Significance of databases in research

When conducting experimental investigations, the feasibility of the procedure (whether it be due to financial, spatial, equipment or time constraints), the reaction conditions required, as well as the replicability of the experimental model are all major considerations. These choices need to be made as the experiment is being designed, and it could be inefficient if one has to rely solely on the text of current literature to assist them due to time. A database is useful for experimental planning because it concisely summarizes the information of a multitude of experimental designs, reagent/reaction condition combinations, and has utility to current researchers whether the motive is the replication of a past experiment, or filling in the gaps of current research. Large databases that are sortable by experimental parameters can also facilitate the field's ability to discern bigger pictures and larger chemical trends as evidenced by Barbier et al. (Reference Barbier, Huang, Andreani, Tao, Hao, Eleish, Prabhu, Minhas, Fontaine, Fox and Daniel2020) using the data set published by Huang et al. (Reference Huang, Barbier, Tao, Hao, de Real, Peuble, Merdith, Leichnig, Perrillat, Fontaine, Fox, Andreani and Daniel2020).

In the context of carbon reduction, there are many prebiotic conditions that have not been tested and/or modelled, and there is a plethora of parameters which need be accounted for, as they all have the potential to affect the outcomes of the investigations conducted. This generally makes carbon reduction experiments extremely difficult to conduct, as the equipment needed to recreate hydrothermal conditions, prepare the minerals to be used, and analyse the products (which are often in trace concentrations) can be expensive to obtain and use. In addition, there is field work that has been performed on analogue sites that could be relevant for experimental and modelled tests. Thus, knowing conditions that have and have not been tested and typical experimental setups and product yields to expect is imperative for facilitating efforts of new researchers looking to experimentally explore the field of carbon reduction, particularly in the origin of life field. While the work done by Huang et al. (Reference Huang, Barbier, Tao, Hao, de Real, Peuble, Merdith, Leichnig, Perrillat, Fontaine, Fox, Andreani and Daniel2020) includes 30 papers related to serpentinization, we aimed to design a data set related to experimental, field work, and modelled results within the field of carbon reduction.

The importance of carbon reduction

Carbon is a necessary element for life on Earth and can exist in various oxidized and reduced forms. Abiotically, the movement of carbon through oxidized and reduced phases constitutes the foundational carbon cycle on Earth. A dominant form of carbon on early Earth was carbon dioxide gas (CO₂), as it made up the majority of the Hadean atmosphere (Kasting, Reference Kasting1993; Kasting and Catling, Reference Kasting and Catling2003; Trail et al., Reference Trail, Watson and Tailby2011; Armstrong et al., Reference Armstrong, Frost, McCammon, Rubie and Boffa Ballaran2019). CO₂ is an oxidized form of carbon as is carbon monoxide (CO), which was also likely present in the early atmosphere, albeit in lower amounts/over shorter time scales (Kasting, Reference Kasting1990; DiSanti et al., Reference DiSanti, Mumma, Russo, Magee-Sauer, Novak and Rettig1999; Zahnle et al., Reference Zahnle, Lupu, Catling and Wogan2020). However, biomass is composed of organic molecules; therefore, if these oxidized gases were important prebiotic carbon sources they would have first had to have been reduced prior to the synthesis of prebiotic molecules like amino acids and nucleobases. Given this, reduction/fixation of CO₂ and CO may have served as a significant source of organic molecules on the prebiotic Earth through a variety of mechanisms. The resulting reduced carbon materials (including methane, formaldehyde, methanol, formic acid, acetate and pyruvate) could then have served as starting materials for prebiotic reactions, and their synthesis may have thus been an important process for the origins of life (Butlerov, Reference Butlerov1861; Miller, Reference Miller1953; Nuevo et al., Reference Nuevo, Augar, Blanot and d'Hendecourt2008; Cleaves, Reference Cleaves and Gargaud2011; Kopetzki and Antonietti, Reference Kopetzki and Antonietti2011; McCollom, Reference McCollom2013; Becker et al., Reference Becker, Thoma, Deutsch, Gehrke, Mayer, Zipse and Carell2016; Stubbs et al., Reference Stubbs, Yadav, Krishnamurthy and Springsteen2020; Ruiz et al., Reference Ruiz, Fernández, Ifandi, Eloy, Meza-Trujilo, Devred, Gaigneaux and Tsikouras2021).

