Meteorite Impact-Induced Rapid NH3 Production on Early Earth: Ab Initio Molecular Dynamics Simulation

NH3 is an essential molecule as a nitrogen source for prebiotic amino acid syntheses such as the Strecker reaction. Previous shock experiments demonstrated that meteorite impacts on ancient oceans would have provided a considerable amount of NH3 from atmospheric N2 and oceanic H2O through reduction by meteoritic iron. However, specific production mechanisms remain unclear, and impact velocities employed in the experiments were substantially lower than typical impact velocities of meteorites on the early Earth. Here, to investigate the issues from the atomistic viewpoint, we performed multi-scale shock technique-based ab initio molecular dynamics simulations. The results revealed a rapid production of NH3 within several picoseconds after the shock, indicating that shocks with greater impact velocities would provide further increase in the yield of NH3. Meanwhile, the picosecond-order production makes one expect that the important nitrogen source precursors of amino acids were obtained immediately after the impact. It was also observed that the reduction of N2 proceeded according to an associative mechanism, rather than a dissociative mechanism as in the Haber-Bosch process.


Results
From previous simulation studies [5][6][7] , time scale of shock compression is assumed to be 10 ps. In our simulation (5 km/s shock-wave simulation), three NH3 molecules were produced on the surface of Fe36 slab during 4 ps, i.e., about 0.21 NH3 molecules are produced per Fe atom during 10 ps. The estimated amount of meteorite accretion 4 × 10 24 g during 4.4 to 3.8 billion years ago 2 is also used. Assuming that the whole accretion occurred by ordinary chondrites contained 10 wt.% iron 3 , Fe atoms of about 1.2×10 13 mol were annually supplied to the early Earth during 4.4 to 3.8 billion years ago. We therefore consider that NH3 of about 2.5×10 12 mol yr -1 (or 4.3×10 7 tons yr -1 ) were produced. It should be noted that the amount is larger than 1.08×10 7 tons yr -1 estimated from the experimental result by Nakazawa et al. 1 , which corresponds to our lower shockenergy simulation (4 km/s shock-wave simulation). This implies that shocks with greater impact velocities would provide further increase in the yield if all of them can survive after quenching.
If we assume that the volume of early sea is 10 21 L 8 and all of the NH3 produced during shock compression (about 2.5 × 10 12 mol yr -1 ) are dissolved into the sea, the concentration of NH3 in the early sea can be estimated as about 2.5×10 -8 mol/L, which is much smaller than that in other proposed mechanisms such as reductions of NO 2− and NO 3− by oceanic Fe 2+ and of crustal N2 on the mineral surfaces around submarine hydrothermal systems (7×10 -5 -8×10 -7 mol/L) 9 . However, we emphasize that meteorite impacts would form locally NH3-enriched areas in the early ocean, and the produced NH3 S5 would be directly involved in the subsequent amino acid production reaction, taking account of the MSST-AIMD results by Goldman et al. 10 Thus, the discussion above of the average concentration on the early ocean may not be relevant in the Earth-science context.  At 0.242 ps, N4 bonds to Fe5 (ON4-Fe5(t) shows ~0.5), and this leads to weakening the strength of N3-N4 bond (ON3-N4(t) shows ~1.25), and QN3(t) and QN4(t) have negative S7 charges. At the same time, N3 begins to interact with H8 as ON3-H8(t) increases gradually.

