New Estimates of Nitrogen Fixation on Early Earth

Abstract

Fixed nitrogen species generated by the early Earth’s atmosphere are thought to be critical to the emergence of life and the sustenance of early metabolisms. A previous study estimated nitrogen fixation in the Hadean Earth’s N₂/CO₂-dominated atmosphere; however, that previous study only considered a limited chemical network that produces NO_x species (i.e., no HCN formation) via the thermochemical dissociation of N₂ and CO₂ in lightning flashes, followed by photochemistry. Here, we present an updated model of nitrogen fixation on Hadean Earth. We use the Chemical Equilibrium with Applications (CEA) thermochemical model to estimate lightning-induced NO and HCN formation and an updated version of KINETICS, the 1-D Caltech/JPL photochemical model, to assess the photochemical production of fixed nitrogen species that rain out into the Earth’s early ocean. Our updated photochemical model contains hydrocarbon and nitrile chemistry, and we use a Geant4 simulation platform to consider nitrogen fixation stimulated by solar energetic particle deposition throughout the atmosphere. We study the impact of a novel reaction pathway for generating HCN via HCN₂, inspired by the experimental results which suggest that reactions with CH radicals (from CH₄ photolysis) may facilitate the incorporation of N into the molecular structure of aerosols. When the HCN₂ reactions are added, we find that the HCN rainout rate rises by a factor of five in our 1-bar case and is about the same in our 2- and 12-bar cases. Finally, we estimate the equilibrium concentration of fixed nitrogen species under a kinetic steady state in the Hadean ocean, considering loss by hydrothermal vent circulation, photoreduction, and hydrolysis. These results inform our understanding of environments that may have been relevant to the formation of life on Earth, as well as processes that could lead to the emergence of life elsewhere in the universe.

1. Introduction

One of the greatest scientific mysteries is the origin of life. On Earth, life may have emerged in hydrothermal systems at the bottom of the Hadean ocean [1,2,3,4]. It is hypothesized that this nascent life would have used fixed nitrogen species as a high-potential electron acceptor to oxidize hydrogen and methane and help reduce CO₂ to organic carbon [5,6,7]. Early metabolisms may have continued to use nitrogen oxides (NO_x) as one source of high-potential electron acceptors [8,9,10]. Other hypotheses for the origin of life place a large emphasis on hydrogen cyanide (HCN) chemistry for protein and nucleotide synthesis [11].

NO_x can form abiotically on ancient Earth via production in the atmosphere by lightning, which is capable of breaking down CO₂ and N₂, followed by photochemistry and rainout to surface waters (Figure 1) [12,13]. Lightning discharges decompose CO₂ and N₂ as follows:

CO₂ → CO + O

O + N₂ → NO + O

N + CO₂ → NO + CO.

Because of diatomic nitrogen’s strong triple bond, very few processes are able to decompose N₂ in the way lightning does. As lightning is discharged into the atmosphere, its current-carrying channel heats the air around it. The shockwave that follows, commonly referred to as thunder, carries that heat to distant particles that dissociate under the heat and pressure of the shockwave. Molecules that form via lightning discharges become kinetically frozen, unable to revert back to their original state because the atmosphere cools back to ambient temperatures in less than a second, kinetically inhibiting reverse reactions [14]. Similarly, HCN is known to form on present-day Earth through impacting solar energetic particles. These break down N₂ into N and N(²D), an excited state of nitrogen, which then react with CH₃ radicals to form H₂CN, which then yields HCN [15]. These species are soluble and dissolve in condensed water, which then rains on to the surface.

The atmospheric production and oceanic concentrations of NO_x were estimated for plausible early-Earth conditions by Wong et al. [12]. This study considered lightning production and rainout fluxes to estimate the amount of nitrogen oxides raining out into the ocean, and then only used hydrothermal vent circulation to determine the loss of aqueous nitrogen oxides in the ocean. Adams et al. [16] improved upon the methods of Wong et al. [12] to estimate the concentrations of both nitrogen oxide and hydrogen cyanides on early Mars. The new reactions and processes included in Adams et al.’s [16] study showed that solar energetic particle (SEP) deposition dominates HCN formation in early terrestrial atmospheres, and that photoreduction dominates over hydrothermal circulation as a loss mechanism for aqueous NO_x, as first described by Ranjan et al. [17].

Here, we utilize the modeling framework of Adams et al. [16] and apply it to early Earth to update and expand upon the findings of Wong et al. [12]. We present state-of-the-art estimates of hydrogen cyanide and nitrogen oxides on early Earth to estimate the amount of fixed nitrogen available at the time of life’s emergence and early evolution. We also compute the abundance of carbon monoxide (CO), a molecule that could have added to conducive conditions for the formation of organic molecules and therefore life [18,19].

