4:15 PM - 4:30 PM
[PPS07-19] Mid-infrared imaging spectroscopy measurements and denaturation experiments of low-temperature sulfur dioxide solids for the surface environment of Jupiter's moon Io
Recent high-sensibility, high-dispersion spectroscopic observations by spacecraft and astronomical telescopes are revealing not only the chemical composition of the surface layers and atmospheres of solar system bodies, but also the atmospheric composition of exoplanets. Detailed spectroscopic data acquired in the laboratory are essential for the interpretation of these spectroscopic observations. Therefore, we have developed a liquid nitrogen-cooled cryostat that can reproduce the low-temperature and low-pressure astronomical environment. Then, a low-temperature spectroscopic imaging system adaptable to a variety of chemical species was assembled (Koga et al. (2024)). In this presentation, focused on SO2, which can be measured with this system and is dominant in the atmosphere and surface of Jupiter's moon Io, which is the most volcanically active object in the solar system.
Io is supplied with the mainly SO2 gas by volcanic eruptions, forming a tenuous atmosphere about 10-3 Pa. Io's surface cools to about 90 K at nighttime and during Jupiter's eclipse, and SO2 gas in the atmosphere condenses to form frost-like SO2 solids on the surface. On the other hand, in the daytime, sunlight heats the surface to about 120 K, causing some of the SO2 solids to sublimate. The Galileo spacecraft has observed the near-infrared spectroscopy of the Io surface and found that the size and density of the SO2 solids vary with the location of the Io surface (Douté et al. (2001)). A small number of simulated mid-infrared spectroscopy experiments (e.g., Nash and Betts (1995)) have also been performed in the past to elucidate the physical properties of SO2 solids. However, the mechanism of denaturation of SO2 solids by heat or UV light in the Io’s surface environment and its correspondence to the spectra have not been elucidated yet.
In this study, SO2 solids were deposited in the developed cryostat to reproduce the low-temperature and low-pressure environment of the Io surface, and the annealing and UV irradiation processes were measured spectroscopically to elucidate the denaturation mechanism of the SO2 solids. An imaging Fourier transform mid-infrared spectrometer (2D FT-MIR), based on near-common-path wavefront-division phase-shift interferometry (Qi et al. (2015)), was used as the spectrometer to acquire mid-infrared transmission absorption spectra in spatial two-dimensions. The following operations were performed in this experiment. (1) Depressurize a vacuum chamber of the cryostat to about 10-3 Pa, (2) ZnSe substrate on the sample holder was cooled down to about 90 K, (3) SO2 gas was jetted into the vacuum chamber to deposit SO2 solids on the cooled ZnSe substrate, (4) Annealing at ~90 K−120 K and irradiation with UV light (~190 nm−340 nm) from a deuterium lamp while performing spectral measurements. A strong band at ~7.5 μm (SO2 ν3 region) and a weak band at ~8.7 μm (SO2 ν1 region) were observed, but the band intensity ratio in the ν1 region was irreversibly enhanced versus temperature after one annealing. In this presentation, the change in the band intensity ratio after multiple annealing will be discussed. Furthermore, the formation of SO3 and an unknown absorption material with a peak around 7.4 µm was confirmed by UV irradiation (Negishi et al., The 23rd Symposium on Molecular Spectroscopy). For SO3, the bond dissociation energy of the SO2 molecule is 218.7 nm, suggesting that it was formed by a competitive reaction between dissociation-recombination reactions and a concerted reaction (Ito et al. (2023)) caused by the excess energy captured by the intersystem crossing associated with the electronic excitation (B←X band). In this presentation, UV irradiation with a long-pass filter with a cut-on wavelength of 250 nm will be carried out to discuss whether dissociation-recombination reactions or concerted reactions are the predominant reaction pathway for the formation of the unknown absorption material.
Io is supplied with the mainly SO2 gas by volcanic eruptions, forming a tenuous atmosphere about 10-3 Pa. Io's surface cools to about 90 K at nighttime and during Jupiter's eclipse, and SO2 gas in the atmosphere condenses to form frost-like SO2 solids on the surface. On the other hand, in the daytime, sunlight heats the surface to about 120 K, causing some of the SO2 solids to sublimate. The Galileo spacecraft has observed the near-infrared spectroscopy of the Io surface and found that the size and density of the SO2 solids vary with the location of the Io surface (Douté et al. (2001)). A small number of simulated mid-infrared spectroscopy experiments (e.g., Nash and Betts (1995)) have also been performed in the past to elucidate the physical properties of SO2 solids. However, the mechanism of denaturation of SO2 solids by heat or UV light in the Io’s surface environment and its correspondence to the spectra have not been elucidated yet.
In this study, SO2 solids were deposited in the developed cryostat to reproduce the low-temperature and low-pressure environment of the Io surface, and the annealing and UV irradiation processes were measured spectroscopically to elucidate the denaturation mechanism of the SO2 solids. An imaging Fourier transform mid-infrared spectrometer (2D FT-MIR), based on near-common-path wavefront-division phase-shift interferometry (Qi et al. (2015)), was used as the spectrometer to acquire mid-infrared transmission absorption spectra in spatial two-dimensions. The following operations were performed in this experiment. (1) Depressurize a vacuum chamber of the cryostat to about 10-3 Pa, (2) ZnSe substrate on the sample holder was cooled down to about 90 K, (3) SO2 gas was jetted into the vacuum chamber to deposit SO2 solids on the cooled ZnSe substrate, (4) Annealing at ~90 K−120 K and irradiation with UV light (~190 nm−340 nm) from a deuterium lamp while performing spectral measurements. A strong band at ~7.5 μm (SO2 ν3 region) and a weak band at ~8.7 μm (SO2 ν1 region) were observed, but the band intensity ratio in the ν1 region was irreversibly enhanced versus temperature after one annealing. In this presentation, the change in the band intensity ratio after multiple annealing will be discussed. Furthermore, the formation of SO3 and an unknown absorption material with a peak around 7.4 µm was confirmed by UV irradiation (Negishi et al., The 23rd Symposium on Molecular Spectroscopy). For SO3, the bond dissociation energy of the SO2 molecule is 218.7 nm, suggesting that it was formed by a competitive reaction between dissociation-recombination reactions and a concerted reaction (Ito et al. (2023)) caused by the excess energy captured by the intersystem crossing associated with the electronic excitation (B←X band). In this presentation, UV irradiation with a long-pass filter with a cut-on wavelength of 250 nm will be carried out to discuss whether dissociation-recombination reactions or concerted reactions are the predominant reaction pathway for the formation of the unknown absorption material.