Because of this, the fixation or reduction of CO and CO₂ has been the focus of a broad body of research in several fields, including planetary science and the origins of life, over the past 50 years. This work spans a significant amount of interdisciplinary research that includes theoretical modelling, laboratory experimentation, field work and analysis of mission data that relates to the origins of life on early Earth or a Martian environment. Modelling is often the focus of this research, and there are limited experimental results due to the challenges related to such work (e.g. high pressures require special reactors, isotopically labelled materials are expensive, and synthesizing pure/contaminant-free minerals is difficult). Experimental research also relies heavily on modelling to deduce which conditions are most promising to explore. In regards to these reactions, there are a number of parameters to investigate (temperature, pressure, pH, mineral source). In order to deduce plausible carbon reduction reactions that could have taken place on early Earth or Mars, it is important that both modelling and experimental work aim to constrain the conditions under which carbon reduction takes place.

We report on a summary of work explored on the reduction of CO and CO₂ under geological contexts relevant to Mars and the early Earth. These results are aimed at experimental researchers who are looking for modelled reactions that have not yet been tested in a laboratory setting. However, this table includes modelled results, field work, theoretical studies, data from missions and experimental work and is therefore useful, across a variety of research techniques. In addition, we highlight gaps within the modelling literature that would be fruitful areas for future work. The experimental conditions under which observations took place can be applied to models and modified to different planetary conditions relevant to the search for life by altering the parameters, such as temperature, pressure, phase of the reaction, depth from surface, catalysts used and partial pressure of relevant atmospheric gases to better simulate worlds/environments of interest.

Mechanisms and locations of interest

CO₂ has a variety of possible mechanisms for reduction and those mechanisms often depend on the environmental conditions. On the early Earth, atmospheric CO₂ would have readily dissolved in the near neutral to mildly acidic oceans (Morse and Mackenzie, Reference Morse and Mackenzie1998; Sleep et al., Reference Sleep, Zahnle and Neuhoff2001; Holland and Turekian, Reference Holland, Turekian and Elderfield2006; Halevy and Bachan, Reference Halevy and Bachan2017; Krissansen-Totton et al., Reference Krissansen-Totton, Arney and Catling2018; MacLeod et al., Reference MacLeod, McKeown, Hall and Russell1994) and existed as a mixture of dissolved CO₂ gas, bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions. H sources for reducing CO₂ to organic molecules could have been derived from atmospheric H₂ (predicted to have been at least ~1% of the Hadean atmosphere; Kasting and Catling, Reference Kasting and Catling2003; Tian et al., Reference Tian, Toon, Pavlov and De Sterck2005; Liggins et al., Reference Liggins, Shorttle and Rimmer2020), water/water vapour, trace gases such as H₂S, NH₄⁺ and/or CH₄. In general, CO₂ reduction requires an electron source, an energy source to drive the reaction (typically thermal, electrochemical, radiation or electrical), and often a catalyst or mediator (e.g. Fe⁰; dissolved metals, iron/zinc minerals, certain organics, etc.); although, large amounts of energy (e.g. electric discharges) can sufficiently drive CO₂ reduction in the absence of a catalyst (i.e. Miller–Urey chemistry; Cleaves et al., Reference Cleaves, Chalmers, Lazcano, Miller and Bada2008; Rodriguez et al., Reference Rodriguez, House, Smith, Roberts and Callahan2019). There are various mechanisms by which CO₂ or CO can be reduced, but the ones of focus for origins of life research include reverse water–gas shift reactions (forms CO), hydrogenation (forms CH₄, CH₃OH) or a series of gas-phase reactions such as Miller–Urey chemistry, Fischer Tropsch (FT) reactions, and free radical chain reactions (forms CO, CH₄ and hydrocarbons) (Pirronello et al., Reference Pirronello, Brown, Lazerotti, Marcantonio and Simmons1982; Riedel et al., Reference Riedel, Schaub, Jun and Lee2001; Jiang et al., Reference Jiang, Liu, Geng, Xu and Liu2018; Cleaves et al., Reference Cleaves, Chalmers, Lazcano, Miller and Bada2008; Porosoff et al., Reference Porosoff, Yan and Chen2016; Miyakawa et al., Reference Miyakawa, Yamanashi, Kobayashi, Cleaves and Miller2002).

