Long-Term Sustainability of a High-Energy, Low-Diversity Crustal Biome[END_REF][START_REF] Chivian | Environmental Genomics Reveals a Single-Species Ecosystem Deep Within Earth[END_REF]. The calculated sulfatereducing cell densities are compared to those found in Earth's deep subsurface [START_REF] Onstott | Indigenous and contaminant microbes in ultradeep mines[END_REF][START_REF] Cockell | Impact Disruption and Recovery of the Deep Subsurface Biosphere[END_REF][START_REF] Magnabosco | The biomass and biodiversity of the continental subsurface[END_REF][START_REF] Onstott | Paleo-Rock-Hosted Life on Earth and the Search on Mars: A Review and Strategy for Exploration[END_REF] (Fig. 2).

The amount of radiolytic hydrogen and sulfate that would be produced in each of the different martian meteorite source regions is calculated based on their individual K, Th, and U concentrations (Fig. 1e, Table 2), sulfide concentrations (Fig. 1f, Table 1), average sulfide grain sizes (Fig. 1f, Table 1), and estimated range of porosity (Fig. 1c, Table 2) before the impact events that ejected these rocks from the martian gravity well. Based on the produced amounts of these redox nutrients, the number of supportable sulfate-reducing cells per kg rock is calculated for each of these meteorite source regions using the range of sulfate-reducing metabolic rates observed in Earth's deep subsurface [START_REF] Wang | Long-Term Sustainability of a High-Energy, Low-Diversity Crustal Biome[END_REF][START_REF] Chivian | Environmental Genomics Reveals a Single-Species Ecosystem Deep Within Earth[END_REF]. Porosity range assumptions are based on density estimates of different martian terrains from orbital gravimetry (Fig. 1c-d; Section 2.2) [START_REF] Goossens | Evidence for a low bulk crustal density for Mars from gravity and topography[END_REF], a range that is supported by rover gravimetry [START_REF] Lewis | A surface gravity traverse on Mars indicates low bedrock density at Gale crater[END_REF]. The radiolysis model applied here (Section 2.3) is similar to the one employed in Lin et al., (2005b); [START_REF] Li | Sulfur mass-independent fractionation in subsurface fracture waters indicates a long-standing sulfur cycle in Precambrian rocks[END_REF]; [START_REF] Dzaugis | Radiolytic H2 Production in Martian Environments[END_REF]; [START_REF] Tarnas | Radiolytic H2 production on Noachian Mars: Implications for habitability and atmospheric warming[END_REF]and Warr et al., (2019).

Radiolysis model

The model used here follows the radiolytic H 2 and SO 4 production models presented in Lin et al., (2005a);Lin et al., (2005b); [START_REF] Li | Sulfur mass-independent fractionation in subsurface fracture waters indicates a long-standing sulfur cycle in Precambrian rocks[END_REF]; [START_REF] Tarnas | Radiolytic H2 production on Noachian Mars: Implications for habitability and atmospheric warming[END_REF]and Warr et al., (2019).

This calculates radiogenic dose rate as

(1)
where is , or radiation, is the net dosage absorbed by pore water (eV kg -1 s -1 ), is the apparent dosage from radioactive element decay (eV kg -1 s -1 ), is the weight ratio of pore water to rock, and is the stopping power of minerals within the rock matrix with respect to each type of radiation particle/ray, where = 1.5, , (Lin et al., 2005a;Lin et al., 2005b;[START_REF] Tarnas | Radiolytic H2 production on Noachian Mars: Implications for habitability and atmospheric warming[END_REF][START_REF] Warr | Mechanisms and rates of 4He, 40Ar, and H2 production and accumulation in fracture fluids in Precambrian Shield environments[END_REF]. The stopping power of minerals can be estimated using Bragg's law [START_REF] Bragg | On the α particles of radium, and their loss of range in passing through various atoms and molecules[END_REF] and typically varies by <0.5 for silicate minerals [START_REF] Nogami | The absorption factor in counting alpha rays from thick mineral sources[END_REF].

, the weight ratio of pore water to rock, primarily depends on the rock porosity with additional minor variations induced by differences in water density in solid (~900 kg m -3 ) or liquid (~1000 kg m -3 ) phase. Values from Adamiec and Aitken, 1998 are used for for 1 wt. % K, 1 ppm Th, and 1 ppm U for , and radiation, which are = 0 (Gy ka -1 ), = 0.782 (Gy ka -1 ), (Gy ka -1 ), 0.061 (Gy ka -1 ), 0.027 (Gy ka -1 ), (Gy ka -1 ), (Gy ka -1 ), (Gy ka -1 ), (Gy ka -1 ). 1 Gy = 6.2415 10 18 eV kg -1 .

H 2 yield is calculated as

(2)
where is the total yield of H 2 molecules (molecules kg -1 s -1 ) and is the number of H 2 molecules produced per 100 eV of radiation. The conversion factor between (kg water) and (kg water and rock matrix) is , the weight ratio of pore water to rock. For H 2 O, H 2 molecules (100 eV) -1 , H 2 molecules (100 eV) -1 , H 2 molecules (100 eV) - 1 (Lin et al., 2005a;Lin et al., 2005b;[START_REF] Tarnas | Radiolytic H2 production on Noachian Mars: Implications for habitability and atmospheric warming[END_REF][START_REF] Warr | Mechanisms and rates of 4He, 40Ar, and H2 production and accumulation in fracture fluids in Precambrian Shield environments[END_REF]. The value of changes depending on the liquid or solid substance filling the pore space, and has received extensive study due to its importance in the context of nuclear waste storage. significantly increase H 2 and oxidant production efficiency from radiolysis [START_REF] Kumagai | Hydrogen production in gamma radiolysis of the mixture of mordenite and seawater[END_REF]. Zeolites are found in both bedrock [START_REF] Ehlmann | Identification of hydrated silicate minerals on Mars using MRO-CRISM: Geologic context near Nili Fossae and implications for aqueous alteration[END_REF] and dust [START_REF] Ruff | Spectral evidence for zeolite in the dust on Mars[END_REF] on Mars, thus this mineral is expected to enhance radiolytic H 2 and oxidant production in some parts of the crust, but this effect is not modeled here in order to focus on the most conservative estimates possible.

SO 4 production is calculated as

(3)
where is the total yield of SO 4 molecules (moles m -2 sulfide year -1 ) and is the number of SO 4 molecules produced per m 2 sulfide per Gy radiation. A value from [START_REF] Lefticariu | Anoxic pyrite oxidation by water radiolysis products -A potential source of biosustaining energy[END_REF] is used that is also used in [START_REF] Li | Sulfur mass-independent fractionation in subsurface fracture waters indicates a long-standing sulfur cycle in Precambrian rocks[END_REF] and [START_REF] Altair | Microbial habitability of Europa sustained by radioactive sources[END_REF], which is 2.1 10 -9 mol m -2 Gy -1 . However, while the experiments of [START_REF] Lefticariu | Anoxic pyrite oxidation by water radiolysis products -A potential source of biosustaining energy[END_REF] were performed under high-energy radiation, they showed that increases under low-radiation-dose conditions, which correspond to typical radiation doses in natural systems [START_REF] Li | Sulfur mass-independent fractionation in subsurface fracture waters indicates a long-standing sulfur cycle in Precambrian rocks[END_REF]. Furthermore, these experiments only investigated the oxidation, structural damage, and chemical weathering of sulfides exposed to γ radiation. As [START_REF] Lefticariu | Anoxic pyrite oxidation by water radiolysis products -A potential source of biosustaining energy[END_REF] note, accounting for increased oxidation, structural damage, and chemical weathering of sulfides from α and β radiationwhich typically represent >90% of total irradiated energy-would increase the value, perhaps significantly. This means once again that the calculated sulfate production rates, and thus supportable sulfate-reducing cell densities herein, are likely conservative estimates. The concentration of sulfate in fluid from radiolytic production is expressed as

(4)
where is the production rate of dissolved sulfate (M year -1 ), is the concentration of sulfide in the rock matrix (volume ratio), and is the average specific surface area of sulfides in the rock matrix (m -2 sulfide/kg). values used in this study are listed in Table 2, which encompasses the values found in martian meteorites (Greenwood et al., 2000b;Greenwood et al., 2000a;[START_REF] Lorand | Pyrrhotite and the remanent magnetization of SNC meteorites: a changing perspective on Martian magnetism[END_REF][START_REF] Lorand | Sulfide mineralogy and redox conditions in some shergottites[END_REF][START_REF] Gattacceca | Matching Martian crustal magnetization and magnetic properties of Martian meteorites[END_REF][START_REF] Chevrier | Sulfide petrology of four nakhlites: Northwest Africa 817, Northwest Africa 998, Nakhla, and Governador Valadares[END_REF][START_REF] Lorand | Nickeliferous pyrite tracks pervasive hydrothermal alteration in Martian regolith breccia: A study in NWA 7533[END_REF][START_REF] Baumgartner | The role of sulfides in the fractionation of highly siderophile and chalcophile elements during the formation of martian shergottite meteorites[END_REF]Jean-Pierre Lorand et al., 2018b;Jean-Pierre Lorand et al., 2018a). values ranging from 2.26 10 1 m 2 kg -1 and 1.41 10 3 m 2 kg -1 are used, which correspond to average sulfide grain sizes of 2-125 µm. The values for each meteorite class are assigned based on the range of sulfide grain sizes measured in those martian meteorites (Fig. 1f; Tables 1 &2). Finally, following [START_REF] Altair | Microbial habitability of Europa sustained by radioactive sources[END_REF], minimum and maximum sulfate-reducing metabolic rates of 5.5 10 -18 -3.6 10 -17 moles cell -1 year -1 are assumed, which correspond to the minimum and maximum sulfatereducing metabolic rates estimated in deep subsurface fracture fluids of the gold mines from the Witwatersrand Basin [START_REF] Wang | Long-Term Sustainability of a High-Energy, Low-Diversity Crustal Biome[END_REF][START_REF] Chivian | Environmental Genomics Reveals a Single-Species Ecosystem Deep Within Earth[END_REF].