Additionally, this study is the first to consider the production of hydrogen cyanide on Earth through a formation pathway involving HCN₂ (Figure 2), motivated by Berry et al. [20]. These reactions were previously used in Krasnopolsky and Cruikshank [21] to model haze formation on Pluto, but to our knowledge they have never been used in a terrestrial context. To simulate a more accurate and complete atmosphere and estimate of hydrogen cyanide, we have added these reactions to improve estimates of HCN production on early habitable worlds.

2. Methods

Due to the vast uncertainty with regard to Hadean atmospheric composition, we modelled 12 different possible Hadean Earth atmospheres. We varied the total pressure between 1 bar, 2 bars, and 12 bars, with background compositions of 90% N₂, 10% CO₂ in the 1-bar case, 1 bar N₂ and 1 bar CO₂ in the 2-bar case, and 10 bars N₂ and 2 bars CO₂ in the 12-bar case. We varied the amounts of H₂ and CH₄ in each atmosphere, allowing the surface mixing ratio of each gas to take values of 0.1%, 0.3%, 1%, or 3%. To form HCN and NO_x in an N₂/CO₂-dominated atmosphere, an energy source is required. We took into account three sources of energy: lightning, SEPs, and photochemistry.

2.1. Thermochemical and Energetic Particle Deposition Modeling

We used the Chemical Equilibrium with Applications (CEA) model [22] to calculate the effect of lightning and its thermal energy (3000 K) and pressure on each of the 12 atmospheric cases. Thermodynamic equilibrium at this immense heat and pressure favors the creation of HCN and NO. The CEA output for HCN and NO was used as input fluxes for the lower boundary conditions in our photochemical model.

SEPs are energetic protons and electrons streaming from the Sun that impact planetary atmospheres. They deposit their energy in the atmosphere and are energetic enough to split the strong triple bond of N₂. We used the Geant4 simulation platform to run new energy deposition profiles for the atmosphere of early Earth in each of the cases described above. Similar to Adams et al. [16], we assumed a coronal mass ejection (CME) frequency of 1 event per day, as the early Sun would have been much more magnetically active than today, with each event having an average differential energy flux similar to that of the 29 October 2003 CME event [23]. The Geant4 simulation platform computes the flux profiles of N and N(²D) produced by SEPs deposited at each altitude layer on early Earth by considering the total SEP energy flux divided by the N₂ bond-dissociation energy and finally multiplied by the concentration of N₂ at that altitude. These profiles of reactive N atoms are then included in our photochemical model.

2.2. Photochemical Modeling

After computing the lightning-induced fluxes for NO and HCN using CEA and the N and N(²D) profiles produced by SEP deposition, we modelled how these species react to produce HNO_x and HCN. To do this, we used the Caltech–JPL photochemistry–transport model KINETICS [24], which solves the continuity equation:

$$\frac{d{n}_i}{dt}=P_i-L_i-\frac{\partial {\mathsf{\phi }}_i}{dz},$$

where n_i is the number density of species i, φ is the vertical flux, P_i is the chemical production rate, and L_i is the chemical loss rate evaluated at time t and altitude z. The vertical flux is given by the following:

$${\phi }_i=-\frac{d{n}_i}{dt}(D_i+K_{zz})-n_i(\frac{D_i}{H_i}+\frac{K_{zz}}{H_{\mathrm{a}\mathrm{t}\mathrm{m}}})-n_i\frac{\partial \mathrm{T}}{dz}[\frac{(1+{\mathsf{\alpha }}_i)D_i+K_{zz}}{dT}],$$

where D_i is the species’ molecular diffusion coefficient, H_i is the species’ scale height, H_atm is the atmospheric scale height, ${\mathsf{\alpha }}_i$ is the thermal diffusion coefficient, K_zz is the vertical eddy diffusion coefficient, and T is the temperature [25].

Our model considers 495 chemical reactions. We varied the H₂ and CH₄ content of the atmosphere, and for each case, the lightning-induced fluxes of the reduced gases computed by CEA were used as lower boundary conditions for NO and HCN in KINETICS. The complete list of boundary conditions in our model is presented in

Table 1. The boundary conditions used in this study. If a range is shown, that value varies between atmospheric cases. If the number is in bold, it represents a deposition velocity (cm s⁻¹); standard text, a fixed flux (molecules cm⁻² s⁻¹); and italicized, a fixed mixing ratio. The HCN and NO_x ranges displayed are referenced from Figure 3.

