Abiotic Nitrogen Fixation on Terrestrial Planets: Reduction of NO to Ammonia by FeS

Abstract

Understanding the abiotic fixation of nitrogen and how such fixation can be a supply of prebiotic nitrogen is critical for understanding both the planetary evolution of, and the potential origin of life on, terrestrial planets. As nitrogen is a biochemically essential element, sources of biochemically accessible nitrogen, especially reduced nitrogen, are critical to prebiotic chemistry and the origin of life. Loss of atmospheric nitrogen can result in loss of the ability to sustain liquid water on a planetary surface, which would impact planetary habitability and hydrological processes that shape the surface. It is known that NO can be photochemically converted through a chain of reactions to form nitrate and nitrite, which can be subsequently reduced to ammonia. Here, we show that NO can also be directly reduced, by FeS, to ammonia. In addition to removing nitrogen from the atmosphere, this reaction is particularly important as a source of reduced nitrogen on an early terrestrial planet. By converting NO directly to ammonia in a single step, ammonia is formed with a higher product yield (∼50%) than would be possible through the formation of nitrate/nitrite and subsequent conversion to ammonia. In conjunction with the reduction of NO, there is also a catalytic disproportionation at the mineral surface that converts NO to NO₂ and N₂O. The NO₂ is then converted to ammonia, while the N₂O is released back in the gas phase, which provides an abiotic source of nitrous oxide. Key Words: Nitrogen fixation—Mars—Earth—Terrestrial—Abiotic—Prebiotic—Nitrogen—Planetary habitability—Biosignatures. Astrobiology 12, 107–114.

1. Introduction

N itrogen is an essential element for life as we know it, and without it, key compounds in biochemistry, such a proteins, RNA, and DNA, would not be possible. For the origin and early evolution of life, an abiotic source of biochemically accessible nitrogen, especially reduced nitrogen, is necessary. Also, the abiotic fixation of nitrogen on terrestrial planets can have an enormous impact on the habitability and evolution of a planet. For example, loss of atmospheric nitrogen can lower atmospheric pressure and leave a planet unable to support liquid water. This leaves the surface of the planet unable to support life, and a lack of a hydrological cycle has an important effect on how the surface is, or is not, reshaped. The importance of understanding the nitrogen chemistry of terrestrial planets is clear.

For example, the current martian atmosphere contains only 0.2 mbar of nitrogen, while early Mars is thought to have had 10–1000 times the current pressure (McKay and Stoker, 1989), which would have been enough to support the presence of liquid water on the surface (Pieri, 1980; Carr, 1981; Squyres, 1984; Christiansen, 1989; Mancinelli and Banin, 2003; Bell et al., 2004; Christensen et al., 2004; Herkenhoff et al., 2004; Klingelhöfer et al., 2004; Squyres et al., 2004a, 2004b). Two possible outcomes for the fate of nitrogen are sequestration in the regolith and ejection into space by impactors (which is known as impact erosion; see Chyba, 1990; Mancinelli, 1996; Mancinelli and Banin, 2003; Manning et al., 2009). For example, pathways that allow nitrogen to be converted to ammonia could have caused nitrogen to become sequestered in clay minerals (Bada and Miller, 1968; Mancinelli et al., 1998; Bishop et al., 2002). Such formation of reduced ammonium would also have provided a source of nitrogen for prebiotic chemistry and for the formation of life. To understand early Mars, the potential for prebiotic chemistry and life on Mars, and when it was that Mars may have had the capacity to sustain life, a critical question must be considered: What happened to Mars' nitrogen?

Nitrogen fixation by atmospheric shock heating and subsequent chemistry is one important route for nitrogen fixation. On terrestrial planets with a nonreducing atmosphere (Mattioli and Wood, 1986; Wood and Vigo, 1989; Delano, 2001), that is, a CO₂/N₂ atmosphere with little or no CO and H₂, shock heating leads to NO formation (Chameides, 1979; Yung and McElroy, 1979; Chameides and Walker, 1981; Miller and Schlesinger, 1983; Borucki and Chameides, 1984; Fegley et al., 1986; Kasting, 1990; Zahnle, 1990; Navarro-González et al., 1998, 2001; Nna Mvondo et al., 2001; Miyakawa et al., 2002). Thus, chemical routes that fix NO into forms that are chemically accessible for the origin of life or into forms that become sequestered into the crust become important. It is known that NO can become photochemically converted to nitrate and nitrite, which can subsequently be reduced to ammonia (Summers and Chang, 1993; Summers and Lerner, 1998; Summers and Khare, 2007). Since both nitrate and nitrite can be reduced by iron(II), in the form of FeS, we became interested in whether FeS could lead to the direct reduction of NO to ammonia. The direct reduction of NO could be the most direct route to convert NO to reduced nitrogen on Mars, early Earth, and other terrestrial planets.

