Radionuclide-induced defect sites in iron-bearing minerals may have accelerated the emergence of life

Abstract

The emergence of life on Earth (and elsewhere) must have occurred in a milieu that is far from equilibrium, such as at alkaline hydrothermal vents that would have harboured built-in gradients in temperature, redox potential and pH along with precipitated iron-bearing minerals capable of separating these gradients, concentrating reactants and catalysing requisite protobiotic reactions. Iron-bearing minerals such as mackinawite, greenalite and fougèrite have been investigated as catalysts for protobiotic reactions, including amino acid synthesis. In the field of heterogeneous catalysis, it is well known that defect sites in the crystal structure are often the most active sites for catalysis, and mineral catalysts that have been exposed to ionizing radiation are known to exhibit increased reactivity due to radiation-induced defect sites. In this work, we (i) review the literature on the radioactive environment of the Hadean era, (ii) highlight the role of radionuclide ionizing radiation from ²³⁸U, ²³²Th and ⁴⁰K in generating defect sites with high catalytic activity for the chemical evolution of organic molecules, and (iii) hypothesize that these processes accelerated the emergence of life.

What is life's job? … To hydrogenate CO₂ —Michael J. Russell

It is at a surface where many of our most interesting and useful phenomena occur. —Walter H. Brattain, ‘Surface properties of semiconductors', Nobel Lecture (11 December 1956). In Nobel Lectures, Physics 1942–1962 (1967), 377.

We think there is color, we think there is sweet, we think there is bitter, but in reality there are atoms and a void. —Democritus (460–370 BCE)

…though the senses retort: ‘Poor intellect, do you hope to defeat us while from us you borrow your evidence? Your victory is your defeat.’ —Galen (129–210 CE)

1. Introduction

All organisms alive today can (in principle) be traced, via an unbroken, approximately 4 billion year-long chain of metabolism and replication, to the last universal common ancestor (LUCA) (figure 1) [8,10,11]. Each element of the chain (i.e. organism) must be, and must have been, far from equilibrium with its environment to sustain the chemistry of life. Upon death, due to the breakdown of one or more critical biochemical pathways, the cellular remains begin to equilibrate with surroundings, and despite having a composition nearly identical to its living brethren, the postmortem processes never reanimate the organism. Life requires external disequilibria and the engines to couple them so that the endergonic metabolic pathways that lead to growth, reproduction and self-replication may be driven. That requirement would have been even more stringent for the first, seemingly improbable link in the chain, which represents the transition from protobiotic geochemistry to the emergence of life.

Figure 1.

High-level representation of Russel's AHV model for the emergence of life [1–5], shown coupled to the ring of life schematic [6,7]. Radionuclide ionizing radiation impacts prebiotic chemistry by radiolysis of water, organics and minerals, which diversifies the reactive species, intermediates and product distribution, and accelerates reactions at mineral defect sites. Permissions for the use of graphics from [8,9] are in process of being obtained. (Online version in colour.)

Sometime during the Hadean Eon, when life emerged, the environment on Earth was much more energetic and dynamic than at the present day. Volcanism was more prevalent as the heat of Earth formation was just beginning to dissipate, the tides were larger and more frequent as our moon was much closer and the days were shorter, surface solar UV was more intense from a young sun and in the absence of atmospheric ozone, and radioactivity was greater given the decay rates of the radionuclides. Putative microcontinents and basaltic islands may have risen above the ocean surface, but probably constituted less than 5% of the present continental area [12], and the composition of the atmosphere was anoxic, dominated by N₂ and CO₂, but also contained other volatiles such as SO₂ and water vapour [13]. The ocean was mildly acidic, owing to the high concentration of atmospheric CO₂ that forms carbonic acid in water [14–16], which enabled increased dissolution of transition metals and actinides. And in the deep subseafloor, carbonic acid-laden waters reacted with rock at elevated temperatures to produce an alkaline effluent with molecular hydrogen, formate and methane that vented back into the acidic ocean [1].