Species	Lower Boundary	Upper Boundary
O	0	0
O2	−1.00 × 10−6	0
O3	−1.00 × 10−2	0
H2	1.00 × 10−3– 3.00 × 10−2	0
HOx	0	0
NxHy	−1.00 × 10−2	0
HNOx	−1.00 × 10−2	0
NxOy	0	0
CHx (x ≠ 4)	0	0
CH4	1.00 × 10−3– 3.00 × 10−2	0
CO	−1.00 × 10−6	0
N	0	−1.60 × 109
HxCyO	−1.00 × 10−2	0
NO	1.07 × 108– 1.49 × 108	0
HCN	3.43 × 100– 6.67 × 100	0
CxHy (y = even)	−1.00 × 10−3	0
CxHy (y = odd)	0	0
CN	−1.00 × 10−2	0
CxN	−1.00 × 10−2	0
HxCyNz	−1.00 × 10−2	0

.

KINETICS then computes the change of species concentration over time, considering both chemical production/loss terms and transport. The model runs for upwards of 10⁸ simulated years to ensure the concentrations converge to a steady state. The outputs of the chemical reactions over time were graphed for analysis (see Section 3).

2.3. New Reactions Involving HCN₂

To create a more realistic and holistic atmospheric reaction network, three reactions were added to the KINETICS database:

CH + N₂ + M → HCN₂ + M (R1)

H + HCN₂ → HCN + NH (R2)

H + HCN₂ → CH₂ + N₂ (R3)

with k₁ = 10⁻³⁰ cm⁶ molecule⁻² s⁻¹, k₂ = 10⁻¹³ cm³ molecule⁻¹ s⁻¹, k₃ = 10⁻¹² cm³ molecule⁻¹ s⁻¹, respectively, and where M denotes background molecules that remove energy from the system. From the lab work of Berry et al. [20] and Trainer et al. [26], it was hypothesized that these reactions would increase the production of HCN in the Hadean Earth’s atmosphere. In this work, we added them to our chemical network and quantified their effect on HCN production (see Section 3.3).

2.4. Oceanic Concentrations

As the motivation behind this study is to understand how early life may have formed on Earth, we used the rainout rates of NO_x and HCN, calculated by KINETICS, to estimate oceanic concentrations of NO_x and HCN during the Hadean period. We computed equilibrium concentrations by balancing delivery from the atmosphere with loss due to photoreduction, hydrolysis, and hydrothermal vents, with an assumed Hadean ocean depth of 5.33 × 10⁵ km [27].

Two photoreduction reactions for NO_x were considered:

NO₃⁻ + hv = NO₂⁻ + ½ O₂ (R4)

NO₂⁻ + H₂O + hv = NO + OH + OH⁻ (R5)

with rate constants of k₄ = 2.3 × 10⁻⁸ s⁻¹ and k₅ = 1.2×10⁻⁶ s⁻¹, respectively. These reactions were chosen because they are well constrained and are good representations of the reaction pathway in the ocean [17]. Additionally, when considering loss of HCN within hydrothermal vents, we assumed a water mass of flux through high-temperature vents of 7.2 × 10¹² kg yr⁻¹. We also considered the loss of HCN through hydrolysis, as discussed by Miyakawa et al. [28]. Specifically, the reaction:

$$\mathrm{HCN}\stackrel{\mathrm{H}2\mathrm{O},k1\left(\mathrm{H}\mathrm{C}\mathrm{N}\right)}{\to }{\mathrm{HCONH}}_2\stackrel{\mathrm{H}2\mathrm{O},k1\left(\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{m}\mathrm{i}\mathrm{d}\mathrm{e}\right)}{\to }\mathrm{HCOOH}+{\mathrm{NH}}_3,$$

where k₁(HCN) and k₁(formamide) are derived from the laboratory hydrolysis rates from the study by Miyakawa et al. [28], which, when fit to an Arrhenius equation, finds a hydrolysis kinetic rate of 2.265 × 10⁻¹² molecules/cm²/s, corresponding to a temperature of 273 K. These loss processes for NO_x and HCN, combined with our production rates calculated via KINETICS, allowed us to solve for the equilibrium concentrations of these species in the Hadean ocean.

3. Results

3.1. Lightning-Induced HCN and NO

After running the CEA thermochemical model to calculate the creation of NO (the precursor to NO_x species) and HCN by lightning under different background atmospheric compositions, it was clear that NO and HCN respond inversely to increases in background H₂ and CH₄ surface mixing ratios (Figure 3). (Note that NO and HCN are on massively different scales of concentration within the atmosphere.) HCN increases as CH₄ increases, which is to be expected as HCN forms via reactions with CH₃, a photochemical product of CH₄. NO decreases as both CH₄ and H₂ increase, as both CH₄ and H₂ make the environment more chemically reduced, while NO is thermodynamically more favorable in more oxidized environments.