Porosity model

The average porosity ranges for the martian meteorite source regions are estimated using density estimates for the different regions of Mars' crust from orbital gravimetry [START_REF] Goossens | Evidence for a low bulk crustal density for Mars from gravity and topography[END_REF] that are validated by rover gravimetry in Gale crater that estimates a porosity range of 40 ± 6% for the top ~300 meters of bedrock [START_REF] Lewis | A surface gravity traverse on Mars indicates low bedrock density at Gale crater[END_REF]. Porosity in this context includes both fracture and pore space within rocks. The orbital gravimetry estimates give average densities of ~1600 kg m -3 for the southern highlands, 2400 kg m -3 for the northern lowlands, and ~2800 kg m -3 for the volcanic bulges. It is assumed that the volcanic bulges and northern lowlands are the possible source regions for the SNCs and the southern highlands are the source region for the regolith breccias [START_REF] Cassata | Chronology of martian breccia NWA 7034 and the formation of the martian crustal dichotomy[END_REF]. In the estimation, an average solid crust matrix density range of 2000-3200 kg m -3 is used for the southern highlands crust, consistent with its mineralogy [START_REF] Ehlmann | Mineralogy of the Martian Surface[END_REF], and 2600-3200 kg m -3 is used for the northern lowlands and volcanic bulges, consistent with volcano-specific orbital gravimetry [START_REF] Broquet | The Gravitational Signature of Martian Volcanoes[END_REF]. It is assumed the difference between the gravimetry density measurements and rock matrix density estimates is caused by pore space filled with ice or water.

The densities add linearly such that where is porosity. This means the composition of the southern highlands rock is assumed to vary, on average, between pure phyllosilicate (~2000 kg m -3 ) and dense basalt (~3200 kg m -3 ), while the composition of the northern lowlands and volcanic bulges rocks are assumed to vary between 50-50 phyllosilicatebasalt mixture (~2600 kg m -3 ) and dense basalt (~3200 kg m -3 ). Given uncertainties in density estimates from orbital gravimetry, to be conservative, estimated porosities were reduced by 10%.

This does not model depth dependence of porosity, but rather the average porosity of the source regions for the individual martian meteorites. This gives approximate average porosity ranges of 15-35% for the regolith breccias, 5-15% for the SNCs. The SNC porosity assumptions are supported by images of porous float vesicular basalt rocks in Gusev crater imaged by the Spirit rover [START_REF] Mcmahon | The habitability of vesicles in martian basaltMars habitability[END_REF][START_REF] Mcsween | Petrology on Mars[END_REF].

Sulfide concentrations

Martian meteorites contain sulfides of both magmatic (pyrrhotite) [START_REF] Lorand | Sulfide mineralogy and redox conditions in some shergottites[END_REF]Lorand et al., 2018b) and hydrothermal (pyrite) [START_REF] Lorand | Nickeliferous pyrite tracks pervasive hydrothermal alteration in Martian regolith breccia: A study in NWA 7533[END_REF][START_REF] Wittmann | Petrography and composition of Martian regolith breccia meteorite Northwest Africa 7475[END_REF]Lorand et al., 2018) origins (Sections 2.4 & 2.5). Sulfide abundances range from <0.01-1.0 wt.

%. The assumed sulfide concentration ranges are derived based on these sulfide concentrations found in martian meteorites (Greenwood et al., 2000b;Greenwood et al., 2000a;[START_REF] Lorand | Pyrrhotite and the remanent magnetization of SNC meteorites: a changing perspective on Martian magnetism[END_REF][START_REF] Lorand | Sulfide mineralogy and redox conditions in some shergottites[END_REF][START_REF] Gattacceca | Matching Martian crustal magnetization and magnetic properties of Martian meteorites[END_REF][START_REF] Chevrier | Sulfide petrology of four nakhlites: Northwest Africa 817, Northwest Africa 998, Nakhla, and Governador Valadares[END_REF][START_REF] Franz | Isotopic links between atmospheric chemistry and the deep sulphur cycle on Mars[END_REF][START_REF] Lorand | Nickeliferous pyrite tracks pervasive hydrothermal alteration in Martian regolith breccia: A study in NWA 7533[END_REF][START_REF] Hewins | Regolith breccia Northwest Africa 7533: Mineralogy and petrology with implications for early Mars[END_REF][START_REF] Wang | Chalcophile elements in Martian meteorites indicate low sulfur content in the Martian interior and a volatile element-depleted late veneer[END_REF][START_REF] Baumgartner | The role of sulfides in the fractionation of highly siderophile and chalcophile elements during the formation of martian shergottite meteorites[END_REF]Jean-Pierre Lorand et al., 2018b;Lorand et al., 2018a;[START_REF] Franz | A new type of isotopic anomaly in shergottite sulfides[END_REF][START_REF] Hewins | Northwest Africa 8694, a ferroan chassignite: Bridging the gap between nakhlites and chassignites[END_REF]) (Fig. 1f; Table 1). Martian meteorites contain variable abundances of sulfides with relatively consistent grain sizes for grains >10 µm. Published abundances for sulfides in martian meteorites, and reported average grain sizes and grain size range, are presented in Table 1. Magmatic sulfides (pyrrhotite)

and hydrothermal sulfides (pyrite) are typically associated with SNCs and regolith breccias, respectively (Greenwood et al., 2000b;Greenwood et al., 2000a;[START_REF] Lorand | Pyrrhotite and the remanent magnetization of SNC meteorites: a changing perspective on Martian magnetism[END_REF]Lorand et al., 2005;[START_REF] Gattacceca | Matching Martian crustal magnetization and magnetic properties of Martian meteorites[END_REF][START_REF] Chevrier | Sulfide petrology of four nakhlites: Northwest Africa 817, Northwest Africa 998, Nakhla, and Governador Valadares[END_REF][START_REF] Lorand | Nickeliferous pyrite tracks pervasive hydrothermal alteration in Martian regolith breccia: A study in NWA 7533[END_REF][START_REF] Baumgartner | The role of sulfides in the fractionation of highly siderophile and chalcophile elements during the formation of martian shergottite meteorites[END_REF]Lorand et al., 2018b;Lorand et al., 2018a). Pyrite is much less abundant than pyrrhotite as it was described only in an orthopyroxenite (ALH84001), two chassignites and a few nakhlites (as minor pyrrhotite replacement product), as well as in regolith breccias (as primary hydrothermal precipitate). Sulfide abundances are typically higher in regolith breccias than in SNCs. The listed sulfide abundances are also minimum abundances, as desert weathering of martian meteorite samples causes oxidation of sulfides and partial S losses (Greenwood et al., 2000b;Greenwood et al., 2000a;[START_REF] Lorand | Pyrrhotite and the remanent magnetization of SNC meteorites: a changing perspective on Martian magnetism[END_REF][START_REF] Lorand | Sulfide mineralogy and redox conditions in some shergottites[END_REF][START_REF] Gattacceca | Matching Martian crustal magnetization and magnetic properties of Martian meteorites[END_REF][START_REF] Chevrier | Sulfide petrology of four nakhlites: Northwest Africa 817, Northwest Africa 998, Nakhla, and Governador Valadares[END_REF][START_REF] Lorand | Nickeliferous pyrite tracks pervasive hydrothermal alteration in Martian regolith breccia: A study in NWA 7533[END_REF][START_REF] Baumgartner | The role of sulfides in the fractionation of highly siderophile and chalcophile elements during the formation of martian shergottite meteorites[END_REF]Lorand et al., 2018b;Lorand et al., 2018a).

The original report of sulfides in Gale crater via CheMin XRD data is no longer supported after recalibration of the data [START_REF] Morrison | Crystal chemistry of martian minerals from Bradbury Landing through Naukluft Plateau, Gale crater, Mars[END_REF], but evolved gas analysis-mass spectroscopy (EGA-MS) [START_REF] Wong | Detection of reduced sulfur on Vera Rubin ridge by quadratic discriminant analysis of volatiles observed during evolved gas analysis[END_REF] and sulfur isotope measurements using the Sample Analysis at Mars (SAM) instrument [START_REF] Franz | Large sulfur isotope fractionations in Martian sediments at Gale crater[END_REF] support the presence of sulfides in multiple Gale crater samples. Sulfides in Gale crater are diagenetic/epigenetic, likely resulting from thermal reduction of preexisting sulfates [START_REF] Franz | Large sulfur isotope fractionations in Martian sediments at Gale crater[END_REF].

