Protoplanets formed by repeated collisions of interplanetary dust and asteroids have firstly a primitive atmosphere consisting of mainly hydrogen and helium, called as a “primary” atmosphere. Such an atmosphere was dissipated soon into the outer space by the solar wind due to light molecules. Later, secondary components such as H2O, CO2, and N2 were added into the atmosphere as originated by the degassing of planetary minerals. Carbonate minerals like calcite (CaCO3) are considered to have been the major source of the CO2 component1, 2. The crater chronology of the Moon and the oxygen isotope ratios for zircon indicate that intense meteorite impact events occurred between ∼3.8 and ∼4.1 billion years ago, which is the so-called late heavy bombardment (LHB) period3, 4. The impact flux on the Earth during the LHB was at least ∼103 times more than that on the present Earth. Moreover, it is believed that collisions exceeding 10 km/s frequently occurred in the LHB period. Therefore, the high-velocity planetary impact-induced degassing of carbonates plays a critical role in the planetary carbon cycle that affects the formation of planetary atmosphere and ocean and the habitability.
Calcite (CaCO3) of carbonates is a typical component in chondritic asteroids5 and rocky planet surfaces. In fact, the large-scale, catastrophic impact evidence in places abundant in carbonates is visible also on the present Earth as the Chicxulub crater. The shock-compressed calcite behaviors have been investigated only up to 100 GPa pressure, corresponding to an 8 km/s impact velocity. In this study, the Hugoniot pressure-density-temperature (P-ρ-T) relation of calcite was precisely determined to 1000 GPa using the decaying shock method, allowing us to predict the post-impact residual temperatures and the degassed greenhouse gas species.
We performed laser shock experiments on targets illustrated in Fig. 1a and obtained the Hugoniot for calcite shocked between 200 and 1000 GPa (Table S1) using the impedance mismatching technique. Typical images recorded by the velocity interferometer system for any reflector (VISAR) for shock velocity (Us) and streak optical pyrometer (SOP) (see Method section) for temperature (T) are shown in Fig. 1b and 1c.
Figure 2a shows the relationship between the shock velocity and the particle velocity (up) determined in the present and previous studies. The previous results showed the linear relation; Us (km/s) = (3.71 ± 0.03) + (1.43 ± 0.01) up (km/s)6 and the extrapolation is in excellent agreement with the SESAME model. The present results are well-approximated also by a linear relation; Us (km/s) = (3.79 ± 0.33) + (1.26 ± 0.03) up (km/s). This linear relation appears to shift downwards parallel to the previous linear extrapolation.
From the present Us-up data, the Hugoniot pressure-density relation was determined through the Hugoniot conservation relations, as shown in Fig. 2b. Our experimental results demonstrate that the SESAME model overestimates shock pressure by more than 100 GPa or density by more than 0.5 g/cc when particle velocity is above ~6 km/s corresponding to ~12 km/s calcite collision velocity. Since the previous experimental data below 100 GPa agree with the SESAME model, the large gap between the present results and the SESAME suggests that any state changes occur with a significant volume change in the 100-200 GPa pressure range.
Figure 3a shows the measured optical reflectivity (R) as a function of the shock pressure. The calcite reflectivity along the decaying shock-velocity history was extracted from the VISAR data. The shock pressure is estimated from the measured shock velocity using the calcite Hugoniot determined in this work. Calcite is found to be reflective over ~200 GPa and the reflectivity steeply increases over ~350 GPa due to the metallization. Similar tendency was observed for shocked quartz and some other insulators.
Figure 3b shows the decaying shock pressure-temperature relationship on calcite, including the previous experimental results7 and the EOS models8. Below ~350 GPa, the obtained Hugoniot P-T curve follows the predicted melting line9. The trend of the P-T relationship changes at ~350 GPa, suggesting that the melt completion of calcite along its Hugoniot occurs at ~350 GPa. Above 350 GPa, calcite would be an electronically conducting fluid.