Early Earth atmospheric CO₂ could have been reduced via lightning (i.e. Miller–Urey chemistry; Cleaves et al., Reference Cleaves, Chalmers, Lazcano, Miller and Bada2008; Rodriguez et al., Reference Rodriguez, House, Smith, Roberts and Callahan2019) or impact events involving catalytic metals within the impactor (Kasting, Reference Kasting1990; DiSanti et al., Reference DiSanti, Mumma, Russo, Magee-Sauer, Novak and Rettig1999; Sekine et al., Reference Sekine, Sugita, Kadono and Matsui2003; Kress and McKay, Reference Kress and McKay2004; Zahnle et al., Reference Zahnle, Lupu, Catling and Wogan2020); CO₂ adsorbed onto catalytic minerals at Earth's surface could have also been reduced if it were subjected to radiation, thermal or electrochemical energy sources (Hudson et al., Reference Hudson, de Graaf, Rodin, Ohno, Lane, McGlynn, Yamada, Nakamura, Barge, Braun and Sojo2020; Li et al., Reference Li, Li, Liu, Wu, Wu, Wang, Ye, Jia, Wang, Li, Zhu, Ding, Lai, Wang, Dick and Lu2020; Tsiotsias et al., Reference Tsiotsias, Charisiou, Yentekakis and Goula2020). CO₂ reduction in aqueous solutions is more limited as water often poisons metal catalysts (Porosoff et al., Reference Porosoff, Yan and Chen2016). Carbon reduction in the deep sea (e.g. at deep-sea vents) and deep subsurface sediments is even more restricted considering the lack of sunlight. Thus, at these locales CO₂ reduction is driven by either thermal (e.g. via Fischer Tropsch Type reactions; FTT) or electrochemical energy. It is debated to what extent the early Earth would have had land above sea level (Mojzsis et al., Reference Mojzsis, Harrison and Pidgeon2001; Wilde et al., Reference Wilde, Valley, Peck and Graham2001; Kemp et al., Reference Kemp, Wilde, Hawkesworth, Coath, Nemchin, Pidgeon, Vervoort and DuFrane2010; Reimink et al., Reference Reimink, Davies, Chacko, Stern, Heaman, Sarkar, Schaltegger, Creaser and Pearson2016; Hawkesworth et al., Reference Hawkesworth, Cawood and Dhuime2020; Rosas and Korenaga, Reference Rosas and Korenaga2021), so CO₂ reduction in the deep sea may have been critical for facilitating abiogenesis events. Of the deep-sea environments, hydrothermal vents are the most promising given that FTT reactions require high temperatures and pressures such as those found at vents; vents also generate strong electrochemical gradients (redox / pH) which, depending on the conditions, can drive CO₂ reduction (Martin and Russell, Reference Martin and Russell2007; Martin et al., Reference Martin, Baross, Kelley and Russell2008; Sojo et al., Reference Sojo, Herschy, Whicher, Camprubi and Lane2016). Accordingly, there is a plethora of work which has demonstrated how hydrothermal vents, particularly black smokers and alkaline vents, could generate conditions conducive for CO₂ reduction and the formation of biologically relevant compounds including amino acids (Russell et al., Reference Russell, Daniel, Hall and Sherringham1994; Russell and Hall, Reference Russell and Hall1997; Braun and Libchaber, Reference Braun and Libchaber2004; Kelley et al., Reference Kelley, Karson, Früh-Green, Yoerger, Shank, Butterfeild, Hayes, Schrenk, Olson, Proskurowski, Jakauba, Bradley, Larson, Ludwig, Glickson, Buckman, Bradley, Brazelton, Roe, Elend, Delacour, Bernasconi, Lilley, Baross, Summons and Sylva2005; Russell et al., Reference Russell, Barge, Bhartia, Bocanegra, Bracher, Branscomb, Kidd, McGlynn, Meier, Nitschke, Shibuya, Vance, White and Kanik2014; Li et al., Reference Li, Kitadai and Nakamura2018; Barge et al., Reference Barge, Flores, Baum, VanderVelde and Russell2019, Reference Barge, Flores, VanderVelde, Weber, Baum and Castonguay2020; Hudson et al., Reference Hudson, de Graaf, Rodin, Ohno, Lane, McGlynn, Yamada, Nakamura, Barge, Braun and Sojo2020). Consequently, hydrothermal sites have been argued as potentially conducive for abiogenesis on early Earth (Baross and Hoffman, Reference Baross and Hoffman1985; Russell and Hall, Reference Russell and Hall1997; Weiss et al., Reference Weiss, Sousa, Mrnjavac, Neukirchen, Roettger, Nelson-Sathi and Martin2016). Indeed, the Iron Sulphur World hypothesis posits that the iron sulphide minerals at black smoker vents were critical for the origins of life as they not only reduce CO₂, but have coordination structures reminiscent of Fe-S clusters in biological metalloenzymes (Wächtershauser, Reference Wächtershauser1990; McGlynn et al., Reference McGlynn, Mulder, Shepard, Broderick and Peters2009; Nitschke et al., Reference Nitschke, McGlynn, Milner-White and Russell2013; White et al., Reference White, Bhartia, Stucky, Kanik and Russell2015; Goldford et al., Reference Goldford, Hartman, Smith and Segrè2017). In addition, alkaline vents have been invoked as potentially relevant for the origins of life given that these systems can generate high temperature and alkaline, H₂-rich fluids that are in disequilibrium with the surrounding near-neutral waters; the resulting pH gradients have been invoked as a mechanism for driving CO₂ reduction akin to how organisms today produce adenosine triphosphate (ATP) via proton gradients (Russell and Hall, Reference Russell and Hall1997; Kelley et al., Reference Kelley, Baross and Delaney2002; Martin et al., Reference Martin, Baross, Kelley and Russell2008; Tivey, Reference Tivey2007; Sojo et al., Reference Sojo, Herschy, Whicher, Camprubi and Lane2016; Hudson et al., Reference Hudson, de Graaf, Rodin, Ohno, Lane, McGlynn, Yamada, Nakamura, Barge, Braun and Sojo2020). Notably, black smoker vents are significantly more acidic (pH 3–5) and hot (up to 350 °C) compared to alkaline vents (pH ~11; temperature 40–75 °C) (Kelley et al., Reference Kelley, Karson, Blackman, Früh-Green, Butterfield, Lilley, Olson, Schrenk, Roe, Lebon and Rivizzigno2001, Reference Kelley, Karson, Früh-Green, Yoerger, Shank, Butterfeild, Hayes, Schrenk, Olson, Proskurowski, Jakauba, Bradley, Larson, Ludwig, Glickson, Buckman, Bradley, Brazelton, Roe, Elend, Delacour, Bernasconi, Lilley, Baross, Summons and Sylva2005) and as such it has been suggested that organics would be more stable at alkaline than black smoker vents; though, metal-sulphide mineral precipitates that are abundant at black smokers may have higher electrochemical reactivity with CO₂ (Roldan et al., Reference Roldan, Hollingsworth, Roffey, Islam, Goodall, Catlow, Darr, Bras, Sankar, Holt, Hogarth and de Leeuw2015; Li et al., Reference Li, Kitadai and Nakamura2018).