In Russell's alkaline hydrothermal vent (AHV) model (figure 1), gradients in temperature, redox potential and pH provide intrinsic disequilibria that have the potential to drive protobiotic chemistry towards organic complexity and the carbon-based superstructures of the cell, replicative chemistry and ultimately the emergence of life. The model describes how alkaline effluents precipitate iron ions upon mixing with the iron-rich ocean waters to form iron-bearing mineral vent structures that separate these gradients, concentrate reactants and could catalyse and drive protobiotic reactions. Such submarine AHVs were first postulated [2] and subsequently discovered, and their abiogenic organic and hydrocarbon production characterized [17–19]. The hydrothermal minerals precipitated in alkaline solutions and invoked in driving putative protobiotic chemistry [3] include iron-bearing minerals such as mackinawite (FeS_1−x), greenalite (∼Fe²⁺, Fe³⁺)₂-3Si₂O₅(OH)₄), and fougèrite ([Fe²⁺_1−x Fe³⁺_xMg_y (OH)_2+2y]^x+[xOH⁻ · mH₂O]x⁻) [20,21] (a naturally occurring form of green rust, ∼[Fe²⁺₆xFe³⁺₆(1 − x)O₁₂H₂(7 − 3x)]²⁺[CO₂⁻³ 3H₂O)]²⁻) [22].

Experimental approaches to study plausible mechanisms for emergence of life have been based on laboratory simulations that recreate geologically and geochemically relevant scenarios and possible conditions of the early Earth (e.g. [23]). The choice of reactants, their concentration, a free energy source and the mineral interfaces are important aspects of experimental design. To test aspects of the AHV model, a series of proposed and performed experiments have been summarized, and provide a roadmap for future experiments that aim to test, falsify or support the many elements of the model [3]. However, one aspect that has not been incorporated in experimental design to date has been the effect of ionizing radiation from radionuclides on the catalytic properties of iron-bearing minerals (figure 1). Since formation, our planet has been exposed to ionizing radiation from space (solar and cosmic rays) and natural radioactivity (radionuclides). Here, we review the radiation environment of early Earth, its possible effects on catalysts, and suggest post-synthesis irradiation of catalysts for future emergence of life experiments on the basis of the hypothesis that ionizing radiation-induced defect sites in iron-bearing minerals result in high catalytic activity that accelerated the emergence of life.

2. Catalysts for the emergence of life

Iron-bearing clay minerals were likely already being generated approximately 4 billion years ago [24], and their presence has been considered essential for protobiotic chemistry [25,26]. Favourable properties include large surface areas that can concentrate reactant organic molecules from dilute sources with pH-dependent adsorption propensity. In acidic pH, the adsorption propensity for organic molecules is high, while at greater than pH 8, the binding of bio-relevant molecules becomes negligible [25]. This may enable reversible binding of reactants and products across the pH gradients separated by layered minerals structures with interstitial voids; reversible binding of substrates is required in enzymes/bio-catalysts. Moreover, clay surfaces may serve as templates that generate ordered adsorption of organic molecules for polymerization, and drive protobiotic chemistry via electron and proton transfer, conformational coupling and the stabilization of transition states and intermediates and block back reactions ([16] and references within).

Montmorillonite is three-layered clay mineral structure, and the interlamellar space may be occupied (i.e. intercalated) by water, free inorganic cations and polar molecules [27], which causes the lattice to expand from 10 to 20–50 Å [28]. In protobiotic chemistry, montmorillonite is the clay mineral type most extensively used to investigate adsorption processes [25]. Indeed, experiments have shown that 1 ton of montmorillonite adsorbed up to 550 kg of protein, 150 kg of fatty acids or 200 kg of carbohydrates [29].