Second Production Process of an NH3 molecule
By the increase in density of H2O molecules due to shock compression, N3 also begins to interact with H9 and H10 at 0.266 ps. An NH3-N molecule consisting of N3, H8, H9, and H10 is formed at 0.295 ps. While N3 forms bonds with the H atoms, N4-Fe5 bond is strengthened as ON4-Fe5(t) reaches ~0.9, accompanied by the increase in positive charge for QFe5(t). In contrast, since ON3-N4(t) decreases to ~0.75, the N3-N4 bond strength is weakened.
However, H10 does not form a stable bond with N4. Thus, ON4-H10(t) begins to decrease after 0.295 step, while ON3-H8(t) and ON3-H9(t) continue to increase. Instead, N4 begins to interact with H11 at around 0.315 ps. ON4-H11(t) increases to ~0.8 at 0.322 ps. On the other hand, ON4-Fe5(t) rapidly decreases to ~0.5. Also, ON3-N4(t) slightly increases to ~0.85. Figure S3 shows the second dissociation reaction of N-N bond observed in 5 km/s shock-wave simulation. The time evolution of the atomic configuration is shown in Figure   S3(a), where N3, N4, H9, H10, and Fe5 are the same atoms as those in Figure S2 Subsequently, H15 is supplied to N3 from an OH fragment on the Fe slab at around 2.142 ps, which indicates that an NH4 + is formed.  Figure S4 shows the production reaction of a hydrazinium (N2H5 + ) observed in 5 km/s shock-wave simulation. The time evolution of the atomic configuration is shown in Figure   S4(a). Figures S4(b) and S4(c) show the time evolution of Oij(t) and Qi(t) for specified atoms. At 1.876 ps, a N2 molecule consisting of N5 and N6 exists in water. QN5(t) and QN6(t) have nearly neutral charges, and ON5-N6(t) has ~1.5 which indicates the strength of a triple bond. As ON5-H16(t) has a positive finite value, N5 has formed a hydrogen bond with H16 of the neighbor H2O molecule up to 1.88 ps. This bond formation makes QN5(t) slightly more negative, and leads to the hydrogen bond formation between N5-H17 at about 1.94 ps. Subsequently, H17 forms a covalent bond with N5 as ON5-H17(t) rapidly increases after 1.953 ps. It should be noted that the used electron to form the covalent bond is supplied from Fe atoms. Figure S5 shows the formation process of a covalent-bond H17-N5. H22 is one of the dissociated H atoms from a H2O molecule existing on the Fe slab surface (see the S11 number of H-Fe bonds shown in Figure 2(f) of the main text). In contrast to the charges of H atoms in H2O molecules (e.g. QH17(t)), QH22(t) has much more negative charge at 1.888 ps because it receives an electron from Fe atoms. In addition, two H2O molecules exist between H22 and the N2 molecule. Along with the transport of H17 to O1 at 1.931 ps, the interatomic distances between H22-O1, H23-O2, and N5-H17 are shortened and the hydrogen bonds are formed. As a result, since the electron of H22 as well as H17

S10
itself are transferred to N5 via the hydrogen bond network, the covalent bond between N5 and H17 is formed at 1.953 ps. It is considered that this reaction is similar to the proton-coupled electron transfer (PCET) mechanism 11,12 .
Such electron and H-atom transfers via the hydrogen bond networks provide the consecutive hydrogenation of the N2 molecule. At around 1.972 ps, H18 has a covalent interaction with N6, and this gives rise to weakening N5-N6 bond. ON5-N6(t) has ~1.0 which indicates that the bond becomes a double bond, and the sum of QN5(t), QN6(t), QH17(t), and QH18(t) becomes nearly zero, i.e., a diazene (N2H2) molecule is produced. Although the formations of the N5-H19 and N6-N6 bonds occur at about 2.057 ps, these are not covalent but the former is a coordinate bond using a lone pair of N5 and the latter is a hydrogen bond, considering that ON5-N6(t) maintains ~1.0 and QN5(t) and QN6(t) does not decrease from ~-0.4. Note that H6 belongs to a neighboring NH4 + , which is the same H atom labeled H6 as shown in Figure 4(a) of the main text. After about 2.127 ps, N5 begins to interact with H20. Simultaneously N6-H6 bond is slightly enhanced, while N5-N6 bond begins to weaken. At about 2.137 ps, QN5(t) and QN6(t) reach ~-0.6, and ON5-N6(t) decreases to ~0.5 which indicates that the bond becomes a single covalent bond. Thus, a hydrazinium (N2H5 + ) is formed although ON6-H6(t) shows a smaller value ( ~0.25) at this