Alkaline serpentinite-hosted vents such as Lost City form via serpentinization whereupon oceanic fluids oxidize the iron of minerals, namely olivine and pyroxene, within ultramafic-mafic rocks (i.e. enriched with Mg/Fe and depleted in SiO₂ (<45 wt%)) producing a range of secondary mineral phases including magnetite and serpentine (Schulte et al., Reference Schulte, Blake, Hoehler and McCollom2006; Shibuya et al., Reference Shibuya, Yoshizaki, Sato, Shimizu, Nakamura, Omori, Suzuki, Takai, Tsunakawa and Maruyama2015), and magnesite which can be produced by mineral carbonation in CO₂-containing systems (Klein and McCollom, Reference Klein and McCollom2013). In addition to being reactive, these materials can mediate organic transformations and are capable of preserving organics through a variety of mechanisms. For example, minerals can preserve organics within sheet structures that traps the organics effectively or organics can adsorb onto the surface of minerals (Farmer and Des Marais, Reference Farmer and Des Marais1999; Bonaccorsi, Reference Bonaccorsi2011).

Perhaps even more important for the habitability on early terrestrial bodies, serpentinization also generates molecular hydrogen (H₂) which could have served as an energy source to organisms that may have been present (Schulte et al., Reference Schulte, Blake, Hoehler and McCollom2006). The serpentinization reaction generates heat and hydroxide anions, which can trigger subsequent hydrothermal alteration and precipitation reactions, including the precipitation of brucite, carbonates and iron oxyhydroxides.