2. Materials and Methods

2.1. Apparatus and experimental method

The flask used in the experiment is shown in Fig. 1. FeS (50–400 mg/mL, Sigma-Aldrich -100 mesh, 99.9%) was added to the flask under nitrogen. Then nitrogen-purged high-purity water (Millipore, MilliQ Gradient reverse osmosis system), with added sodium bicarbonate to establish the CO₂/bicarbonate buffer needed to establish the desired pH (0–5 g), was added. The aqueous phase was freeze-pump-thaw degassed four times. The vessel was then filled from a tank containing the gas mixture (CO₂/N₂, 20%/80%, with 1% NO and CO) to a pressure of ∼760 torr, and the system was allowed to react. Additional experiments, in which FeS was added to a flask where the aqueous phase had already been deoxygenated and the nitrogen purge gas replaced with the reaction gas mixture, showed that the order of addition did not affect results. Experiments were conducted in either a 350 mL vessel with 10–20 mL of water or a 40 mL vessel with 1 mL of water (particularly with experiments with isotopically labeled water). For experiments in which pH was studied, samples from the same batch of FeS were used.

FIG. 1.

Diagram of experimental apparatus.

2.2. Product analysis

Gas-phase samples were backfilled into an IR gas cell and measured by Fourier transform infrared (FTIR) spectroscopy. All FTIR spectra were corrected for the pressure change. Analysis of ammonia concentrations was performed on the solution after reaction was over (2–3 h). Analyses were usually by colorimetric methods (Verdouw et al., 1978; Summers, 2005), but some analyses were also done by ion chromatography. Analysis by ion chromatography was done at ambient temperature on a Dionex DX-100 instrument with a Dionex CS-12 column (and a Dionex CG-12 guard column) and 20 mM methane sulfonic acid as the eluent. In experiments analyzed by ion chromatography, all sodium salts were replaced with potassium salts to prevent interference with ammonium detection. Samples were filtered through a 0.2 μm filter prior to injection. Since ammonia and ammonium are in rapid equilibrium when in aqueous solution, the distinction between the two species becomes complicated. We use “ammonia” to refer to the total concentration of ammonia as both NH₃ and \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${ \rm NH}_4^{\,\,+}$$ \end{document} .

2.3. Isotopic labeling experiments

To verify product peak identification in the IR spectra and elucidate the reactions and mechanisms taking place, isotopic labeling experiments were conducted. These were carried out as described above (using the 40 mL flask), substituting a reactant or solvent enriched in a particular isotope (relative to natural abundance). For example, isotopically labeling the water might involve use of water with oxygen atoms that are 100% ¹⁸O, instead of the 0.2% natural abundance. If a reaction is run in such water and a product is observed to be isotopically enriched, then the enriched atoms on the solvent molecules would have been the source of those atoms on the product. Conversely, lack of enrichment of the product would show that those atoms did not come from the enriched atoms in the water.

Isotopic substitution can also verify that vibrational peaks are due to the assigned molecules (and vibrations therein). A bond using atoms of a heavier isotope will vibrate at a different frequency and show a different peak in the IR spectrum, compared to the same bond using atoms of a lighter isotope. Changing the relative isotopic abundances present in the molecules in that sample will change the relative intensity of the different peaks for the different isotopes engaged in that bond. Thus, we can correlate the observed peaks with the atoms and bonds being affected.

