14:45 〜 15:00
[PPS05-15] Nanosecond time-gated Raman spectroscopy of clay minerals for the reconstruction of water chemistry on early Mars
キーワード：火星、ラマン、粘土鉱物、表層進化、地球化学
Lines of evidence suggest that Mars once had liquid water, but how and when it was lost remains a mystery. Reconstructing past aqueous environments by analyzing sedimentary rocks could provide clues for understanding the Martian environmental evolution. One of the key phases is clay minerals, as these could retain records of aqueous environments. NASA’s Perseverance rover, currently in operation on Mars, is equipped with a Raman spectrometer, but the Raman spectra of clay minerals using a time-gated Raman spectrometer has not yet been examined. This is because clay minerals are difficult to analyze with Raman spectroscopy owing to their strong fluorescence. Thus, in this study, we develop a time-gated Raman spectrometer with the same specifications as the Perseverance rover to analyze clay minerals and clarify the conditions under which the Raman signals of clay minerals could be observed. Then, we will discuss the detection possibility on Mars.
The Raman spectrometer developed in this study consists of a light source that irradiates samples with a pulsed laser of 532 nm wavelength at the power of 0.4–40 mJ with beam diameter variable from 50 μm to 10 mm, and an optical system that captures Raman scattering through a spectrometer coupled with an intensifier-mounted CCD camera (ICCD). The irradiation optics are the same as that of the Perseverance rover, except that laser power and beam diameter can be varied, while both are fixed at 9 mJ and 3 mm in the Perseverance rover. The ICCD is time gated by synchronizing its imaging timing with the laser irradiation timing using a signal generator. The exposure time could be controlled at the nanosecond scale with an accuracy of ±1 ns. This enables the separation of fluorescence from the Raman scattering because the typical fluorescence lifetime is longer than Raman scattering. Measurements were conducted in the wavelength range between 532–800 nm. Wavelength was calibrated using a Ne lamp, and sensitivity was corrected using a calibrated halogen light source.
Using the developed measurement system, we measured an acetaminophen reagent as a standard sample and confirmed that obtained Raman spectra matched the spectra in the literature. We then measured the Raman spectra of calcite (CaCO3), a carbonate mineral with strong fluorescence, with different time gate widths (i.e., exposure times) and confirmed that the fluorescence could be separated by controlling the exposure time. The measurement results of three clay mineral samples, montmorillonite, kaolinite, and saponite, showed several peaks when the beam diameter was 100 μm and laser power was 10 mJ. We found many elemental emission lines of the samples, which can be due to emission from a laser-induced breakdown but also other peaks that cannot be explained as elemental lines. These peaks were observed in a wavelength range where even non-time-gated (i.e., continuous-wave) Raman spectra have not been reported previously for these clay minerals, and the effective suppression of fluorescence suggests that these might be Raman scattering originating from clay minerals.
Our results suggest that the signature of clay minerals might be detected by a time-gated Raman spectroscopy. We are planning to analyze the Raman spectra data obtained by the Perseverance rover, as the data up to December 27th, 2022, will be released to the public at the end of March 2023. Although both beam diameter and laser power are fixed in the Perseverance rover, it may be possible to increase the laser power per area by narrowing the beam diameter by intentionally defocusing the laser.
The Raman spectrometer developed in this study consists of a light source that irradiates samples with a pulsed laser of 532 nm wavelength at the power of 0.4–40 mJ with beam diameter variable from 50 μm to 10 mm, and an optical system that captures Raman scattering through a spectrometer coupled with an intensifier-mounted CCD camera (ICCD). The irradiation optics are the same as that of the Perseverance rover, except that laser power and beam diameter can be varied, while both are fixed at 9 mJ and 3 mm in the Perseverance rover. The ICCD is time gated by synchronizing its imaging timing with the laser irradiation timing using a signal generator. The exposure time could be controlled at the nanosecond scale with an accuracy of ±1 ns. This enables the separation of fluorescence from the Raman scattering because the typical fluorescence lifetime is longer than Raman scattering. Measurements were conducted in the wavelength range between 532–800 nm. Wavelength was calibrated using a Ne lamp, and sensitivity was corrected using a calibrated halogen light source.
Using the developed measurement system, we measured an acetaminophen reagent as a standard sample and confirmed that obtained Raman spectra matched the spectra in the literature. We then measured the Raman spectra of calcite (CaCO3), a carbonate mineral with strong fluorescence, with different time gate widths (i.e., exposure times) and confirmed that the fluorescence could be separated by controlling the exposure time. The measurement results of three clay mineral samples, montmorillonite, kaolinite, and saponite, showed several peaks when the beam diameter was 100 μm and laser power was 10 mJ. We found many elemental emission lines of the samples, which can be due to emission from a laser-induced breakdown but also other peaks that cannot be explained as elemental lines. These peaks were observed in a wavelength range where even non-time-gated (i.e., continuous-wave) Raman spectra have not been reported previously for these clay minerals, and the effective suppression of fluorescence suggests that these might be Raman scattering originating from clay minerals.
Our results suggest that the signature of clay minerals might be detected by a time-gated Raman spectroscopy. We are planning to analyze the Raman spectra data obtained by the Perseverance rover, as the data up to December 27th, 2022, will be released to the public at the end of March 2023. Although both beam diameter and laser power are fixed in the Perseverance rover, it may be possible to increase the laser power per area by narrowing the beam diameter by intentionally defocusing the laser.