In Russell's AHV model, iron nickel sulfides minerals, such as mackinawite/greigite, with its cubane iron–sulfur structure strongly reminiscent of the active site of the bioinorganic enzyme acetyl-CoA synthase/carbon monoxide dehydrogenase that mediates electron transfer reactions in metabolism [4] were first invoked as the main catalytically relevant precipitates at the alkaline vent effluent/acid Hadean ocean interfaces [30]. However, the pores, while initially appealing conceptually as a mineral analogue of a cell, do not account for the requirements of pumping ions on a molecular scale, nor removal of waste products. These requirements can be realized with the double-layer hydroxides, which have a structure consisting of alternating positively charged hydroxide layers and hydrated anion layers. Double-layer hydroxides are a close relative of the more common layered double hydroxides, of which magnesium layer hydroxides are an example [31]. Specifically, the iron mixed-valent minerals, such as greenalite and fougèrite (i.e. green rust), along with the subordinate iron sulfide mackinawite were ‘proposed to have functioned as the electrochemical nano-engines acting to convert the imposed external proton and redox disequilibria into the internal disequilibria necessary to bring life into being’ [1,3,32].

Mixed-valence minerals may have been prevalent in the pre-oxygen environment, and fougèrite (green rust) was the likely precursor to banded iron formations preceding the ‘Great Oxidation Event’ [33]. The characteristic bluish green colour of hydromorphic soils arises from the metal-to-metal charge transfer absorption band that results from the presence of mixed Fe²⁺/Fe³⁺ hydroxide species [34,35].

While numerous laboratory experiments that have aimed to reproduce the conditions of early Earth and investigate reactions catalysed by clay minerals have been published, the impact of defect sites (i.e. crystal imperfections such as vacancies, atom replacements, etc.) in mineral structures on catalytic reaction rates and selectivity has been considered in only a few protobiotic chemistry investigations [23,36,37]. Mineral defect sites may have been particularly important on early Earth, when the ionizing radiation of radionuclides was much greater than present day. And our thesis is that that radiation would have generated ‘uncountable’ defect sites for accelerating protobiotic geochemistry.

3. Mineral defect sites and their effects on catalysis

In heterogeneous catalysis, a solid catalyst promotes the activity and selectivity of gas or liquid phase chemical reactions. Heterogeneous catalysts have enabled a number of large-scale industrial chemical processes, including the Haber nitrogen to ammonia, steam reforming, water gas shift, Sabatier, Fischer–Tropsch and methanol synthesis [38–41]. More recently, the climate change-driven rise of the CO₂ capture and conversion industry, which aims to close the anthropogenic CO₂ production cycle to the chemical and fuel industry, has led to substantial investments in the discovery of highly active, selective and stable catalysts that can efficiently and economically enable CO₂ conversions to high-value chemicals and fuels on a massive hundred megaton per year scale [42].

Approaches to modify catalyst activity, selectivity and stability characteristics typically include both experimental and theoretical methods to evaluate structural and/or electronic augmentations of a given catalyst of interest. Structural changes include modifications in the surface structure and area, accessibility to active sites and sintering of the catalyst [40,43], while electronic changes include modification in the electronic band structure through charge transfer and re-hybridization, changes in adsorption energies of reactants, intermediates and products, and modifications of the free energy reaction profile. It is the defects in crystalline structures that control their electronic and catalytic properties [43–45].

From 2015 to 2018, the number of ‘defect engineering’ publications listed in the Web of Science have nearly quadrupled, and our understanding and development of methods has correspondingly grown. In defect engineering, the type and population of defects may be engineered into heterogeneous catalysts to tailor and optimize their catalytic properties. For the intensely investigated crystalline metal oxides (thanks to the semiconductor industry), various point and extended defects introduced into the lattices modify electrical, optical, magnetic, thermal and mechanical properties [46]. Less well known is the effect of defect sites on heterogeneous catalysis [43,47]. The heterogeneous catalysis characteristics imparted by defect sites is largely governed by the position and nature of the defect level in the electronic band gap of a material, as well as the effective work function and density of states, which determines whether a defect behaves as an electron acceptor (Lewis acid) or an electron donor (Lewis base), as well as the strength of its acidity or basicity. Understanding and controlling these properties is at the heart of designing new defect-based heterogeneous catalytic materials [43] (figure 2).

