11:30 〜 11:45
[PPS08-10] The survival of organic matter in large impacts: approaches from sub-second ultra-high temperature and pressure experiments
キーワード：隕石、炭素質コンドライト、衝突、FT-IR、XANES、高温高圧実験
It has long been hypothesized that the extraterrestrial materials may have delivered the pre-biotic organic matter to the early Earth during the late heavy bombardment. This hypothesis is supported by recent studies showing that organic matter is more ubiquitous in extraterrestrial materials, i.e., comets, chondrites, and asteroids than previously thought. On the other hand, however, organic matter is believed to be mostly metamorphosed by high temperatures induced by the impact shock on the Earth’s surface toward being inorganic carbon (graphitization) eventually. According to the theoretical simulation, the organic matter can be in extreme conditions of > 2000 K and several tens of GPa for a short time, typically less than 1 second during the impacts. While previous studies estimated the survival rate of organic matter based on the kinetic data taken at relatively low-pressure and -temperature conditions, the effects of pressure and high temperature above 2000 K were poorly known so far. Although earlier studies have conducted shock experiments to simulate the impact of extraterrestrial materials, laboratory shock experiments can only cover limited pressure-temperature-time (P-T-t) space. On the other hand, static compression experiments can generate various P-T-t conditions, while short-time experiments are not straightforward by conventional apparatus. In this study, we performed state-of-the-art experiments using a laser-heated diamond anvil cell (LHDAC) to quantify the chemical reactions of organic matter at extreme conditions equivalent to the impact.
We used a synthetic chondritic organic matter prepared by a previous study simulating the aqueous alteration in the carbonaceous chondrites. High-pressure and temperature experiments were conducted using a newly designed LHDAC experimental system installed at SPring-8 BL10XU. Two lasers were used to heat the sample from both sides. We conducted in situ X-ray diffraction measurements (XRD) by a CdTe hybrid pixel array detector, which can take images in <1000 frames per second. A spectrometer with an electron-multiplying CCD was used for quick temperature determinations. An integrated control system can synchronize laser heating, XRD measurements, and spectroradiometry. We have successfully generated high temperatures above 2000 K at 16 - 56 GPa for a short time, which widely covers conditions equivalent to the impact. Recovered samples were analyzed by Fourier transform infrared (FT-IR) micro-spectroscopy and Raman spectroscopy. Selected samples were further analyzed by X-ray absorption near edge structure measurements (XANES) at KEK.
We obtained the degradation rates of aliphatic C-H bonds based on the FT-IR measurements. The degradation rates were generally higher at high temperatures and lower at high pressures, and the value reached almost 100 % in the sample heated at a higher temperature. The obtained rates were well-fitted by the diffusion-controlled reaction model, which was used in the previous study at atmospheric pressure and at relatively low temperatures. The results of Raman measurements were generally consistent with the FT-IR measurements. Using the C-XANES spectra, we obtained the intensity of 1s-σ* exciton, which is a measure of the metamorphism of the meteoritic organic matter. We observed a clear correlation between exciton intensity and degradation rates of aliphatic C-H bonds. Current results showed that the organic materials were not perfectly converted to graphene structure even when the aliphatic C-H bonds were totally degraded. The incompleteness of the graphitization may be because of pressure and the short heating durations. We will discuss the possible survival of the organic matter in large impacts based on the kinetic data taken at high-pressure and -temperature conditions.
We used a synthetic chondritic organic matter prepared by a previous study simulating the aqueous alteration in the carbonaceous chondrites. High-pressure and temperature experiments were conducted using a newly designed LHDAC experimental system installed at SPring-8 BL10XU. Two lasers were used to heat the sample from both sides. We conducted in situ X-ray diffraction measurements (XRD) by a CdTe hybrid pixel array detector, which can take images in <1000 frames per second. A spectrometer with an electron-multiplying CCD was used for quick temperature determinations. An integrated control system can synchronize laser heating, XRD measurements, and spectroradiometry. We have successfully generated high temperatures above 2000 K at 16 - 56 GPa for a short time, which widely covers conditions equivalent to the impact. Recovered samples were analyzed by Fourier transform infrared (FT-IR) micro-spectroscopy and Raman spectroscopy. Selected samples were further analyzed by X-ray absorption near edge structure measurements (XANES) at KEK.
We obtained the degradation rates of aliphatic C-H bonds based on the FT-IR measurements. The degradation rates were generally higher at high temperatures and lower at high pressures, and the value reached almost 100 % in the sample heated at a higher temperature. The obtained rates were well-fitted by the diffusion-controlled reaction model, which was used in the previous study at atmospheric pressure and at relatively low temperatures. The results of Raman measurements were generally consistent with the FT-IR measurements. Using the C-XANES spectra, we obtained the intensity of 1s-σ* exciton, which is a measure of the metamorphism of the meteoritic organic matter. We observed a clear correlation between exciton intensity and degradation rates of aliphatic C-H bonds. Current results showed that the organic materials were not perfectly converted to graphene structure even when the aliphatic C-H bonds were totally degraded. The incompleteness of the graphitization may be because of pressure and the short heating durations. We will discuss the possible survival of the organic matter in large impacts based on the kinetic data taken at high-pressure and -temperature conditions.