Importantly, serpentinization occurs wherever there is ultramafic rock subjected to aqueous alteration – on early Earth such rocks were likely prevalent in the oceanic crust, especially the deep subsurface, given that hotter, younger mantles are more conducive towards generating ultramafic melts as evidenced by Archaean rocks and Martian meteorites, respectively (Griffin et al., Reference Griffin, Belousova, O'Neill, O'Reilly, Malkovets, Pearson, Spetsuis and Wilde2014; Shibuya et al., Reference Shibuya, Yoshizaki, Sato, Shimizu, Nakamura, Omori, Suzuki, Takai, Tsunakawa and Maruyama2015; Santosh et al., Reference Santosh, Arai and Maruyama2017; Drabon et al., Reference Drabon, Byerly, Byerly, Wooden, Keller and Lowe2021). While mafic rocks, namely basalt, have substantially less ferrous mineral content (e.g. pyroxene and olivine) compared to ultramafic rocks, serpentinization and the generation of H₂ of mafic rocks can still occur (Stevens and McKinley, Reference Stevens and McKinley2000; Xiong et al., Reference Xiong, Wells, Menefee, Skemer, Ellis and Giammar2017). Studying these reactions via computer modelling and laboratory experiments provides critical analogues for early habitable systems on both Earth and Mars. While serpentinization does not reduce CO₂, the resulting high temperatures, H₂, and pH gradients from the reaction can drive CO₂ reduction (Sleep et al., Reference Sleep, Meibom, Fridriksson, Coleman and Bird2004); thus, serpentinizing systems may have been conducive for origins of life events (Russell et al., Reference Russell, Hall and Martin2010). Serpentinization is suggested to occur in the Martian subsurface (Hand, Reference Hand2009; Brown et al., Reference Brown, Viviano-Beck, Bishop, Cabrol, Andersen, Sobron, Moersch, Templeton and Russell2016; Tarnas et al., Reference Tarnas, Lin, Mustard and Zhang2018a, Reference Tarnas, Mustard, Sherwood Lollar, Bramble, Cannon, Palumbo and Plesa2018b) and other worlds hosting water:rock interactions including Ceres, Europa, and Enceladus (Glein et al., Reference Glein, Baross and Waite2015; Vance et al., Reference Vance, Hand and Pappalardo2016; Castillo-Rogez et al., Reference Castillo-Rogez, Neveu, Scully, House, Quick, Bouquet, Miller, Bland, De Sanctis, Ermakov, Hendrix, Prettyman, Raymond, Russell, Sherwood and Young2020).

The primordial Martian environment can be considered an analogue to early Earth in some ways: it once had a magnetic field (Langlais et al., Reference Langlais, Purucker and Mandea2004), an atmosphere dominated by CO₂ with transient periods of no O₂ (Sholes et al., Reference Sholes, Smith, Claire, Zahnle and Catling2017), and water flowing over a basaltic crust (Bibring et al., Reference Bibring, Langevin, Mustard, Poulet, Arvidson, Gendrin, Gondet, Mangold, Pinet and Forget2006; Carr, Reference Carr2012). Notably, large portions of the Martian surface that are older than 3.7 Ga (i.e. rocks from the Noachian eon) have been preserved (Bibring et al., Reference Bibring, Langevin, Mustard, Poulet, Arvidson, Gendrin, Gondet, Mangold, Pinet and Forget2006). Given the preservation potential by minerals present on both the early Martian crust and the early Earth, the Martian surface may provide a window to observe abiotic chemistry unimpacted by a biosphere. This could be especially interesting to explore reactions involving CO and CO₂ reduction given that organics and minerals that catalyse such reactions have been identified (Michalski et al., Reference Michalski, Onstott, Mojzsis, Mustard, Chan, Niles and Stewart Johnson2018; Liu et al., Reference Liu, Michalski, Tan, He, Ye and Xiao2021).