Reactions were conducted where the water was enriched in ¹⁸O and ²D, where bicarbonate/carbon dioxide was enriched in ¹³C, and where NO was enriched in ¹⁸O. No difference was observed with ¹³C or ²D labeling. NO did not exchange with H₂O over the timescale, ∼1.5 h, of these experiments (which can be verified by the lack of increased intensity for N¹⁸O peaks in the IR spectra), so transfer of label between NO and the water did not occur during these experiments. Conversely, in experiments where N¹⁸O was used, the isotopically labeled gas mixture was produced by exposing NO to \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${ \rm H}_2^{\;18}{ \rm O}$$ \end{document} for 72 h to produce 50% labeled N¹⁸O (as determined by IR spectroscopy). The formation of a N¹⁸O peak verified the exchange of ¹⁸O and formation of N¹⁸O. The resulting gas was used to run a reaction over natural abundance water. Unfortunately, it was not possible to cleanly subtract this gas mixture from the FTIR spectra after this reaction, so that peaks covered by water could not be followed in those experiments.

3. Results

3.1. Formation of ammonia and yield

The products of shock heating, NO and CO, were exposed to an aqueous suspension of FeS under a CO₂ and N₂ atmosphere. The formation of ammonium in the aqueous phase was observed (see experimental section). For example, in one experiment with 95 mg/mL of FeS suspended in water at pH 7.3, 64% of the NO that reacted was converted to ammonia. NO was consumed at a rate of 5×10⁻⁸ mol g⁻¹ s⁻¹, and ammonium was correspondingly formed at a rate of 3×10⁻⁸ mol g⁻¹ s⁻¹. The formation of ammonia from NO reduction was also measured by ion chromatography. After reaction of NO with 57 mg/mL of FeS at pH∼6.6, ammonia was detected with a yield of 40%. If no FeS is present, formation of ammonium is not detected.

In Table 1, the yield of ammonia over a series of experiments (with 185 mg/mL FeS in each experiment) is shown. The highest yields occur near neutral pH, and the yield declines as conditions become more acidic or alkaline (particularly in the alkaline direction).

Table 1.

Product Yield for the Conversion of Nitric Oxide to Ammonia

pH	NO→NH₃ conversion	Amount of FeS (mg/mL)
∼6.6^a	40%	57
7.3	64%	64
6.3	23%	185^b
6.8	32%	185^b
7.3	47%	185^b
7.8	11%	185^b
8.3	6%	185^b

Reaction time, 2–3 h.

pH is approximate because no bicarbonate was added; the pH was buffered only by FeS.

These experiments were all performed using FeS from the same batch.

3.2. Gas phase products

Figure 2A shows the changes that occurred in the gas phase during this reaction. (Peaks due to water have been mathematically subtracted, except where noted.) One interesting observation in many experiments is the formation and subsequent disappearance of peaks due to NO₂ (at 1602 and 1629 cm⁻¹).

FIG. 2.

FTIR absorption spectra of gas phases (1% NO, 1% CO, 19% CO₂, 79% N₂, 0.04 L) that had been allowed to react with a suspension (1 mL) of FeS in water (370 mg/mL). (A) Normal absorbance spectrum. (B) Difference spectrum. All spectra are corrected for the pressure change upon transfer to the IR cell. The spectrum of water vapor has been digitally subtracted.

There are peaks at 2235, 2210, 1270, and 1300 cm⁻¹. The peaks at 2235 and 2210 cm⁻¹ are hard to see because they are right next to the large CO₂ peak, but they become clearer in the difference spectra (Fig. 2B). In a difference spectrum, the initial gas spectrum is subtracted from the spectrum obtained after reaction, showing only the changes in absorbance upon reaction. Species that were produced are seen as positive peaks, and those that were consumed as negative peaks. These peaks are due to the formation of N₂O. There is no evidence that CO was consumed or produced in the reaction. The small positive and negative changes in absorbance that can occasionally be seen can be attributed to small errors in subtraction.

In the pH experiments that produced the data for Table 1, the yield of N₂O was independent of pH (within experimental error). In these experiments, the yield was ∼35% (based on N). Without FeS, no reaction was observed, so all gaseous species must have been formed by reactions with, or at, the FeS in the aqueous phase.