3.2. Photochemistry

As the photochemical model, KINETICS, runs to a steady state (for over a hundred million simulated years), it computes the amount of each species present in the atmosphere and as a function of altitude. In our 1-bar, 2-bar, and 12-bar atmospheres, we assumed surface temperatures of 280 K, 332 K, and 388 K, respectively, in accordance with the Hadean Earth general circulation model results of Wong et al. [12]. The lower atmospheric temperature profile follows the moist adiabatic lapse rate. Upon reaching a stratospheric temperature of 142.8 K, 139.6 K, or 169 K (again, corresponding to the 1-bar, 2-bar, and 12-bar cases), we set the temperature profile to an isotherm. The eddy diffusion parameter K_zz describes vertical transport within the atmosphere and is computed following the methods of Ackerman and Marley [29]. The H₂O profile is fixed to the saturation vapor pressure (Figure 4). The common occurrence of carbon monoxide buildup was also seen in this atmosphere (see Section 4.2).

To effectively compute the concentrations of HCN and NO_x, seven species are the focus throughout the discussion of the results: HCN, NO, HNO, HNO₂, HNO₃, N, and N(²D) (Figure 5). All of the NO_x and HNO_x species react with and affect the concentrations of each other (Figure 1 and Figure 2). Throughout the atmosphere, the concentrations of HNO, HNO₂, and HNO₃ are all similar, with HNO₃ having slightly lower concentrations than the other two. These species all dissolve in water droplets to rain out on to surface waters, and the equilibrium oceanic concentrations are directly proportional to the rainout rates. N and N(²D) vary slightly depending on the atmospheric pressure but are within a consistent range for all starting concentrations.

3.3. The Effect of Adding HCN₂ Reactions

We ran all atmospheric cases with and without the new HCN₂ reactions to compare the rainout rate of HCN between the atmospheres with and without these reactions and assess the importance of including HCN₂ pathways in photochemical modeling relevant to the origins of life.

Figure 6 displays the rainout rates, without and with the new reactions, for both NO_x (magenta) and HCN (green) per each pressure case. Interestingly, the HCN rainout rate rose by a factor of five in the 1-bar case and is about the same in the 2- and 12-bar cases when the HCN₂ reactions were added.

3.4. Oceanic Concentrations

Once NO_x species and HCN reach the Hadean ocean, they are subject to a number of loss processes. We determined their equilibrium concentrations via the procedure outlined in Section 2.4, and these are shown in Figure 7. An atmosphere with more H₂ and CH₄ results in a higher oceanic concentration HCN and a lower ocean concentration of NO_x. This is because N(²D) attacking methane acts as a bottleneck for HCN formation: CH₄ is the source of CH₃ radicals, which are essential for making H₂CN and eventually HCN (Figure 2). Additionally, CH₃ radicals will react with HO_x species, removing HO_x species needed to make NO_x (Figure 1). Therefore, more CH₄ results in less HO_x and less NO_x. Our sensitivity studies showed that increasing H₂ alone, on the other hand, does not contribute to CH₃ radicals, and thus is less impactful on the HCN and NO_x concentrations.

4. Discussion

4.1. The Relevance of NO_x and HCN

We presented updated estimates of NO_x and new estimates of photochemically derived oceanic HCN on early Earth, including the effects of SEP events on both NO_x and HCN and the photoreduction of NO_x in aqueous solution. Additionally, the new HCN₂ reactions increase the overall amount of HCN in the atmosphere, and hence the HCN rainout into the early Earth’s ocean. This was expected, as these reactions provide an additional pathway of HCN formation, increasing the rainout rate of HCN by a factor of five in the 1-bar case. These results provide an update to the previous results of Wong et al. [12] and Adams et al. [16], and they are compatible with the current scientific narrative of HCN and NO_x being created in the troposphere before raining out and being accessible to any potential life in the ocean.

Our model considered a CO₂- and N₂-dominated Hadean atmosphere. Other studies have investigated the nature of the Hadean atmosphere post-giant impact, which causes the atmosphere to take on a much more reduced state. In this post-impact state, Wogan et al. [30] found a slower deposition rate (<1 × 10⁵ HCN/cm²/s) than our results for low-CH₄ (<10%) cases, and Wogan et al. [30] and Pearce et al. [31] found the in high-CH₄ cases, the HCN production and deposition rates were much larger.