The importance of carbon cycling on other terrestrial planets beyond Earth, and the implications of that putative carbon cycle for the habitability of those worlds, is still an open question. For at least portions of its history, Mars possessed liquid water, photo and chemical energy sources to power potential microbial metabolisms, water of amenable pH and salinity, and the biogenic elements (carbon, hydrogen, oxygen, nitrogen, phosphorus and sulphur; i.e. CHONPS; Knoll and Grotzinger, Reference Knoll and Grotzinger2006). Indeed, Mars' atmosphere is currently dominated by CO₂ (Franz et al., Reference Franz, Trainer, Malespin, Mahaffy, Atreya, Becker, Benna, Conrad, Eigenbrode, Freissinet, Manning, Prats, Raaen and Wong2017), making reduction in the atmosphere a possible pathway relevant to abiogenesis (Heinrich et al., Reference Heinrich, Khare and McKay2007; Franz et al., Reference Franz, Mahaffy, Flesch, Raaen, Freissinet, Atreya, House, McAdams, Knudson, Archer, Stern, Steele, Stutter, Eigenbrode, Glavin, Lewis, Malespin, Millan, Ming, Navarro-González and Summons2020). Furthermore, organic carbon has been detected on the Martian surface as evidenced by the organic content in Martian meteorites and the Sample Analysis at Mars (SAM) instrument (Callahan et al., Reference Callahan, Burton, Elsila, Baker, Smith, Glavin and Dworkin2013; Eigenbrode et al., Reference Eigenbrode, Summons, Steele, Freissinet, Millan, Navarro-González, Sutter, McAdam, Franz, Glavin, Archer, Mahaffy, Conrad, Hurowitz, Grotzinger, Gupta, Ming, Sumner, Szopa, Malespin, Buch and Coll2018; Steele et al., Reference Steele, Benning, Siljeström, Fries, Hauri, Conrad, Rogers, Eigenbrode, Schreiber, Needham, Wang, Mccubbin, Kilcoyne and Rodriguez Blanco2018). By providing a source of organics, this could have sustained extant life on the planet.