3.3. Isotopic labeling

The reaction was also studied with ¹⁸O isotopically labeled water and NO. Figure 3 shows the difference spectrum obtained when the reaction was run using \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${ \rm H}_2^{\,\,18}{ \rm O}$$ \end{document} . No peaks due to the formation of N₂ ¹⁸O are observed. Indeed, for labeled N₂ ¹⁸O, we would expect significant peaks due to the ¹⁴N-¹⁸O vibrations (at 1235 and 1265 cm⁻¹, see below). More interestingly, there are also no peaks due to the formation of N¹⁸O¹⁶O. On the other hand, when a reaction was run with N¹⁸O (Figs. 4 and 5), the formation of \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${ \rm N}_2^{\,\,18}{ \rm O}$$ \end{document} can be clearly seen by the presence of lower frequency N-O vibrations at 1235 and 1265 cm⁻¹ (see Fig. 4). (For unknown reasons, it was not possible to cleanly subtract the mixture of \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${ \rm H}_2^{\,\,16}{ \rm O}$$ \end{document} and \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${ \rm H}_2^{18}{ \rm O}$$ \end{document} peaks from the spectrum. NO₂ was not observed in these experiments, at least in sufficient quantities to be reliably observed.) The peaks at 2235 and 2210 cm⁻¹, which in N₂O should be primarily due N-N vibrations but will have a little N-O character, show a smaller shift (Fig. 5). When reactions were run in D₂O or with ¹³CO₂/NaH¹³CO₃, no significant differences are observed in peak positions of any of the species discussed above.

FIG. 3.

FTIR difference spectra of gas phases (1% NO, 1% CO, 19% CO₂, 79% N₂, 0.04 L) that had been allowed to react with a suspension of FeS in ¹⁸O water (0.5 mL, 100% \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${ \rm H}_2^{\,\,18}{ \rm O}$$ \end{document} ), 370 mg/mL. All spectra are corrected for the pressure change upon transfer to the IR cell. The spectrum of water vapor has been digitally subtracted.

FIG. 4.

FTIR difference spectra of a gas phase (1% NO [50% ¹⁸O], 1% CO, 19% CO₂, 79% N₂, 0.04 L) that had been allowed to react with a suspension of FeS in water (1 mL), 370 mg/mL. All spectra are corrected for the pressure change upon transfer to the IR cell. The spectrum of water vapor has been digitally subtracted. Color images available online at www.liebertonline.com/ast

FIG. 5.

FTIR difference spectra of gas phases (1% NO, 1% CO, 19% CO₂, 79% N₂, 0.04 L) that had been allowed to react with a suspension of FeS in water (0.5–1 mL), 370 mg/mL. Experiment used \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${ \rm NO} / { \rm H}_2^{\,\,18}{ \rm O}$$ \end{document} , NO/H₂O, and N¹⁸O/H₂O as indicated. All spectrum are corrected for the pressure change upon transfer to the IR cell. The spectrum of water vapor has been digitally subtracted. Color images available online at www.liebertonline.com/ast

4. Discussion

A consideration of the gaseous products and the isotope labeling experiments informs about the chemistry that is occurring in conjunction with the formation of ammonia. The transient nature of NO₂ formation is not surprising. Nitrogen dioxide is reactive with water (to form nitrate/nitrite; see the work of Gmelin, 1936; Lee and Schwartz, 1981), and it would be expected to be destroyed as it is formed (if it is not reduced to ammonia by FeS). Thus, it ends up being another intermediate species in the reduction of NO to ammonia. Since reaction of NO and, hence, formation of any products from it (such as NO₂) is not observed in the absence of FeS (Summers and Khare, 2007), NO₂ must be produced by FeS in an aqueous phase. Clearly, however, under the right conditions, some NO₂ can escape up into the gas phase. It is quite possible, given similarities to other nitrogen oxides, that NO₂ is reduced by FeS to ammonia. However, it is known that, if it is not directly reduced by FeS, it will be converted to nitrite/nitrate (Gmelin, 1936; Lee and Schwartz, 1981), which will, under these conditions, be reduced by FeS (Summers, 2005). Whether it is directly reduced to ammonia by FeS or converted to nitrate/nitrite and then reduced is not clear (and may be a somewhat academic distinction).

No incorporation of oxygen from \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${ \rm H}_2^{\,\,18}{ \rm O}$$ \end{document} , into either N₂O or NO₂, was observed. Similarly, use of N¹⁸O to form \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${ \rm N}_2^{\,\,18}{ \rm O}$$ \end{document} showed that ¹⁸O atoms from NO were not exchanged with O from water, over the timescale of the reaction, in the formation of N₂O. If such exchange were occurring, the ¹⁸O atoms would have been lost into the water (which was present in much greater quantities than NO and, as we have just mentioned, was not a source of O for N₂O) and thus would not have been incorporated significantly into NO₂. In spite of the aqueous environment, the second oxygen on NO₂ did not come from H₂O. We can also rule out incorporation from CO₂. CO₂ rapidly exchanges with water (through carbonic acid), so if the water is 50% enriched in ¹⁸O, then any carbonic acid and CO₂ present must also be similarly enriched. No reaction of CO was seen to account for the oxygen. Thus, the second oxygen in the formation of NO₂ came from NO. This implies that both N₂O and NO₂ were produced solely from NO by a disproportionation reaction (see Eq. 1). \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}3{ \rm NO} \rightarrow { \rm N}_2{ \rm O} + { \rm N}{ \rm O}_2 \tag{1}\end{align*} \end{document}