Sulfide grain sizes

Average grain sizes for sulfides in martian meteorites range from 5-100 µm with occasional outliers up to 400 µm (Fig. 1f; [START_REF] Hewins | Northwest Africa 8694, a ferroan chassignite: Bridging the gap between nakhlites and chassignites[END_REF].

Radionuclide concentrations

Martian meteorites and remote measurements by the Gamma Ray Spectrometer (GRS) onboard Mars Odyssey [START_REF] Boynton | Concentration of H, Si, Cl, K, Fe, and Th in the low-and midlatitude regions of Mars[END_REF], which is sensitive to the top ~tens of cm of the martian crust, provide first-order constraints for radionuclide concentrations in the martian subsurface (Fig. 1). Radionuclide concentrations in the top ~tens of cm in the martian crust vary with latitude and longitude (Fig. 1a-b), and volcanic shergottite-nakhlite-chassignite (SNC) martian meteorites generally have lower radionuclide contents than regolith breccias (Fig. 1e) that are likely representative of the Noachian crust [START_REF] Cassata | Chronology of martian breccia NWA 7034 and the formation of the martian crustal dichotomy[END_REF] (Fig. 1). Elemental concentrations in martian meteorites range from ~0.01-0.4 wt. % K, ~0.01-2 ppm Th, and ~0.003-0.36 ppm U [START_REF] Onstott | Martian CH4: Sources, Flux, and Detection[END_REF][START_REF] Meyer | The Martian Meteorite Compendium[END_REF] (Fig. 1e). Some U enrichment in these samples is possible through desert contamination [START_REF] Tarnas | Radiolytic H2 production on Noachian Mars: Implications for habitability and atmospheric warming[END_REF]. K and Th concentrations estimated from GRS measurements range from ~0.07-0.6 wt. % and ~0.015-1.1 ppm [START_REF] Boynton | Concentration of H, Si, Cl, K, Fe, and Th in the low-and midlatitude regions of Mars[END_REF][START_REF] Tarnas | Radiolytic H2 production on Noachian Mars: Implications for habitability and atmospheric warming[END_REF], respectively (Fig. 1a-b,e).

For calculations of SO 4 and H 2 production in martian meteorites, the measured values for K, Th, and U in each individual meteorite class were used [START_REF] Onstott | Martian CH4: Sources, Flux, and Detection[END_REF][START_REF] Meyer | The Martian Meteorite Compendium[END_REF], integrating both the maximum and minimum radionuclide concentration values measured (Fig. 1e; Table 2). The K and Th concentration ranges for SNCs are substantially lower than K and Th concentrations measured in GRS data (Fig. 1e), meaning the majority of the martian crust at depths shallower than ~0.5 m typically has higher radionuclide concentrations than are represented by the SNCs. Following [START_REF] Hahn | Martian surface heat production and crustal heat flow from Mars Odyssey Gamma-Ray spectrometry[END_REF] and [START_REF] Tarnas | Radiolytic H2 production on Noachian Mars: Implications for habitability and atmospheric warming[END_REF], the GRS data are normalized to correct for contributions from Cl, H, and S. K concentrations measured by Mars rovers range from 0.05 wt. % measured by Spirit to 4.4 wt. % measured by Curiosity [START_REF] Gellert | Chemistry of Rocks and Soils in Gusev Crater from the Alpha Particle X-ray Spectrometer[END_REF][START_REF] Deit | The potassic sedimentary rocks in Gale Crater, Mars, as seen by ChemCam on board Curiosity[END_REF].

Results

Habitable redox conditions in martian meteorite source regions

The results presented here show that the shergottite, nakhlite, chassignite, and regolith breccia martian meteorite source localities would all produce sufficient redox energy to support sulfate-reducing life where liquid groundwater exists within these lithologies. In all cases, based on the calculations herein, sulfate is the limiting redox nutrient in comparison with H 2 . The regolith breccia source locality could support significantly more sulfate-reducing life than the SNC localities because it has higher concentrations of both radionuclides and sulfides. Of the SNCs, the shergottite and nakhlite source localities could support the highest densities of sulfatereducing cells. Key findings are illustrated in Fig. 2, displayed in Table 3, and are summarized below as:

(1) Regolith breccia source locality: The sulfate production rates for the regolith breccia source locality vary from ~[2 × 10 -14 ] -[7 × 10 -12 ] moles (kg rock) -1 year -1 , while H 2 production rates range from ~[1-3] 10 -11 moles (kg rock) -1 year -1 , which could support [7 × 102 ]-[1 × 10 6 ] sulfate-reducing cells per kg rock. The regolith breccia source locality would generate the most redox energy for sulfate-reducing microorganisms compared to the other martian meteorite source localities.

(2) Shergottite source locality: The sulfate production rates for shergottite source localities vary from ~[8 × 10 -18 ] -[2 × 10 -13 ] moles (kg rock) -1 year -1 , while H 2 production rates range from ~[4 × 10 -14 ]-[4 × 10 -12 ] moles (kg rock) -1 year -1 , which could support [2 × 10 - from ~[5 × 10 -13 ]-[6 × 10 -12 ] moles (kg rock) -1 year -1 , which could support [START_REF] Lorand | Sulfides are concentrated by hydrothermal fluids that are Srich and reducing. Many hydrothermally-formed sulfide deposits are associated with serpentinites, due to the highly reducing fluids formed during the process of serpentinization[END_REF]-[3 × 10 4 ] sulfate-reducing cells per kg rock.

(4) Chassignite source locality: The sulfate production rates for chassignite source localities vary from ~[1 × 10 -17 ]-[6 × 10 -15 ] moles (kg rock) -1 year -1 , while H 2 production rates range from ~[2 × 10 -13 ]-[2 × 10 -12 ] moles (kg rock) -1 year -1 , which could support [4 × 10

Regolith breccia ~[1-3] 10 -11 ~[2 × 10 -14 ] -[7 × 10 -12 ] [7 × 10 2 ]-[1 × 10 6 ] Shergottite ~[4 × 10 -14 ]-[4 × 10 -12 ] ~[8 × 10 -18 ] -[2 × 10 -13 ] [2 × 10 -1 ]-[3 × 10 4 ] Nakhlite ~[5 × 10 -13 ]-[6 × 10 -12 ] ~[1 × 10 -16 ] -[2 × 10 -13 ] [3]-[3 × 10 4 ] Chassignite ~[2 × 10 -13 ]-[2 × 10 -12 ] ~[1 × 10 -17 ]-[6 × 10 -15 ] [4 × 10 -1 ]-[1 × 10 3 ]
Table 3 | Estimated H 2 and sulfate production rates for martian meteorite source regions and associated supportable sulfate-reducing cell densities.

The sulfate production rates from these calculations are several orders of magnitude lower than those of [START_REF] Lefticariu | Anoxic pyrite oxidation by water radiolysis products -A potential source of biosustaining energy[END_REF] because those authors assumed 5-10 wt. % pyrite in the martian crust, while this study uses lower sulfide concentrations based on measured values from martian meteorites (Fig. 1f, Table 1). Because the model uses the lowest and highest measured sulfate-reducing metabolic rates in Witwatersrand Basin for calculating cell density estimates [START_REF] Wang | Long-Term Sustainability of a High-Energy, Low-Diversity Crustal Biome[END_REF][START_REF] Chivian | Environmental Genomics Reveals a Single-Species Ecosystem Deep Within Earth[END_REF], there is a difference between the estimated maximum and minimum range for sulfate production rates and the estimated maximum and minimum range for number of supportable sulfate-reducing cells per kg rock. H 2 production estimates, in units of H 2 dissolved in water, range from 0.002-0.165 nM H 2 /year. This compares to estimates by [START_REF] Dzaugis | Radiolytic H2 Production in Martian Environments[END_REF] of 0.001-1.2 nM H 2 /year for proposed Mars landing sites. The differences in the dissolved H 2 estimates between this study and [START_REF] Dzaugis | Radiolytic H2 Production in Martian Environments[END_REF] derive from differences in radionuclide concentrations and porosity used for calculations in the two studies.