Oceanic concentrations of HCN and NO_x are of astrobiological interest because they are possible sources of fixed nitrogen—a necessity for the formation, maintenance, and evolution of early life. For HCN to be helpful in protein synthesis, local concentrations of at least 0.01 M are required [32]. Our estimated concentrations of NO_x and HCN on early Earth may be helpful to future lab work to determine the habitability, genesity, and urability of the Hadean environment [33,34].

4.2. CO Runaway

Our models reproduce the previously observed [35] “carbon monoxide (CO) runaway” (Figure 8). Because of the relatively large amount of carbon dioxide present in the Hadean atmosphere, ultraviolet light can photolyze the carbon dioxide, forming atomic oxygen and carbon monoxide. If very little to no water is present at the relevant altitudes, then HO_x, the main family of compounds that reacts with carbon monoxide to cycle it back to carbon dioxide, is unable to react with enough CO to eliminate the runaway via the following process:

CO + OH → CO₂ + H (R6)

H + O₂ + M → HO₂ + M (R7)

O + HO₂ → O₂ + OH (R8)

Net: CO + O → CO₂

As a result, carbon monoxide can accumulate in the atmosphere over time and dominate the composition of the atmosphere. However, after analyzing the model output, it became clear that the lower layer of the atmosphere was not nearly as dominated by CO as the top. This is expected, as the bottom of the atmosphere, where rain forms, has much more water vapor than at the top of the atmosphere.

Based on our modeling, it was possible that an upper-atmosphere CO runaway would be present during the Hadean eon. Although CO does not necessarily build up to a “runaway” state in the lower atmosphere, a significant fraction (over 10%) of the atmosphere is CO in the photochemical steady state across all of our atmospheric cases (Figure 8). This could have profound implications on the creation of organic compounds via CO-related species and pathways [18,19]. Our results motivate further investigation of organic synthesis under high-CO conditions.

We acknowledge that the photolysis rate of water (i.e., production rate of HO_x) is not well constrained (e.g., two largely conflicting rates are presented in Ranjan et al. [36] and Burkholder et al. [37]). Future laboratory work could re-examine H₂O photolysis rates and compare their results and photolysis rates in practice, allowing the photolysis rates to be more constrained.

4.3. Other Considerations

In the future, adding HCN reactions with iron in surface waters, as a loss mechanism to HCN, would add more accuracy to the survival rates and thereby equilibrium concentrations of HCN. Additionally, lab work performed on the NO_x reaction rates with iron would also add immensely to the accuracy of the survival rates of NO_x, as those rates are currently poorly constrained [17].

Further research can build upon the biological context for these new estimates by adding more to our understanding of metabolisms involving nitrate. It is likely that NO_x is important for microbiological metabolisms and the origin of life, but it is unknown how much NO_x is needed for this to be important. Recent work [38] has shown that the precursor molecules pyruvate and nitrate can be converted into the amino acid glutamine via reductive amination on ferroan brucite, a common mineral in hydrothermal vent systems, but these system effects have not been analyzed as a full system. Clarification of these reactions and processes would help determine the potential for the emergence of life in these oceanic environments.

In conclusion, the additional HCN₂ reactions (R1–R3) and updated atmospheric processes added here improve upon HCN and NO_x rainout estimates from that of past research. The common “problem” of a CO runaway is present in our model, but only dominates the upper, thinner part of the atmosphere. A high-CO lower atmosphere may be of interest to future work on prebiotic chemistry. Further lab work on this as well as widening information on chemical networks and microbial metabolisms could help us eventually reach the goal of understanding the roots of life on early Earth and, potentially, early Mars and Earth-like exoplanets.

5. Conclusions

Understanding the processes of nitrogen fixation remains one of the keys to understanding the emergence of life on Earth and other planets. We estimated Hadean NO and HCN lightning-induced fluxes of approximately 10⁸ and a few to ~70 cm⁻²s⁻¹, respectively, by computing the thermochemical equilibrium with the CEA model. We computed Hadean N and N(²D) flux profiles based on solar energetic particle events using the Geant4 simulation platform. These were used as inputs for our photochemical model, the Caltech–JPL model KINETICS, which we used to derive the Hadean precipitation rates of HNO_x and HCN (on the order of 10⁸ molecules/cm²/s and 3 × 10⁵–1 × 10⁷ molecules/cm²/s, respectively). With the assumed losses to photodestruction and hydrothermal vent circulation, we found concentrations of nitrate of about 1 × 10⁻⁹–6 × 10⁻⁹ M and hydrogen cyanide of about 1 × 10⁻²–1 × 10⁻⁴ M. We suggest more laboratory research to fully understand the astrobiological implications of these concentrations.