Since there was no exchange with water during the reaction, it seems highly unlikely that the formation of NO₂ proceeded by the cleavage of the N-O bond and formation of a surface bound oxygen species (which would be expected to readily exchange with water). This makes the formation of a NO dimer and its reaction with a third NO the most likely mechanism. \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}{ \rm ON} \hbox{- }{ \rm NO} + { \rm NO} \rightarrow { \rm N}_2{ \rm O} + { \rm N}{ \rm O}_2 \tag{2}\end{align*} \end{document}

Since N₂O is not formed in sufficient amounts to account, by Eq. 1, for all the NO that reacts, it seems unlikely that ammonia formation could be going entirely by the formation of NO₂ (and its subsequent conversion to ammonia). Similarly, it seems unlikely that FeS, which will bind and reduce nitrate and nitrite, would also bind NO (well enough to catalyze its disproportionation) but would not reduce it. For that reason, we believe that both the direct reduction of NO to ammonia and also the formation of NO₂ (and subsequent conversion to ammonia) occurs.

The pH dependence is similar to the reduction of nitrate/nitrite by FeS (Summers, 2005), which also decreases as the pH rises (probably due to the interference from bicarbonate ions). However, in this case reduction of NO shows a decrease in yield in acidic solutions not seen with nitrate/nitrite.

If we add this route to the already known fixation processes leading from NO, we can construct the following schematic of abiotic nitrogen cycling from atmospheric processes on terrestrial planets (Fig. 6).

FIG. 6.

Simplified schematic of abiotic nitrogen fixation pathways proceeding from the formation of NO. Color images available online at www.liebertonline.com/ast

5. Conclusions

It has been previously shown that NO could be photochemically converted to nitrate and nitrite by two pathways, one through the gas-phase formation of HNO and the other by the gas-phase formation of NO₂ (Summers and Khare, 2007). These results show that another pathway for the abiotic fixation of nitrogen on terrestrial planets exists, the reduction of NO to ammonia by FeS. This pathway probably represents the most efficient route from NO to reduced nitrogen, which makes it of particular prebiotic importance. Such fixation can result in sequestration of nitrogen in the crust. Ammonium would be quite stable sequestered in clays (Bada and Miller, 1968; Mancinelli et al., 1998; Bishop et al., 2002).

On young planets, about the age at which life originated on Earth, it is thought that the amount of continental crust is limited, which leads to a picture of a world with high levels of island volcanism and other types of volcanoes in close proximity to marine/lacustrian environments. It is also interesting to note that, in such environments, hydrothermal activity that produces FeS is possible in close proximity to NO generation by volcanically generated lightning (Navarro-González et al., 1998). Since this pathway does not involve a photochemical step, this means that the NO generated would only need to be transported downward into aqueous phases that contain FeS in order to react. The close proximity of sources of NO and FeS makes this an interesting environment for the formation of prebiotic reduced nitrogen (as illustrated in Supplementary Fig. S1, available online at www.liebertonline.com/ast).

FeS also catalyzes the disproportionation of NO into N₂O and NO₂. The biogenic formation of N₂O has been proposed as a signature of biological activity in the atmospheres of extrasolar planets (Segura et al., 2005). However, this brings up the issue as to which abiotic sources of N₂O might exist (for example, see; Nna-Mvondo et al., 2005; Summers and Khare, 2007). This chemistry would be an abiotic source of N₂O and thus has potential implications for the use of nitrous oxide as a biosignature.

Footnotes

Acknowledgments

The authors would like to thank the NASA Astrobiology Institute (NNA04CC05A) and Exobiology Program for support.

Author Disclosure Statement

No competing financial interests exist.

Abbreviation

FTIR, Fourier transform infrared.

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