The number of supportable sulfate-reducing cells per kg rock in the nakhlite, shergottite, and regolith breccia source localities are comparable to those found in Earth's deep subsurface (~10 4 -10 6 cells per kg rock; Onstott et al., 2019) (Fig. 2). Estimates for the chassignite source locality meanwhile are slightly lower (~10 1 -10 3 cells per kg rock). In all cases, sulfate is the redox nutrient that limits sustainable cell densities, and as such estimated cell densities are most sensitive to estimated sulfate production rates. The martian regolith breccia NWA 7034 and one of its paired samples, NWA 7533, contain the highest radionuclide and sulfide concentrations of the currently known martian meteorites, and thus their source locality would host sulfate production rates capable of supporting the greatest density of sulfate-reducing microorganisms relative to other known martian meteorites. Regolith breccia meteorites are likely representative of the southern highlands, which is a major fraction of Mars' total crustal volume. The results presented here support growing evidence that the subsurface is potentially the largest, longestlived, and most consistently habitable environment on Mars [START_REF] Michalski | The Martian subsurface as a potential window into the origin of life[END_REF][START_REF] Michalski | Groundwater activity on Mars and implications for a deep biosphere[END_REF][START_REF] Onstott | Paleo-Rock-Hosted Life on Earth and the Search on Mars: A Review and Strategy for Exploration[END_REF][START_REF] Tarnas | Radiolytic H2 production on Noachian Mars: Implications for habitability and atmospheric warming[END_REF] and could serve as a refugia for any life that ever existed on the martian surface (National Academies of Sciences, Engineering, and Medicine et al., 2018;[START_REF] Michalski | The Martian subsurface as a potential window into the origin of life[END_REF]. Other viable processes could produce redox nutrients to sustain additional metabolisms in the martian subsurface, including H 2 and CH 4 production via serpentinization and Fischer-Tropsch-type synthesis, respectively (e.g. [START_REF] Warr | Mechanisms and rates of 4He, 40Ar, and H2 production and accumulation in fracture fluids in Precambrian Shield environments[END_REF], atmospheric O 2 dissolution [START_REF] Stamenković | O 2 solubility in Martian near-surface environments and implications for aerobic life[END_REF], atmospheric H 2 and CO dissolution [START_REF] Weiss | Atmospheric energy for subsurface life on Mars?[END_REF], as well as Fe 2+ and Fe 3+ from minerals [START_REF] Onstott | Paleo-Rock-Hosted Life on Earth and the Search on Mars: A Review and Strategy for Exploration[END_REF]. If sulfides are not present in the rock matrix, these other reactions may be favorable over sulfate reduction.

These results provide scientific justification for future missions to search for signs of extant life in the martian subsurface-which is an intriguing next frontier for planetary exploration (National Academies of Sciences, Engineering, and Medicine et al., 2018;[START_REF] Stamenković | The next frontier for planetary and human exploration[END_REF]-and identifies key characteristics of optimal regions for future landing sites.

Even the relatively small sampling of Mars' petrologic diversity via four martian meteorite classes (shergottites, nakhlites, chassignites, regolith breccias) shows a wide range in redox energy production rates from these different rock types/localities (Fig. 2). Some of these localities, such as the source regions for regolith breccias, shergottites, and nakhlites could support comparable concentrations of extant life to those found in Earth's deep subsurface [START_REF] Onstott | Indigenous and contaminant microbes in ultradeep mines[END_REF][START_REF] Cockell | Impact Disruption and Recovery of the Deep Subsurface Biosphere[END_REF][START_REF] Onstott | Paleo-Rock-Hosted Life on Earth and the Search on Mars: A Review and Strategy for Exploration[END_REF]. As is demonstrated by the diversity of radionuclide concentrations, sulfide concentrations, and sulfide grain sizes in martian meteorites, some regions of the martian crust will support higher sulfate-reducing cell densities than others. Because sulfate is likely the limiting nutrient for hydrogenotrophic sulfate-reducing metabolisms in a martian subsurface biosphere rather than H 2 , the highest concentrations of sulfate-reducing microorganisms would exist in a water-bearing crustal section with high sulfide abundance, low sulfide grain sizes, and relatively high radionuclide concentrations. Regions rich in zeolites, which enhance radiolytic gas production [START_REF] Kumagai | Hydrogen production in gamma radiolysis of the mixture of mordenite and seawater[END_REF], would also sustain higher sulfate-reducing bacteria concentrations. Future missions should focus on characterizing where these criteria are met, as these would be prime landing site targets for extant life investigation from a redox energy perspective.

Discussion

Potential for higher cell densities elsewhere on Mars

The possibility of relatively high sulfide concentrations on Mars has been recognized for decades due to the presence of ultramafic lithologies there [START_REF] Burns | Evolution of sulfide mineralization on Mars[END_REF][START_REF] Baumgartner | Magmatic controls on the genesis of Ni-Cu±(PGE) sulphide mineralisation on Mars[END_REF][START_REF] Humayun | Sulfide Assimilation and Mineralization in Ancient (2.4 Ga) Shergottites[END_REF]) (Section 4.2). Concentration of magmatic sulfides are assumed to derive from assimilation of crustal sulfates and/or S-rich regolith in lava flows, as sampled by nakhlites, which likely crystallized from a sulfide-saturated melt after this assimilation [START_REF] Franz | Isotopic links between atmospheric chemistry and the deep sulphur cycle on Mars[END_REF][START_REF] Mari | Syneruptive incorporation of martian surface sulphur in the nakhlite lava flows revealed by S and Os isotopes and highly siderophile elements: implication for mantle sources in Mars[END_REF]. Additional evidence for the existence of concentrated martian sulfides has continued to accrue in the form of sulfides in martian meteorites (Table 1 andrefs . therein), in Gale crater [START_REF] Franz | Large sulfur isotope fractionations in Martian sediments at Gale crater[END_REF][START_REF] Wong | Detection of reduced sulfur on Vera Rubin ridge by quadratic discriminant analysis of volatiles observed during evolved gas analysis[END_REF], the prevalence of sulfate-hematite assemblages on the martian surface (e.g. [START_REF] Wiseman | Phyllosilicate and sulfate-hematite deposits within Miyamoto crater in southern Sinus Meridiani, Mars[END_REF], which may or may not have formed as weathering products of sulfides (e.g. [START_REF] Zolotov | Formation of jarosite-bearing deposits through aqueous oxidation of pyrite at Meridiani Planum, Mars[END_REF], verifiable orbital detections of serpentine outcrops in Noachian terrains [START_REF] Leask | Challenges in the Search for Perchlorate and Other Hydrated Minerals With 2.1-μm Absorptions on Mars[END_REF], which are often associated with formation of reduced sulfides and metal alloys on Earth (e.g. [START_REF] Economou | Sulfides associated with podiform bodies of chromite at Tsangli, Eretria, Greece[END_REF][START_REF] Thalhammer | Postmagmatic, hydrothermal origin of sulfide and arsenide mineralizations at Limassol Forest, Cyprus[END_REF]Shiga, 1987;[START_REF] Auclair | Mineralogy, geochemistry, and paragenesis of the Eastern Metals serpentinite-associated Ni-Cu-Zn deposit, Quebec Appalachians[END_REF][START_REF] Wafik | Les Minéralisations Sulfurées à Cu-Fe et les Altérations Associées dans les Ophiolites Protérozoiques de Bou Azzer et de Khzama (Anti-Atlas, Maroc) (Cu-Fe sulfureous mineralisations and the associated alterations in the Bou Azzer and Khzama Proterozoic ophiolites, Anti-Atlas, Maroc)[END_REF][START_REF] Marques | Sulfide mineralization in an ultramaficrock hosted seafloor hydrothermal system: From serpentinization to the formation of Cu-Zn-(Co)-rich massive sulfides[END_REF], Cu concentrations in Gale crater that are likely caused by presence of sulfides [START_REF] Payré | Copper enrichments in the Kimberley formation in Gale crater, Mars: Evidence for a Cu deposit at the source[END_REF], and the low oxygen fugacity (Mari et al., 2019 and refs. therein) and high sulfur fugacity of the ancient martian mantle relative to Earth's [START_REF] Wang | Chalcophile elements in Martian meteorites indicate low sulfur content in the Martian interior and a volatile element-depleted late veneer[END_REF]Mari et al., 2019 and refs. therein). Pyrrhotite hosts magnetization in multiple classes of martian meteorites (Rochette et al., 2005 and refs. therein) and thus at high enough concentrations its subsurface presence could explain Mars' crustal magnetic field anomalies [START_REF] Langlais | A New Model of the Crustal Magnetic Field of Mars Using MGS and MAVEN[END_REF][START_REF] Johnson | Crustal and time-varying magnetic fields at the InSight landing site on Mars[END_REF].

High concentrations of sulfides could form via differentiation of sulfide-saturated lavas that were contaminated by crustal assimilation of sulfates [START_REF] Burns | Evolution of sulfide mineralization on Mars[END_REF][START_REF] Baumgartner | Magmatic controls on the genesis of Ni-Cu±(PGE) sulphide mineralisation on Mars[END_REF] as occurred in nakhlite lava flows [START_REF] Franz | Isotopic links between atmospheric chemistry and the deep sulphur cycle on Mars[END_REF][START_REF] Mari | Syneruptive incorporation of martian surface sulphur in the nakhlite lava flows revealed by S and Os isotopes and highly siderophile elements: implication for mantle sources in Mars[END_REF], differentiation of impact melt accompanied by crustal sulfur assimilation as occurred in Sudbury, Ontario (e.g. [START_REF] Therriault | The Sudbury Igneous Complex: A Differentiated Impact Melt Sheet[END_REF], later hydrothermal sulfide precipitation with or without serpentinization, as is recorded in regolith breccia martian meteorites [START_REF] Lorand | Nickeliferous pyrite tracks pervasive hydrothermal alteration in Martian regolith breccia: A study in NWA 7533[END_REF], or differentiated intrusions (Fig. 3; Section 4.3). Once formed, fluid fracture networks within such sulfide-rich regions in the subsurface of Mars could support high concentrations of radiolytically-fueled sulfate-reducing microorganisms and would be key locations to investigate the possibility of extant life on Mars.