Methane was first detected in the Martian atmosphere by Mars Express (Formisano et al., Reference Formisano, Atreya, Encrenaz, Ignatiev and Giuranna2004; Webster et al., Reference Webster, Mahaffy, Atreya, Moores, Flesch, Malespin, McKay, Martinez, Smith, Martin-Torres, Gomez-Elvira, Zorzano, Wong, Trainer, Steele, Archer, Sutter, Coll, Freissinet, Meslin, Gough, House, Pavlov, Eigenbrode, Glavin, Pearson, Keymeulen, Christensen, Schwenzer, Smith, Harri, Genzer, Hassler, Lemmon, Crisp, Sander, Zurek and Vasavada2018; Yung et al., Reference Yung, Chen, Nealson, Atreya, Beckett, Blank, Ehlmann, Eiler, Etiope, Ferry, Forget, Gao, Hu, Kleinböhl, Klusman, Lefèvre, Miller, Mischna, Mumma, Newman, Oehler, Okumura, Oremland, Orphan, Popa, Russell, Shen, Sherwood Lollar, Staehle, Stamenković, Stolper, Templeton, Vandaele, Viscardy, Webster, Wennberg, Wong and Worden2018) at levels near the instrumental detection limit. The Curiosity rover recently observed methane in the vicinity of Gale Crater on Mars (Webster et al., Reference Webster, Mahaffy, Atreya, Flesch, Mischna, Meslin, Farley, Conrad, Christensen, Pavlov, Martín-Torres, Zorzano, McConnochie, Owen, Eigenbrode, Glavin, Steele, Malespin, Archer, Sutter, Coll, Freissinet, McKay, Moores, Schwenzer, Bridges, Navarro-Gonzalez, Gellert and Lemmon2015; Eigenbrode et al., Reference Eigenbrode, Summons, Steele, Freissinet, Millan, Navarro-González, Sutter, McAdam, Franz, Glavin, Archer, Mahaffy, Conrad, Hurowitz, Grotzinger, Gupta, Ming, Sumner, Szopa, Malespin, Buch and Coll2018; Giuranna et al., Reference Giuranna, Viscardy, Daerden, Neary, Etiope, Oehler, Formisano, Aronica, Wolkenberg, Aoki, Cardesín-Moinelo, Marín-Yaseli de la Parra, Merritt and Amoroso2019). However, the Trace Gas Orbiter has not detected any atmospheric methane despite having a much lower detection limit, leading to an inconsistency yet to be resolved (Korablev et al., Reference Korablev, Vandaele, Montmessin, Fedorova, Trokhimovskiy, Forget, Lefèvre, Daerden, Thomas, Trompet, Erwin, Aoki, Robert, Neary, Viscardy, Grigoriev, Ignatiev, Shakun, Patrakeev, Belyaev, Bertaux, Olsen, Baggio, Alday, Ivanov, Ristic, Mason, Willame, Depiesse, Hetey, Berkenbosch, Clairquin, Queirolo, Beeckman, Neefs, Patel, Bellucci, López-Moreno, Wilson, Etiope, Zelenyi, Svedham and Vago2019). While methane detections remain hotly debated, the SAM instrument aboard Curiosity has detected a suite of other organics, as well, including chlorinated organics (e.g. chloromethane, dichloromethane, chlorobenzene) and S-bearing organics (both aliphatic and aromatic) (Mahaffy et al., Reference Mahaffy, Webster, Cabane, Conrad, Coll, Atreya, Arvey, Barciniak, Benna, Bleacher, Brinckerhoff, Eigenbrode, Carignan, Cascia, Chalmers, Dworkin, Errigo, Everson, Franz, Farley, Feng, Frazier, Freissinet, Glavin, Harpold, Hawk, Holmes, Johnson, Jones, Jordan, Kellogg, Lewis, Lyness, Malespin, Martin, Maurer, McAdam, McLennan, Nolan, Noriega, Pavlov, Prats, Raaen, Sheinman, Sheppard, Smith, Stern, Tan, Trainer, Ming, Morris, Jones, Gundersen, Steele, Wray, Botta, Leshin, Owen, Battel, Jakosky, Manning, Squyres, Navarro-González, McKay, Raulin, Sternberg, Buch, Sorensen, Kline-Schoder, Coscia, Szopa, Teinturier, Baffes, Feldman, Flesch, Forouhar, Garcia, Keymeulen, Woodward, Block, Arnett, Miller, Edmonson, Gorevan and Mumm2012, Glavin et al., Reference Glavin, Freissinet, Miller, Eigenbrode, Brunner, Buch, Sutter, Archer, Atreya, Brinckerhoff, Cabane, Coll, Conrad, Coscia, Dworkin, Reanz, Grotzinger, Leshin, Martin, McKay, Ming, Navarro-González, Pavlov, Steele, Summons, Szopa, Teinturier and Mahaffy2013, Williams et al., Reference Williams, Eigenbrode, Floyd, Wilhelm, O'Reilly, Stewart Johnson, Craft, Knudson, Andrejkovičová, Lewis, Buch, Glavin, Freissinet, Willians, Szopa, Millan, Summons, McAdam, Benison, Navarro-González, Malespin and Mahaffy2019; Millan et al., Reference Millan, Williams, McAdam, Eigenbrode, Freissinet, Glavin, Szopa, Buch, Williams, Navarro-Gonzalez, Lewis, Fox, Bryk, Bennet, Steele, Teinturier, Malespin, Johnson and Mahaffy2021). The mechanism by which all these Martian organics were generated remains uncertain (Miller et al., Reference Miller, Eigenbrode, Freissinet, Glavin, Kotrc, Francois and Summons2016; Szopa et al., Reference Szopa, Freissinet, Glavin, Millan, Buch, Franz, Summons, Sumner, Sutter, Eigenbrode and Williams2020). One possibility is that they were derived from reduced gases, such as H₂ and CH₄, which have been hypothesized to have originated from various sources including serpentinization of ultramafic minerals such as olivine, subsurface hydrothermal environments, photocatalysis and volcanic activity (Sherwood Lollar et al., Reference Sherwood Lollar, Lacrampe-Couloume, Slater, Ward, Moser, Gihring, Lin and Onstott2006; Amador et al., Reference Amador, Bandfield and Thomas2018; Tarnas et al., Reference Tarnas, Lin, Mustard and Zhang2018a, Reference Tarnas, Mustard, Sherwood Lollar, Bramble, Cannon, Palumbo and Plesa2018b). Methane on Mars can be hypothetically produced by biological reactions (Boston et al., Reference Boston, Ivanov and McKay1992; Weiss et al., Reference Weiss, Yung and Nealson2000) or produced abiotically by water-rock reactions (Wallendahl and Treimann, Reference Wallendahl and Treimann1999; Max and Clifford, Reference Max and Clifford2000), volcanic outgassing (Wong et al., Reference Wong, Atreya and Encrenaz2003) or even exogenous delivery such as comets (Kress and McKay, Reference Kress and McKay2004). Further complicating this, methane is easily trapped in the subsurface, persisting in minerals or gas pockets through geologic time (Max and Clifford, Reference Max and Clifford2000). On Mars, methane could have been produced recently or trapped in the ice-bearing subsurface in the Noachian or Hesperian as the planet cooled (Kerr, Reference Kerr2004). The non-uniform atmospheric detections of methane in the Martian atmosphere are indicative of localized sources and/or localized sinks (Formisano et al., Reference Formisano, Atreya, Encrenaz, Ignatiev and Giuranna2004), emphasizing the need for more precise constraints on the reactions creating or uptaking Martian methane (Fig. 1).