The previous experimental results agree with the SESAME model, but not with the ANEOS and PANDA models. These show that the decomposition reaction of calcite into CaO and CO2 in the solid phase does not appear in Hugoniot as predicted by the ANEOS and PANDA models at ~80 GPa, because SESAME model does not consider the decomposition reaction. Sekine et al. suggested that the decomposition of shocked MgCO3 occurred at 107 GPa from their recovery experiment results and the Hugoniot and released state measurements10. Considering the binding energies of MgCO3 to MgO and CO2, CaCO3 may decompose at slightly higher shock pressure than MgCO3. Due to the melting and this decomposition reaction, therefore, the previous data points may show a small deviation from the SESAME Hugoniot along the predicted melting line of CaCO3. The Hugoniot P-T data in the previous and present studies can be smoothly connected along the melting line. No optical reflectivity observed below 200 GPa, as shown in Fig.3a, is consistent with the presumption that the melting and the decomposition reaction dominate in the 100-200 GPa pressure range. Finally, in the pressure range of 100-350 GPa along the melt line, the reactions of melting, decomposition, and dissociation of calcite may sequentially occur.
Implications for natural impacts: carrier of atmospheric components
The adiabatic release paths from Hugoniot state of calcite were calculated using the Mie-Grüneisen equation of state (see Method section) at pressures of 100, 200, and 300 GPa based on our experiments in Fig. 4a (pink, green, and blue diamonds). The Hugoniot temperature TH and the residual (post-shock) temperature TR are shown in Figs. 4a and 4b, respectively. After calcite decomposed into CaO and CO2, the chemical reaction CO2 = CO + 1/2O2 occurs depending on temperature. The CO/CO2 ratio produced from calcite shocked at 100, 200, and 300 GPa were estimated to be 6, 10, and 11, respectively, at a total pressure of 1 atm as a function of the residual temperature according to the equilibrium gas-phase diagram11. This result shows that the amount of CO generated from CaCO3 increases with increasing shock temperature (shock pressure), because CO is more thermodynamically stable than CO2 at high temperature.
According to the previous sample recovery experimental results, calcite should be completely degassed at shock pressures above 70 GPa, although there are some variations in the threshold pressure dependent on the starting material of single crystal or powders and capsuled conditions of open or close system (e.g., 10 GPa and 70 GPa by Lange and Ahrens, 1986; at 30-35 GPa and 45 GPa by Martinez et al., 1995; at 20 GPa and 33 GPa by Ohno et al., 2008, respectively12, 13, 14). The numerical simulations of the Chicxulub impact reported the total gas amount by the shock-induced volatilization as (0.8–7.6) × 1016 mol15, 16. If the CO/CO2 ratio from shocked calcite is ~10 at an impact pressure of 200 GPa (in a case of our results), the amounts of CO and CO2 gases produced by the impact become (0.7–6.9) × 1016 mol and (0.1–0.7) × 1016 mol, respectively. According to the previous studies using a light-gas gun, the CO/CO2 from single crystal calcite shocked at peak shock pressures of below ~100 GPa was estimated to be about 0.117. The residual temperature of the present study was much higher than those of the previous studies. Kawaragi et al. (2009) reported that a CO dominant gas (CO/CO2 = 2.02) increased ~3 K in temperature compared with that in case of only CO2 in the Chicxulub impact event18, 19. CO is known to produce CH4 and tropospheric O3 with the strong greenhouse effect through photochemical reactions and has more intense indirect-radiative-forcing to the heat uptake than CO220. Our results suggest that the indirect greenhouse effect of CO should be taken into account in considering the climate change after the high-speed planetary impact events.
Figure 4b shows that CO/CO2 ratio in released gas from calcite and the residual temperature as a function of impact pressure. The collision velocity required to generate shock pressures of 100-300 GPa was calculated to be 7-14 km/s for ordinary chondrites21. These velocities are typical in planetary impacts22. The results suggest the CO gas was produced more significantly in large-scale planetary impacts than the previously estimated. In natural impacts, CO production proceeds more efficiently than our experimental conditions, since meteorites should relatively increase temperature due to certain amounts of porosity23. Especially, it is possible that more CO than expected had been supplied on the Earth by planetary impacts during the LHB period.
Our results may prompt a reconsidering of whole planetary evolution processes on the planetary surface, especially the estimation of the atmospheric compositions and the greenhouse effect, and the seawater temperature. Furthermore, a large amount of CO generated from impacted carbonate rocks may contribute to model the global warming, the planetary atmosphere chemistry, and the biological mass extinction24 on the Earth triggered by bolide impacts.