Evidence for concentrated sulfides on Mars

The conclusions from this study do not require concentrated sulfides on Mars, but rather are based on the large body of data for disseminated sulfides on Mars. As such, the estimates are appropriately conservative and are consistent with the body of lithological and geochemical data available. The presence of concentrated sulfide deposits on Mars can be considered, however, and supporting evidence includes (1) sulfur assimilation and sulfide saturation in nakhlite melts [START_REF] Franz | Isotopic links between atmospheric chemistry and the deep sulphur cycle on Mars[END_REF][START_REF] Mari | Syneruptive incorporation of martian surface sulphur in the nakhlite lava flows revealed by S and Os isotopes and highly siderophile elements: implication for mantle sources in Mars[END_REF], (2) geologic evidence for high sulfide concentrations elsewhere in the martian crust based on expected mantle oxygen [START_REF] Stanley | CO2 solubility in Martian basalts and Martian atmospheric evolution[END_REF][START_REF] Armstrong | Speciation and solubility of reduced C-O-H-N volatiles in mafic melt: Implications for volcanism, atmospheric evolution, and deep volatile cycles in the terrestrial planets[END_REF] and sulfur [START_REF] Ding | Sulfur concentration of martian basalts at sulfide saturation at high pressures and temperatures -Implications for deep sulfur cycle on Mars[END_REF] fugacities, (3) surface evidence for sulfide oxidation [START_REF] Zolotov | Formation of jarosite-bearing deposits through aqueous oxidation of pyrite at Meridiani Planum, Mars[END_REF][START_REF] Dehouck | Evaluating the role of sulfide-weathering in the formation of sulfates or carbonates on Mars[END_REF][START_REF] Vaniman | Mineralogy of a Mudstone at Yellowknife Bay, Gale Crater[END_REF], and (4) orbital detections of past serpentinization [START_REF] Ehlmann | Geologic setting of serpentine deposits on Mars[END_REF][START_REF] Leask | Challenges in the Search for Perchlorate and Other Hydrated Minerals With 2.1-μm Absorptions on Mars[END_REF], which produces reducing fluids and is often associated with generation of hydrothermal sulfide deposits on Earth (e.g. [START_REF] Economou | Sulfides associated with podiform bodies of chromite at Tsangli, Eretria, Greece[END_REF][START_REF] Thalhammer | Postmagmatic, hydrothermal origin of sulfide and arsenide mineralizations at Limassol Forest, Cyprus[END_REF]Shiga, 1987;[START_REF] Auclair | Mineralogy, geochemistry, and paragenesis of the Eastern Metals serpentinite-associated Ni-Cu-Zn deposit, Quebec Appalachians[END_REF][START_REF] Wafik | Les Minéralisations Sulfurées à Cu-Fe et les Altérations Associées dans les Ophiolites Protérozoiques de Bou Azzer et de Khzama (Anti-Atlas, Maroc) (Cu-Fe sulfureous mineralisations and the associated alterations in the Bou Azzer and Khzama Proterozoic ophiolites, Anti-Atlas, Maroc)[END_REF][START_REF] Marques | Sulfide mineralization in an ultramaficrock hosted seafloor hydrothermal system: From serpentinization to the formation of Cu-Zn-(Co)-rich massive sulfides[END_REF].

1. Nakhlites likely crystallized from a sulfide saturated melt after S assimilation from the crust/regolith based on Δ 33 S, δ 34 S, and highly siderophile element (HSE) patterns [START_REF] Mari | Syneruptive incorporation of martian surface sulphur in the nakhlite lava flows revealed by S and Os isotopes and highly siderophile elements: implication for mantle sources in Mars[END_REF]. Some amount of sulfur assimilation from S-rich regolith occurred after the nakhlite lavas were emplaced on or near the martian surface [START_REF] Franz | Isotopic links between atmospheric chemistry and the deep sulphur cycle on Mars[END_REF][START_REF] Mari | Syneruptive incorporation of martian surface sulphur in the nakhlite lava flows revealed by S and Os isotopes and highly siderophile elements: implication for mantle sources in Mars[END_REF]. Nakhlites are the only SNC meteorite class known to likely have crystallized from a sulfide-saturated melt, as shergottites crystallized from sulfide undersaturated parent magmas [START_REF] Wang | Chalcophile elements in Martian meteorites indicate low sulfur content in the Martian interior and a volatile element-depleted late veneer[END_REF]. Still, 1/3 of volcanic martian meteorite classes exhibiting geochemical characteristics consistent with crystallization from a sulfide saturated melt implies that characteristically similar volcanism could have occurred throughout Mars' history. Sulfates are prevalent across the surface of Mars [START_REF] Ehlmann | Mineralogy of the Martian Surface[END_REF], S-rich regolith is effectively ubiquitous across the surface [START_REF] Clark | Inorganic Analyses of Martian Surface Samples at the Viking Landing Sites[END_REF][START_REF] Foley | Final chemical results from the Mars Pathfinder alpha proton X-ray spectrometer[END_REF], and elemental sulfur concentrations measured by GRS range from 0.7-3.2 wt. % at GRS spatial resolution [START_REF] Mclennan | Distribution of Sulfur on the Surface of Mars Determined by the 2001 Mars Odyssey Gamma Ray Spectrometer[END_REF], thus a substantial amount of sulfur for accumulation by lavas commonly exists on the martian surface. Therefore, it is likely that lavas saturated with respect to sulfide after assimilation of S from the crust/regolith-similar to those from which nakhlites crystallized-were common throughout Mars' history, thus resulting in the formation of high sulfide concentration zones.

2. The martian mantle is generally expected to be more reducing than Earth's [START_REF] Stanley | CO2 solubility in Martian basalts and Martian atmospheric evolution[END_REF][START_REF] Armstrong | Speciation and solubility of reduced C-O-H-N volatiles in mafic melt: Implications for volcanism, atmospheric evolution, and deep volatile cycles in the terrestrial planets[END_REF], with estimates for mantle oxygen fugacity based on basalts measured in Gusev crater varying from iron-wüstite (IW) to IW+1 [START_REF] Stanley | CO2 solubility in Martian basalts and Martian atmospheric evolution[END_REF]. Oxygen fugacity estimates from shergottites are estimated to be [QFM-4]-[QFM-1] [START_REF] Herd | The oxygen fugacity of olivine-phyric martian basalts and the components within the mantle and crust of Mars[END_REF], and from nakhlites are ~QFM [START_REF] Mari | Syneruptive incorporation of martian surface sulphur in the nakhlite lava flows revealed by S and Os isotopes and highly siderophile elements: implication for mantle sources in Mars[END_REF]. Earth's mantle has an oxygen fugacity range of [QFM-1]-[QFM+1] [START_REF] Blundy | Carbon-fluid equilibria and the oxidation state of the upper mantle[END_REF][START_REF] Brounce | Redox variations in Mauna Kea lavas, the oxygen fugacity of the Hawaiian plume, and the role of volcanic gases in Earth's oxygenation[END_REF]. Still, on Earth, pyrrhotite is found in volcanic rocks derived from partial melting of a more oxidized mantle relative to Mars (e.g. [START_REF] Desborough | Mineralogy of sulfides from certain Hawaiian basalts[END_REF][START_REF] Whitney | Fugacities of sulfurous gases in pyrrhotite-bearing silicic magmas[END_REF], though concentrations are typically higher in silicic magmas than basalt, the latter of which is more common on Mars. Thus, magmatic sulfides are expected in some volcanic rocks derived from the wide variability in melt oxygen fugacities from the martian mantle. Martian mantle sulfur abundance estimates range from ~400-2200 ppm [START_REF] Wang | Chalcophile elements in Martian meteorites indicate low sulfur content in the Martian interior and a volatile element-depleted late veneer[END_REF], while Earth's mantle sulfur abundance is ~250 ppm [START_REF] Mcdonough | The composition of the Earth[END_REF]. Additional sulfur is incorporated into martian melts via crustal assimilation of sulfates and/or S-rich regolith, as occurred in the nakhlite lavas [START_REF] Franz | Isotopic links between atmospheric chemistry and the deep sulphur cycle on Mars[END_REF][START_REF] Mari | Syneruptive incorporation of martian surface sulphur in the nakhlite lava flows revealed by S and Os isotopes and highly siderophile elements: implication for mantle sources in Mars[END_REF]. As such, sulfur activity in martian melts is typically higher than in most melts on Earth, while oxygen fugacity is lower than in most melts on Earth, both of which increase the likelihood of sulfide precipitation.