Fig. 1. Plausible mechanisms for CO₂ reduction on early Earth or Mars. (A) General CO₂ reduction reaction requires a H source (likely H₂ or H₂O), energy (e.g. thermal, radiation, electric discharge or redox gradient), and some sort of catalyst or mediator (e.g. Fe⁰, Ni²⁺, magnetite). Common products of this reaction include CO (which can be further reduced) and CH₄ (unstable to photolytic degradation) with organics usually produced in lower amounts (methanol, formaldehyde, formic acid and acetic acid are generally produced and are shown). (B) Locations of interest for CO₂ reduction on early Earth or Mars: reduction via Miller–Urey chemistry with (1) H₂ as a H donor or (2) H₂O as a H donor generates a range of organics (Cleaves, Reference Cleaves2008); (3) impactors containing catalytic transition metals can facilitate CO₂ reduction (e.g. Civiš et al., Reference Civiš, Szabla, Szyja, Smykowski, Ivanek, Knížek, Kubelík, Šponer, Ferus and Šponer2016; Steele et al., Reference Steele, Benning, Siljeström, Fries, Hauri, Conrad, Rogers, Eigenbrode, Schreiber, Needham, Wang, Mccubbin, Kilcoyne and Rodriguez Blanco2018); (4) CO₂ dissolution via precipitation or with equilibrium with bodies of water produces carbonic acid, bicarbonate and carbonate ions; (5) reduction of CO₂ adsorbed onto catalytic minerals such as anatase (which contains TiO₂) via photolysis (e.g. Knížek et al., Reference Knížek, Kubelík, Bouša, Ferus and Civiš2020); (6) UV irradiation of CO₂ generates reduced C species; (7) black smoker hydrothermal vents, (8) alkaline vents such as those at Lost City, (e.g. Hudson et al., Reference Hudson, de Graaf, Rodin, Ohno, Lane, McGlynn, Yamada, Nakamura, Barge, Braun and Sojo2020; Preiner et al., Reference Preiner, Igarashi, Muchowska, Yu, Varma, Kleinermanns, Nobu, Kamagata, Tüysüz, Moran and Martin2020). (9) serpentinization in the deep subsurface can generate conditions conducive for CO₂ reduction (e.g. Etiope et al., Reference Etiope, Schoell and Hosgörmez2011; Preiner et al., Reference Preiner, Xavier, Sousa, Zimorski, Neubeck, Lang, Greenwell, Kleinermanns, Tüysüz, Micollom, Holm and Martin2018). *CO₂ indicates a mixture of dissolved CO₂ gas or bicarbonate/carbonate anions.

The source and production of reduced carbon on Earth and its availability on other worlds are not well constrained. Carbon reduction can give a variety of reduced products depending on the complex geological context (Schrenk et al., Reference Schrenk, Brazelton and Lang2013). In addition, there is limited access to extraterrestrial samples, which would help constrain the geologic context as well as provide insight to the chemical reactions occurring. CO and CO₂ could both be sources for reduced carbon via several reaction pathways as both gases would have been available in the atmosphere and are more stable in comparison to methane (Kasting et al., Reference Kasting, Zahnle and Walker1983; Kasting, Reference Kasting1993; Kasting and Catling, Reference Kasting and Catling2003; Sherwood Lollar et al., Reference Sherwood Lollar, Lacrampe-Couloume, Slater, Ward, Moser, Gihring, Lin and Onstott2006).