Figure 2.

(a–f) From [48]. (a) Schematic polyhedral representation of the mixed-valence layered-double-hydroxide (LDH) structure with defective MO₆ octahedra induced by a vacancy site. (b) Biaxial strain for MO₆ octahedra in LDH nanosheets, with (c) undistorted and (d) the corresponding strained MO₆ octahedron. (e) Two-dimensional structural model for an LDH monolayer from density function theory (DFT) calculations viewed from above (vacancy defect site marked by the yellow dot). (f) DFT electron density map of a vacancy defect site showing increased electron density that translates to increased reducing power. Vacancy and distortion defects induce additional energy states, which likely serve as electron-trapping sites that enhance electron transfer. (g) Defect engineering of metal dichalcogenide generates (g,h) vacancies and (i) substitutions, as visualized by electron microscopy and DFT [49]. (j) Mixed-valence LDH after alkaline etching generated defects give rise to superior catalytic activity [50]. Permissions for the use of graphics from [39,51,52] are in process of being obtained. (Online version in colour.)

The simplest and most abundant defects are vacancies, but it is also possible for lattice atoms to be replaced by substituting other atomic species. When considering the extent to which a foreign atom may substitute into a crystal lattice, the ion's relative dimensions, electronegativity, valence and end member crystal structure are all relevant factors [49]. For example, for the case of the transition metal dichalcogenides (the family of elements that form metal ores—i.e. members of group 16 or VI on the periodic table), and for the sulfide of molybdenum (MoS₂) in particular, several metallic dopants have been incorporated, including tungsten ([49] and references within). The substitution of iron in green rust with tungsten through vacancy defect sites, for example, might enable a two-electron process similar to the biological mechanisms of electron bifurcation [53–55]. Defect sites can be deliberately generated post-synthesis by ion/electron irradiation, and sulfur vacancies, for example, have been produced by electron bombardment [56]. In addition, defects have been engineered by α-particle and Mn⁺ ion bombardment, proton beam irradiation, ozone treatment and laser illumination [49].

4. Radionuclide-induced defect sites in catalysts

When life first emerged on Earth, the three radionuclides that dominated radioactivity were uranium-238, thorium-232 and potassium-40. At that time, Earth's total inventory of U, Th and K radionuclides can be calculated from radioactive decay rates. ⁴⁰K was approximately eight times its present abundance, ²³⁸U twice, although ²³²Th was only about 20% more abundant due to its 14.5 billion year half-life (table 1). The effects of ionizing radiation from these radionuclides on chemical reactivity and the generation of high energy reactive intermediates have been considered as possible sources of energy for life on early Earth [57], as well as on the ocean worlds of the outer solar system [58], and even beyond [16]. For example, water radiolysis produces e⁻_aq, HO*, H*, HO₂*, H₃O⁺, OH⁻, H₂O₂ and H₂ [16,59–61], and organic molecules, such as amino acids, can be produced [62] and destroyed by ionizing radiation depending on the sample composition and dosage. However, the ionizing radiation not only generates free radicals from water and organic molecules, but also produces long-lived defect sites in mineral structures that can serve as coordinatively unsaturated binding sites for reactants [63] with the potential for high catalytic activity to produce molecules that may have contributed to the emergence of life on Earth (figure 1).

Table 1.

Radionuclide ionizing radiation characteristics.

radionuclide	half-life (Gyr)	abundance 4 Gyr ago	α-particles in decay chain	Etotal (MeV)e
232Tha	14	+22%	6	42.6
235Ub	0.7	+50×	7	46.4
238Uc	4.5	+1.9×	8	51.7
40Kd	1.25	+9×	0	1.3

^aOnce thorium-232 decays to radon-228 with one α-particle emission, the subsequent decays to form lead-208 occur rapidly (less than 10 years) with six α emissions.