3. Sulfide oxidation has been invoked to explain multiple mineral assemblages found on the martian surface [START_REF] King | Effects of H2O, pH, and oxidation state on the stability of Fe minerals on Mars[END_REF], including the hematite enrichment in Meridiani Planum [START_REF] Zolotov | Formation of jarosite-bearing deposits through aqueous oxidation of pyrite at Meridiani Planum, Mars[END_REF], the Burns formation -a sulfate-rich sandstone in Merdiani Planum [START_REF] Zolotov | Formation of jarosite-bearing deposits through aqueous oxidation of pyrite at Meridiani Planum, Mars[END_REF], sulfate-carbonate assemblages found using orbital hyperspectral data [START_REF] Dehouck | Evaluating the role of sulfide-weathering in the formation of sulfates or carbonates on Mars[END_REF], akaganeite detections made from orbit [START_REF] Carter | Orbital detection and implications of akaganéite on Mars[END_REF], and acidic fluid diagenetic mineral assemblages detected by the Curiosity rover [START_REF] Rampe | Mineralogy of an ancient lacustrine mudstone succession from the Murray formation, Gale crater, Mars[END_REF]. In its type locality on Earth, akaganeite forms as an alteration product of pyrrhotite [START_REF] Nambu | New mineral Akaganeite, β-FeOOH, from Akagane Mine, Iwate Prefecture, Japan[END_REF]. To concentrate the abundances of sulfur needed to form the Burns formation, preconcentration of sulfur via magmatic or hydrothermal sulfides may be required [START_REF] Zolotov | Formation of jarosite-bearing deposits through aqueous oxidation of pyrite at Meridiani Planum, Mars[END_REF]. Furthermore, oxidation of sulfides to form the acid-minedrainage-like mineral assemblages of sulfates and oxides seen on the martian surface provides a functional hypothesis for generation of these assemblages. Formation of concentrated sulfides could have occurred on Mars via differentiation/crustal assimilation from ultramafic lavas [START_REF] Burns | Evolution of sulfide mineralization on Mars[END_REF], the existence of which is supported by possible observations of ultramafic volcanism on the martian surface [START_REF] Ruff | Evidence for a Noachian-aged ephemeral lake in Gusev crater, Mars[END_REF]Ruff et al., 2019;[START_REF] Kremer | A widespread olivine-rich ash deposit on Mars[END_REF], ultramafic lithologies in martian meteorites [START_REF] Burns | Evolution of sulfide mineralization on Mars[END_REF], and evidence that nakhlites crystallized from sulfide-saturated melts after S assimilation from the crust/regolith [START_REF] Franz | Isotopic links between atmospheric chemistry and the deep sulphur cycle on Mars[END_REF][START_REF] Mari | Syneruptive incorporation of martian surface sulphur in the nakhlite lava flows revealed by S and Os isotopes and highly siderophile elements: implication for mantle sources in Mars[END_REF].

4. Mg-rich serpentine has been detected on Mars using CRISM hyperspectral images [START_REF] Ehlmann | Geologic setting of serpentine deposits on Mars[END_REF][START_REF] Leask | Challenges in the Search for Perchlorate and Other Hydrated Minerals With 2.1-μm Absorptions on Mars[END_REF], but most of these detections have been called into question after discovery of a 2.1 µm spectral artifact in CRISM TRR3 data that can resemble the diagnostic absorption feature of Mg-rich serpentine [START_REF] Leask | Challenges in the Search for Perchlorate and Other Hydrated Minerals With 2.1-μm Absorptions on Mars[END_REF]. Still, at least 3 Mg-rich serpentine detections have been validated as unrelated to this artifact [START_REF] Leask | Challenges in the Search for Perchlorate and Other Hydrated Minerals With 2.1-μm Absorptions on Mars[END_REF], demonstrating that serpentinization has occurred on Mars. The fluids generated via serpentinization are highly reducing, which can cause precipitation of reduced alloys and minerals such as awaruite (Lorand, 1985;[START_REF] Sleep | H2-rich fluids from serpentinization: Geochemical and biotic implications[END_REF]) (NiFe alloy) and sulfides [START_REF] Economou | Sulfides associated with podiform bodies of chromite at Tsangli, Eretria, Greece[END_REF][START_REF] Thalhammer | Postmagmatic, hydrothermal origin of sulfide and arsenide mineralizations at Limassol Forest, Cyprus[END_REF]Shiga, 1987;[START_REF] Auclair | Mineralogy, geochemistry, and paragenesis of the Eastern Metals serpentinite-associated Ni-Cu-Zn deposit, Quebec Appalachians[END_REF][START_REF] Wafik | Les Minéralisations Sulfurées à Cu-Fe et les Altérations Associées dans les Ophiolites Protérozoiques de Bou Azzer et de Khzama (Anti-Atlas, Maroc) (Cu-Fe sulfureous mineralisations and the associated alterations in the Bou Azzer and Khzama Proterozoic ophiolites, Anti-Atlas, Maroc)[END_REF][START_REF] Marques | Sulfide mineralization in an ultramaficrock hosted seafloor hydrothermal system: From serpentinization to the formation of Cu-Zn-(Co)-rich massive sulfides[END_REF]. As such, serpentinization would have generated reducing fluids on Mars that could have precipitated and concentrated sulfide minerals, making regions containing serpentine potential 'hotzones' for both ancient chemolithotrophic life that consumed H 2 from serpentinization, as well as modern chemolithotrophic life that may consume H 2 and SO 4 from radiolysis in sulfide-rich zones.

Possible settings of concentrated sulfides on Mars

1. Differentiated impact melt sheets: The Sudbury Impact Structure in Ontario, Canada is one of the three largest discovered impact structures on Earth-the others being Chicxulub and Vredefort-and hosts concentrations of sulfides of high economic value. Sulfides were concentrated via gravitational differentiation from the impact melt sheet [START_REF] Zieg | The Sudbury Igneous Complex: Viscous emulsion differentiation of a superheated impact melt sheet[END_REF]. Sulfur was likely introduced to the impact melt via assimilation from the surrounding country rock during the depressurization melting that formed the Sudbury Igneous Complex [START_REF] Walker | Re-Os isotope systematics of Ni-Cu sulfide ores, Sudbury Igneous Complex, Ontario: evidence for a major crustal component[END_REF][START_REF] Dickin | Osmium isotope evidence for a crustal origin of platinum group elements in the Sudbury nickel ore, Ontario, Canada[END_REF][START_REF] Mungall | Geochemical evidence from the Sudbury structure for crustal redistribution by large bolide impacts[END_REF]. Because of the prevalence of sulfur on Mars in the form of sulfates and S-rich regolith, and the larger number of preserved impact structures on Mars relative to Earth, Sudbury-type differentiated impact melt sheet sulfide concentrations may be more common on Mars than they are on Earth (West and Clarke, 2010) (Fig. 3 Setting 1).

Layered intrusions:

Sulfides are commonly concentrated in ultramafic-to-mafic layered intrusions on Earth, such as the Bushveld Igneous Complex in South Africa [START_REF] Gain | The geological setting of the platinoid and base metal sulfide mineralization in the Platreef of the Bushveld Complex in Drenthe, North of Potgietersrus[END_REF]Mostert, 1982), Voisey's Bay in Canada (Evans-Lamswood et al., 2000), carbonates, chlorite, hydrated silica, and zeolites [START_REF] Ehlmann | Identification of hydrated silicate minerals on Mars using MRO-CRISM: Geologic context near Nili Fossae and implications for aqueous alteration[END_REF], and small amounts of Mg-serpentine [START_REF] Ehlmann | Geologic setting of serpentine deposits on Mars[END_REF][START_REF] Leask | Challenges in the Search for Perchlorate and Other Hydrated Minerals With 2.1-μm Absorptions on Mars[END_REF] are detected on the martian surface from orbit using CRISM. Evidence for past hydrothermal alteration has been seen by every Mars rover. NASA's Opportunity rover observed evidence for multiple diagenetic episodes in the Burns Formation (McLennan et al., 2005), as well as hydrothermal alteration of bedrock due to meteorite impact at Endeavour crater [START_REF] Squyres | Ancient Impact and Aqueous Processes at Endeavour Crater[END_REF][START_REF] Arvidson | Ancient Aqueous Environments at Endeavour Crater[END_REF]. NASA's Spirit rover observed carbonate-bearing olivine-rich rocks (Comanche) formed via alteration of ultramafic tuff (Algonquin) [START_REF] Morris | Identification of Carbonate-Rich Outcrops on Mars by the Spirit Rover[END_REF][START_REF] Ruff | Evidence for a Noachian-aged ephemeral lake in Gusev crater, Mars[END_REF], silica deposits [START_REF] Ruff | Characteristics, distribution, origin, and significance of opaline silica observed by the Spirit rover in Gusev crater, Mars[END_REF] that likely formed in hot spring settings [START_REF] Ruff | Silica deposits on Mars with features resembling hot spring biosignatures at El Tatio in Chile[END_REF]Ruff et al., 2019), and hydrothermal alteration of Home Plate resulting in precipitation of silica [START_REF] Squyres | Detection of Silica-Rich Deposits on Mars[END_REF]. NASA's Curiosity rover has found evidence for multiple diagenetic episodes in the stratigraphy of Mount Sharp, including cross-cutting Ca-sulfate veins [START_REF] Yen | Multiple stages of aqueous alteration along fractures in mudstone and sandstone strata in Gale Crater, Mars[END_REF], jarosite veins dated to 2.1 ± 4 Gyr [START_REF] Martin | A Two-Step K-Ar Experiment on Mars: Dating the Diagenetic Formation of Jarosite from Amazonian Groundwaters[END_REF], and evidence for diagenetic episodes that cross-cut multiple stratigraphically distinct units [START_REF] Sheppard | Mineral Stratigraphy Around Mt. Sharp Suggests Aqueous Processes Affected the Entire Mound: Directions for Upcoming Rover Observations from Orbital Data[END_REF]. Sulfides could also be concentrated via hydrothermal activity during submarine volcanism -as likely occurred in Eridania Basin [START_REF] Michalski | Ancient hydrothermal seafloor deposits in Eridania basin on Mars[END_REF]. As such, hydrothermal alteration has occurred on Mars in a wide variety of geochemical settings, including some that precipitated pyrite in regolith breccias [START_REF] Lorand | Nickeliferous pyrite tracks pervasive hydrothermal alteration in Martian regolith breccia: A study in NWA 7533[END_REF][START_REF] Wittmann | Petrography and composition of Martian regolith breccia meteorite Northwest Africa 7475[END_REF][START_REF] Hewins | Regolith breccia Northwest Africa 7533: Mineralogy and petrology with implications for early Mars[END_REF]Jean-Pierre Lorand et al., 2018a), and some that formed serpentinite [START_REF] Ehlmann | Geologic setting of serpentine deposits on Mars[END_REF][START_REF] Leask | Challenges in the Search for Perchlorate and Other Hydrated Minerals With 2.1-μm Absorptions on Mars[END_REF]. It is plausible that higher concentrations of sulfides than those found in NWA 7533 and NWA 7034-which were ejected from Mars by the same impact event [START_REF] Wittmann | Petrography and composition of Martian regolith breccia meteorite Northwest Africa 7475[END_REF]-could form from hydrothermal alteration in other regions of the martian crust (Fig. 3 Settings 3 & 5).

4. Sulfide-bearing sediments. Mount Sharp in Gale crater is an example of a sulfidebearing sediment mound on Mars [START_REF] Mcadam | Sulfur-bearing phases detected by evolved gas analysis of the Rocknest aeolian deposit, Gale Crater, Mars[END_REF][START_REF] Franz | Large sulfur isotope fractionations in Martian sediments at Gale crater[END_REF][START_REF] Wong | Detection of reduced sulfur on Vera Rubin ridge by quadratic discriminant analysis of volatiles observed during evolved gas analysis[END_REF] (Fig. 3 Setting 4). These sulfides are diagenetic/epigenetic, likely resulting from thermal reduction of preexisting sulfates [START_REF] Franz | Large sulfur isotope fractionations in Martian sediments at Gale crater[END_REF]. It is possible that a similar sulfate thermal reduction process could occur in other sulfate-bearing martian sediments, which are widespread [START_REF] Ehlmann | Mineralogy of the Martian Surface[END_REF], thus generating a higher concentration of sulfides in those localities. Cu-enrichments in Gale crater are potentially due to presence of detrital sulfides [START_REF] Payré | Copper enrichments in the Kimberley formation in Gale crater, Mars: Evidence for a Cu deposit at the source[END_REF]. It is therefore plausible that sulfides could be concentrated in martian sediments in detrital form and also via diagenetic processes.

Differentiated ultramafic-to-mafic lava flows.

Examples of sulfide concentrations in komatiitic lava flows on Earth include the Kambalda and Perseverance deposits in Australia. Sulfides are also concentrated picrites, including the Pechenga deposits in Russia [START_REF] Barnes | Magmatic Sulfide Ore Deposits[END_REF]. Due to the presence of ultramafic lithologies in martian meteorites, the possibility of sulfide concentration in komatiite-type settings has been known for decades [START_REF] Burns | Evolution of sulfide mineralization on Mars[END_REF]. Since then, further evidence for ultramafic volcanism on Mars has been presented [START_REF] Ruff | Evidence for a Noachian-aged ephemeral lake in Gusev crater, Mars[END_REF]Ruff et al., 2019;[START_REF] Kremer | A widespread olivine-rich ash deposit on Mars[END_REF]. The magmas that precipitated the nakhlites assimilated sulfur from the crust, forming a sulfide-saturated melt [START_REF] Franz | Isotopic links between atmospheric chemistry and the deep sulphur cycle on Mars[END_REF][START_REF] Mari | Syneruptive incorporation of martian surface sulphur in the nakhlite lava flows revealed by S and Os isotopes and highly siderophile elements: implication for mantle sources in Mars[END_REF]. Thus, there is already evidence for some degree of sulfide concentration in differentiated ultramafic-to-mafic lava flows on Mars via the martian meteorites (Fig. 3 Setting 6).

Conclusions

The results presented here demonstrate that radiolysis in the martian meteorite source lithologies would produce sufficient H 2 and sulfate to sustain sulfate-reducing bacteria wherever groundwater is present. The regolith breccia source lithology, which is within the southern highlands [START_REF] Cassata | Chronology of martian breccia NWA 7034 and the formation of the martian crustal dichotomy[END_REF], could sustain the highest concentrations of sulfate-reducing bacteria, with shergottite and nakhlite lithologies supporting the second highest concentrations.

The calculated supportable sulfate-reducing bacteria densities in the regolith breccia, shergottite, and nakhlite source lithologies are comparable to those measured in Earth's deep subsurface (Table 3; Fig. 2).

If Earth-like chemolithotrophic life ever existed on Mars and survived until present, it could potentially be sustained in these subsurface regions by harnessing the same radiolytically-derived energy that drives the metabolisms of microbial ecosystems on Earth sustained in groundwaters >10 6 -10 9 years old [START_REF] Wang | Long-Term Sustainability of a High-Energy, Low-Diversity Crustal Biome[END_REF][START_REF] Chivian | Environmental Genomics Reveals a Single-Species Ecosystem Deep Within Earth[END_REF][START_REF] Lollar | Follow the Water': Hydrogeochemical Constraints on Microbial Investigations 2.4 km Below Surface at the Kidd Creek Deep Fluid and Deep Life Observatory[END_REF]. Furthermore, this subsurface environment has likely remained habitable since the Noachian [START_REF] Tarnas | Radiolytic H2 production on Noachian Mars: Implications for habitability and atmospheric warming[END_REF], making it the longest-lived habitable environment on Mars and the most likely refugia for any life on the planet (National Academies of Sciences, Engineering, and Medicine et al., 2018;[START_REF] Michalski | The Martian subsurface as a potential window into the origin of life[END_REF]. The parameters outlined here -radionuclide abundance, sulfide abundance, average sulfide grain size, and porosity-along with characterization of the locations of liquid groundwater, can be used to constrain an optimal landing site for a mission to detect Earth-like extant sulfate-reducing martian life in the subsurface. Additional microbial metabolisms, for which redox energy availability is more difficult to model, could also exist in the martian subsurface, as they do in Earth's subsurface, potentially increasing the measured biomass in any subsurface region that contains groundwater. This should be considered in the context of future extant life detection missions to Mars.

Figures

Figure 1 | Model input data. (a) K concentration (wt. %) from GRS data, corrected for contributions Cl, H, and S [START_REF] Boynton | Concentration of H, Si, Cl, K, Fe, and Th in the low-and midlatitude regions of Mars[END_REF][START_REF] Tarnas | Radiolytic H2 production on Noachian Mars: Implications for habitability and atmospheric warming[END_REF]. (b) Th concentration (ppm) from GRS data, corrected for contributions Cl, H, and S [START_REF] Boynton | Concentration of H, Si, Cl, K, Fe, and Th in the low-and midlatitude regions of Mars[END_REF][START_REF] Tarnas | Radiolytic H2 production on Noachian Mars: Implications for habitability and atmospheric warming[END_REF]. (c) Possible crustal density estimates from orbital gravimetry [START_REF] Goossens | Evidence for a low bulk crustal density for Mars from gravity and topography[END_REF] that are supported by rover gravimetry [START_REF] Lewis | A surface gravity traverse on Mars indicates low bedrock density at Gale crater[END_REF]. (d) Division of Mars into 3 different regions. (e) Radionuclide abundances in martian meteorites [START_REF] Meyer | The Martian Meteorite Compendium[END_REF] and GRS data from 60ºS-60ºN, where near surface ice does not contaminate measurements [START_REF] Boynton | Concentration of H, Si, Cl, K, Fe, and Th in the low-and midlatitude regions of Mars[END_REF][START_REF] Tarnas | Radiolytic H2 production on Noachian Mars: Implications for habitability and atmospheric warming[END_REF]. The lower spatial resolution of GRS data (100s of km) means the calculated K and Th concentrations are more closely clustered than in the martian meteorites. There are also no U concentration measurements from GRS data, thus the color is black. (f) Sulfide concentrations and grain sizes in martian meteorites are compiled and referenced in Table 1.

Black symbols show the maximum and minimum reported grain sizes, while red symbols show the reported average grain size. 2. The different colors of the same datapoints represent the supportable sulfate-reducing cells per kg rock assuming the highest and lowest metabolic rates observed in Earth's deep subsurface (5.5 10 -18 -3.6 10 -17 moles cell -1 year -1 ) [START_REF] Wang | Long-Term Sustainability of a High-Energy, Low-Diversity Crustal Biome[END_REF][START_REF] Chivian | Environmental Genomics Reveals a Single-Species Ecosystem Deep Within Earth[END_REF].