^bOnce uranium-235 decays to thorium-231 with one α-particle emission, the subsequent decay to protactinium-231 with β-emission occurs in days. Then protactinium decays with α-particle emission to actinium-227 with a 33 kyr half-life. The subsequent eight decays in the chain to form lead-207 occur rapidly with five additional α-particle decays.

^cOnce uranium-238 decays to thorium-234 with one α-particle emission, the subsequent decays via protactinium-245 to uranium-234 occur on the timescale of weeks with β-particle emissions. Then uranium-234 decays with α-particle emission and a half-life of 245 kyr. The resultant thorium-230 also decays with α-particle emission and half-life of 75 kyr, which in turn decays with α-particle emission and a half-life of 1.6 kyr. The subsequent eight decays in the chain to form lead-206 occur rapidly with four additional α-particle decays.

^dThe total energy released from potassium-40 with β-decay is 1.3 MeV (89.28% of the time) and with electron capture is 1.5 MeV (10.72% of the time).

^eFor reference, typical covalent bond energies range between 1 and 10 eV.

Because of their technological importance and environmental concern, the properties and distribution of radionuclide-containing minerals have been intensively studied [64,65]. The earliest description of a change in mineral property caused by radiation damage was by Jöns Jacob Berzelius in 1814, when he discovered that some U- and Th-bearing minerals glowed on moderate heating, and released large amounts of energy [66,67]. The discovery of radioactivity by Becquerel in 1896 set the stage for Hamberg in 1914 to suggest that radiation-induced transformation of minerals was caused by α-particles that emanate as a result of the uranium and thorium decay-series [67]. In 1955, Holland & Gottfried applied the understanding of radiation-induced crystal structure damage mechanisms to amorphized zircons to develop an age-dating method [67,68]. The world's oldest known Hadean zircon grains from the Jack Hills district in Western Australia have U and Th concentrations of up to 100s of ppm [65].

While the calculated whole-Earth uranium concentrations are only approximately 10 ppb based on meteorite uranium concentrations and the volume of Earth [65,69,70], the average uranium concentration in Earth's crust is 2–3 ppm, with concentrations rising to percentage levels at some locations to form uranium and thorium minerals (uraninite, UO₂; thorianite, ThO₂; coffinite, USiO₄) [65]. At one point, about 2 billion years ago, uranium-bearing minerals were even enriched to the point of maintaining a natural bio-nuclear reactor [71,72]. It has been postulated that a billion years were required for extensive fluid reworking of the upper mantle and crust, including magmatic hydrothermal activity, to concentrate and enrich uranium/thorium radionuclides for corresponding mineral formation [65].

During the Hadean Eon, the geochemistry of U and Th was dominated by the 4+ oxidation state in magmas and in aqueous solutions, as the 6+ became accessible only after the Great Oxidation Event around 2 billion years ago [65]. Uraninite is highly insoluble as the U⁴⁺ species UO₂(aq) in solutions with pH > 3 and at temperatures up to about 300°C [65]. One recent model for the emergence of life at AHVs has included fougèrite (i.e. green rust) as a key mineral [3], and experiments have shown that U⁶⁺ (as soluble uranyl ion) is readily reduced by green rust to U⁴⁺ in the form of relatively insoluble UO₂ nanoparticles [35]. At the Lost City Hydrothermal Field, which is a serpentinite-hosted AHV field located 15 km west of the spreading axis of the Mid-Atlantic Ridge, fluid sample analyses indicated that effluents contain only 7 ppt U compared to 3 ppb of U in ambient seawater [73]. However, communities of organisms clustered around hydrothermal vents have been exposed to high natural radiation doses as evidenced by high U, and Po–Pb levels compared to non-vent organisms [74]—an indication that present-day hydrothermal effluents and vent structures bear a wide range of radionuclide abundances. These observations of radionuclide abundances and distributions in present-day minerals indicate that mechanisms to concentrate the then more abundant radionuclides were in operation during the Hadean era, and provide plausibility for the hypothesis that substantial ionizing radiation was present during the protobiotic geochemical evolution that led to the emergence of life on Earth.