Cell density measurements from Earth's deep subsurface come from a (Cockell et al., 2012), b (Onstott et al., 2003). Differentiated impact melt sheet similar to sulfide concentrations at the Sudbury Impact Structure in Canada [START_REF] Therriault | The Sudbury Igneous Complex: A Differentiated Impact Melt Sheet[END_REF]. (2) Sulfide concentration in layered intrusion, similar to Bushveld Igneous Complex in South Africa and many other localities on Earth. [START_REF] Lorand | Sulfides are concentrated by hydrothermal fluids that are Srich and reducing. Many hydrothermally-formed sulfide deposits are associated with serpentinites, due to the highly reducing fluids formed during the process of serpentinization[END_REF] Sulfide hydrothermally concentrated in serpentinite, similar to many ophiolite localities on Earth.

Mg-serpentine has been detected on Mars [START_REF] Leask | Challenges in the Search for Perchlorate and Other Hydrated Minerals With 2.1-μm Absorptions on Mars[END_REF]. (4) Sulfide concentrations in sediments, such as those in Gale crater [START_REF] Franz | Large sulfur isotope fractionations in Martian sediments at Gale crater[END_REF][START_REF] Wong | Detection of reduced sulfur on Vera Rubin ridge by quadratic discriminant analysis of volatiles observed during evolved gas analysis[END_REF]. ( 5) Sulfide hydrothermally concentrated without associated serpentinite minerals, such as those in regolith breccia martian meteorites [START_REF] Lorand | Sulfide mineralogy and redox conditions in some shergottites[END_REF][START_REF] Lorand | Nickeliferous pyrite tracks pervasive hydrothermal alteration in Martian regolith breccia: A study in NWA 7533[END_REF]Jean-Pierre Lorand et al., 2018b;Jean-Pierre Lorand et al., 2018a). ( 6) Concentrations of sulfides in differentiated ultramafic-to-mafic lava flows, as occurs in many localities on Earth including the Kambalda and Perseverance deposits in Australia. Sulfides are also concentrated in picrites, including the Pechenga deposits in Russia. This is a possible formation setting for nakhlites, which crystallized from sulfide saturated magmas after assimilation of S from the crust/regolith [START_REF] Franz | Isotopic links between atmospheric chemistry and the deep sulphur cycle on Mars[END_REF][START_REF] Mari | Syneruptive incorporation of martian surface sulphur in the nakhlite lava flows revealed by S and Os isotopes and highly siderophile elements: implication for mantle sources in Mars[END_REF]. ( 7) Concentration of sulfides in ultramafic-to-mafic subvolcanic intrusions beneath volcanic provinces such as Tharsis, Elysium, and Syrtis Major.

  of values for martian meteorites calculations. a Agee et al. (2013), b Wittman et al. (2015), c Lorand et al. (2018a), d Meyer et al. (2016), e Lorand et al. (2018b), f Lorand et al. (2012), g Chevrier et al. (2011), h Lorand et al. (2005), i Le Deit et al. (2016), j Boynton et al. (2007), k Hahn et al. (2011), l Tarnas et al. (2018), m Vaniman et al. (2014). Sulfide grain size excludes the few unusually large sulfide grains reported in each meteorite class.

Figure 2 |

 2 Figure 2 | Radiolytic redox energy production and number of supportable sulfate-reducing

Figure 3 |

 3 Figure 3 | Possible sulfide concentration settings on Mars in addition to those found in

Table 1 | Sulfide concentrations and grain sizes in martian meteorites. Assume density of 5000 kg/m 3 for sulfide and 3000 kg/m 3 for host rock to convert from vol % to wt %.

 1 [START_REF] Franz | Isotopic links between atmospheric chemistry and the deep sulphur cycle on Mars[END_REF][START_REF] Franz | A new type of isotopic anomaly in shergottite sulfides[END_REF] and[START_REF] Wang | Chalcophile elements in Martian meteorites indicate low sulfur content in the Martian interior and a volatile element-depleted late veneer[END_REF] data are calculated from whole-rock S contents assuming 38 wt.% S for pyrrhotite and 54 wt. % for pyrite. Other sulfide abundances are from image analyses on polished thick sections. a(Chevrier et al., 2011), b [START_REF] Wang | Chalcophile elements in Martian meteorites indicate low sulfur content in the Martian interior and a volatile element-depleted late veneer[END_REF],

	Sample Sulfide abundance (wt. %) Average	Grain	Grain	Publications	Type
			grain	size	size
			size (µm)	min	max
				(µm)	(µm)
	NWA 998 0.13	-	20	120	a	Nakhlite
	Nakhla 0.07 (0.08)	-	10	50	a,b	Nakhlite
	Governardor	0.06	-	2	25	a	Nakhlite
	Valadares
	NWA 817 0.03	-	2	30±15	a	Nakhlite
	MIL 03346	0.05	-	-	-	c	Nakhlite
	And paired
	samples
	Y000593 0.08	-	-	-	b	Nakhlite
	ALH A77005 0.3 d , 0.12 c , 0.19 b	100	-	-	b-d	Lherzolitic shergottites
	LEW 88516 0.3	17.5	-	-	d	Lherzolitic shergottites
	Y-293605 0.05	-	-	-	d	Lherzolitic shergottites
	Tissint 0.6 e , 0.35 c	-	<1	200	c,e	Olivine-phyric shergottite
	EET	0.52 b , 0.28 c	-	-	-	c	Olivine-phyric shergottite
	A79001-A
	DaG	0.45	50	10	100	d	Olivine-phyric shergottites
	476/489
	SaU 005/094 0.30 d , 0.16 c , 0.38 b	30	10	50	b-d	Olivine-phyric shergottites
	Dhofar 019 0.5 d , 0.20 c	25	-	-	c, d	Olivine-phyric shergottites
	NWA	0.8	-	-	-	c, d	Olivine-phyric shergottites
	1068/1110
	Y-980459 0.1 c , 0.4 b	30	5	50	b-d, f	Olivine-phyric shergottites
	Zagami 0.7 d , 0.24 c , 0.35 b	<10	100	200	b-d	Basaltic shergottite
	Shergotty 0.4	-	100	200	d	Basaltic shergottite
	EET	0.12	-	-	-	c	Basaltic shergottite
	A79001-B
	QUE 94201 0.3 d , 0.13 c	-	-	-	d	Basaltic shergottite
	NWA 480 0.24	-	100	200	d	Basaltic shergottite
	NWA 856 0.1	-	-	-	d	Basaltic shergottite
	Los Angeles 0.67 d , 0.32 c	-	200	400	d	Basaltic shergottite
	Y000097 0.12	-	-	-	b	Lherzolitic shergottite
	LAR 12011 0.35	-	-	-	b	Lherzolitic shergottite
	LAR12095 0.5	-	-	-	b	Lherzolitic shergottite
	Mean of 37 0.21±0.18	-	-	-	c, g	Shergottites

Table 1 and refs. therein)

. Sulfide grain sizes are relatively consistent across martian meteorite samples (Fig.

1f

). A compilation of published measured sulfide average grain sizes and grain size ranges in martian meteorites is presented in Table

1

and plotted in Fig.

1f

. It is noteworthy that many of these studies do not consider sulfides <10 µm in diameter, thus these results may skew average sulfide grain sizes towards larger-thantrue values. Since sulfate production in the calculations increases as sulfide grain size decreases, as a result of greater specific surface area, this ensures the calculations are conservative. Sulfide grain sizes depend on a wide array of parameters, including temperature, supersaturation of fluids, nucleation rates, and fracturing. For each individual martian meteorite class, the range of measured average sulfide grain sizes in those meteorites are used as the basis for the calculations. c

(Franz et al., 2014), d (Lorand et al., 2005 and refs. therein

), e

[START_REF] Gattacceca | Opaque minerals, magnetic properties, and paleomagnetism of the Tissint Martian meteorite[END_REF]

, f

[START_REF] Baumgartner | The role of sulfides in the fractionation of highly siderophile and chalcophile elements during the formation of martian shergottite meteorites[END_REF]

, g

[START_REF] Franz | A new type of isotopic anomaly in shergottite sulfides[END_REF]

, h

[START_REF] Lorand | Nickeliferous pyrite tracks pervasive hydrothermal alteration in Martian regolith breccia: A study in NWA 7533[END_REF]

, i

(Lorand et al., 2018a)

, j

(Lorand et al., 2018b)

, k

(Lorand et al., 2012), l

]-[3 × 10 4 ] sulfate-reducing cells per kg rock wherever liquid water exists.(3) Nakhlite source locality: The sulfate production rates for nakhlite source localities vary from ~[1 × 10 -16 ] -[2 × 10 -13 ] moles (kg rock) -1 year -1 , while H

production rates range

Acknowledgements

We thank Norman Sleep and an anonymous reviewer for feedback that improved the quality of this manuscript. We thank Paul Niles, Ralph Milliken, and Steven D'Hondt for discussions.

Thanks to the Astromaterial Curation Group at NASA JSC, especially Kevin Righter, for providing a compilation table of martian meteorite elemental abundances. We thank Paul Niles, Ralph Milliken, and Steven D'Hondt for discussions. J.D.T. gratefully acknowledges support from the NASA Postdoctoral Program and a Brown University Dissertation Fellowship. J.F.M., B.S.L., V.S., and J.R.M. acknowledge support from the Canadian Institute for Advanced Research (CIFAR).