Ionizing radiation from naturally occurring radionuclides in the form of β(e⁻), γ(hν) and α(He²⁺) radiation along with the associated ballistic α-recoil events of the parent nucleus are very efficient sources of energy for generating reactive free radicals from water, organic molecules and minerals, apart from the damage they do to crystal lattices of minerals. These mechanisms work in concert to generate catalytically active defect sites that would not be present in perfect crystalline lattices. The large specific surface areas of clay minerals, in particular, provide an increased interaction area with radionuclide-bearing fluids, and consequently a higher sensitivity to natural radiation [75], even after mineral formation. For example, uranium can be adsorbed onto their external surface or occur within associated minerals [35], and radiation may cause complete amorphization [75,76]. The radiation level may be enhanced by the trapping of radionuclides by associated iron oxides or organic matter [9]. Effects of amorphization on properties of clay are significant, particularly on their sorption capacity, swelling, dissolution kinetics [51] and as hypothesized here, on the catalytic potential for accelerating protobiotic geochemistry.

In minerals, the dominant radiation damage mechanism results from α-decays associated with ²³⁸U, ²³⁵U and ²³²Th and radionuclides in their respective decay chains that generate both α-particles and α-recoil nuclei, which are ejected in opposite directions from the decaying nuclei (table 1). Along their trajectories, they transfer energy to the atoms in the crystal structure. The α-particles have approximately 98% of the energy of decay-events with energies in the range of 4.5–5.8 MeV and cause crystal damage along a 10–30 µm path, while the larger α-recoil nuclei have energies in the range of 70–100 keV and cause crystal damage along a 10–20 nm path [52,67]. The energy transfer occurs by ionization, by generating electron–hole pairs that result in bond rupture, by electronic excitation, and by elastic collisions of both α-particles and α-recoil nuclei with the nuclei in the crystal. If the collisions impart sufficient kinetic energy to the crystal atoms (E_kinetic > E_displacement), then atomic displacements from lattice sites dominate in a branching cascade of displaced atoms [52,67,77]. The kinetic energy of α-particles is sufficient to induce several hundred atomic displacements, which form Frenkel defects [78]. And although they are less energetic, α-recoil nuclei generate up to 2000 localized displacements [52,67].

Crystal lattice impurities, chemical and structural defects, including displacements of lattice atoms, can trap unpaired electrons or positive holes produced by ionizing α-, β- and γ-radiation to generate coordinatively unsaturated and activated sites for enhanced catalytic chemistry. These defects may cause coloration of minerals, such as smoky quartz, amethyst (Fe⁴⁺) or the great diversity of fluorite colours arise from trapped electrons and positive holes, which create new energy states and cause specific optical transitions [76,79]. These radiation-induced colour centres have even been used to identify sodium chloride salt on the surface of Jupiter's ocean world, Europa [80]. On Earth, electron paramagnetic resonance (EPR) spectroscopy is the analytical tool of choice for measuring the concentration of these centres as a function of total radiation dose to which the sample was exposed [76].

5. Laboratory investigations of protobiotic catalysis

While radiation effects on protobiotic chemistry have been investigated extensively with laboratory experiments, the impact of pre-irradiated catalytic surfaces that promote these reactions has been merely mentioned in passing [16]. In addition to the radiolytic chemistry in solution, ionizing radiation generates defects on mineral surfaces that enhance heterogeneous catalysis by increased reactant adsorption and catalytic activity due to radiation-induced electron displacement (ionization, excitation and trapping) and atom displacement (vacancies and interstitial). Consequently, the localized nature of ionizing radiation may have produced microenvironments within iron-bearing minerals that are of high activity, where simple organic molecules could have reacted to form complex protobiotic product distributions. Laboratory campaigns that aim to investigate protobiotic chemistry based on heterogeneous catalysis with synthesized minerals will thus require pre-irradiation of the catalyst to emulate the defect distribution that govern reaction rates and product distributions. Because the radiation environment of the early Earth are not known, the pre-irradiation parameters, such as dosage and flux, to emulate defect site formation of the early Earth will need to be determined empirically by monitoring catalysis product formation as a function of these parameters.

The radiation intensity on Hadean Earth arising from long-lived radionuclides in clays was calculated at 1.75 × 10⁻² Gy year⁻¹ (Gy = Gray, unit for absorbed dose, and equivalent to 1 × 10⁻³ J g⁻¹) [81], which generates sufficient energy to induce radiolytic chemical reactions. Of course, the actual radiation dosage at the time of life's emergence will remain unknown, as the differentiation process only had a few hundred million years from Earth's formation to the emergence of life to concentrate radionuclides from the initial dilute abundance of source celestial material as represented by meteorites. Nonetheless, the dosage, being a statistical term, does not need to be exceedingly high to have accelerated protobiotic chemistry, because even a singular radionuclide decay could have generated the microenvironment that fostered life's emergence.

The experimental design for laboratory experiments, such as those summarized and proposed by Russell [3], should include pre-irradiation of iron-bearing catalysts with a variety of dosages and radiation sources. As a guide to radiation dosage, the likely timescale of Hadean AHV activity for a given location can be estimated at 10⁴–10⁵ years, based on current age estimates of AHV at Lost City [73]. Radiation sources for pre-irradiation of catalysts should include α-, β- and γ-radiation sources to emulate and explore the variable space of mineral defects and their impact on reaction rates, selectivity and product distributions.

Diagnostic tools are critical in characterizing catalysts and their defect site populations. Current technology investments in CO₂ capture and hydrogenation need to be leveraged, which include operando methods to characterize both catalyst defects and product distributions. For example, oxygen vacancies have an electron donor effect in the band gap and can be observed by optical absorption–emission, X-ray photoelectron spectra and scanning transmission electron microscopy [43,49]. Another example of pertinent technology development are electrochemical cells that use low volumes and consequently maintain products at high concentrations to enable GC and NMR identification of additional products and intermediates that could provide additional mechanistic insights [82].

Computational screening of new and less ordered catalysts is still challenging, due to the presence of diverse active sites, intermediates and reaction steps. Operando techniques include state-of-the-art analytical optical, X-ray and electron microscopy approaches, and have been invaluable in combining advanced characterization of a catalyst with simultaneous measurements of its activity and selectivity under real working conditions [83]. These techniques employ analytical tools in situ to characterize both product formation and the states of a catalyst during catalysis. Experimental designs that include operando techniques will provide the evidence required to address key issues in protobiotic catalysis, including assessments of catalyst stability under reaction conditions, identification of key intermediates and of catalytically active sites, preferred reaction pathways and selectivity, and effects of reaction environment.

6. Conclusion

Laboratory investigations that probe elements of emergence-of-life models require the simulation of the protobiotic environment in terms of physical conditions, composition and disequilibria, and it should be noted that ionizing radiation impacts all of these parameters. Experimental design for emergence-of-life laboratory studies that include heterogeneous catalysis needs to include pre-irradiation of catalysts to induce defects that emulate plausible Hadean environments. Also, the large investment in operando techniques to support the CO₂ capture and hydrogenation industries represent an opportunity to leverage these tools to gain mechanistic understanding of heterogeneous catalysis of plausible protobiotic chemistries that may have accelerated the emergence of life.

Data accessibility

This article has no additional data.

Competing interests

I declare I have no competing interests.

Funding

This study was funded by NASA and the Jet Propulsion Laboratory through the Mars 2020 Project Science Office, which is supported by NASA's Science Mission Directorate, and the MOXIE Technology Demonstration Project, which is supported by a collaboration between NASA's Human Exploration and Operations Mission Directorate and the Space Technology Mission Directorate.