11:00 AM - 11:15 AM
[PCG21-08] Mid-infrared imaging spectroscopy and alteration experiments of solid sulfur dioxide under simulated surface conditions of the Jovian moon Io
Keywords:Simulation experiment, Infrared spectroscopy, Low-temperature solid, Sulfur dioxide, Jovian moon Io
Jovian moon Io exhibits the most vigorous volcanic activity in the solar system, primarily driven by tidal heating due to its eccentric orbit. Recent studies suggest that Io does not have a globally extensive shallow magma ocean beneath its surface. Instead, a model has been proposed in which magma chambers are dotted within a predominantly solid mantle in Io's interior (Park et al., 2024). The magma is erupted onto the surface, accompanied by the secondary eruption of sulfur dioxide (SO2) gas. This process contributes to the formation of a tenuous atmosphere, primarily composed of SO2 (~90%), with a pressure of ~10-3 Pa.
Io's surface, distant from volcanic hot spots, cools to ~90 K during nighttime, causing SO2 gas to condense and deposit as solid SO2. During daytime, solar irradiation raises the surface temperature to ~120 K, leading to partial sublimation of the solid SO2. The surface SO2 is undergone thermal cycling over Io's ~42.5-hour orbital period and is subjected to photochemical processes induced by ultraviolet (UV) irradiation from space. However, the mechanisms underlying the alteration of solid SO2 and its corresponding spectral variations remain unclear. We have developed a cryostat cooled by liquid nitrogen, capable of simulating low-temperature and low-pressure planetary environments, and established an infrared spectroscopic imaging system for solid particles (Koga et al., 2024). In this study, we deposited solid SO2 under simulated Io surface conditions and investigated its spectral evolution using in-situ infrared spectroscopic imaging while reproducing day-night temperature variations and UV irradiation effects.
Our experiment focused on the mid-infrared (MIR) spectral region, where strong absorptions corresponding to symmetric (ν1) and asymmetric (ν3) stretching vibrations of SO2 molecules are observed. We employed a two-dimensional Fourier-transform MIR spectrometer (2D FT-MIR) based on near-common path wavefront-division phase-shift interferometry (Qi et al., 2015), enabling spectral measurements over the 7 µm–14 µm range with a spectral resolution of ~0.7 µm and a spatial resolution of ~36 µm. The experimental procedure included: (1) Heating and dehydrating the vacuum chamber, followed by evacuation to ~10-3 Pa using a dry pump. (2) Cooling a zinc selenide (ZnSe) infrared transmission substrate to ~90 K using a liquid nitrogen-cooled oxygen-free copper sample holder. (3) Pulsed jetting of SO2 gas into the vacuum chamber to form solid SO2 on the substrate, followed by spectral imaging over a ~3 mmφ region. (4) Spectral imaging of the solid SO2 during temperature cycling between ~90 K and 120 K (annealing). (5) Spectral imaging of solid SO2 under UV irradiation (115 nm–400 nm) from a deuterium lamp.
Experiment (3) confirmed the presence of a strong absorption band at ~7.5 µm (SO2 ν3 region) and a weaker band at ~8.7 µm (SO2 ν1 region) (JpGU 2024, PPS07-19). In experiment (4), a newly implemented precision temperature control mechanism (accuracy≦1 K) attached to the sample holder was used to regulate the heating and cooling rates, thereby inducing changes in the crystal structure. The spectral differences compared to experiment (3) indicated an irreversible enhancement of the ν1 band intensity with temperature. Additionally, experiment (5) revealed the emergence of a new absorption band near 7.2 µm, attributed to the degenerate S=O stretching vibration (ν3) of sulfur trioxide (SO3). The formation of SO3 is interpreted as resulting from the competition between the photodissociation-recombination reactions of SO2 molecules and the concerted reaction associated with the B←X electronic transition (Ito et al., 2023). In this presentation, we plan to report on the relationship between the wavelength of irradiated UV light and the progression of the disproportionation reaction process associated with charge transfer on the surface of solid SO2, using a UV bandpass filter.
Io's surface, distant from volcanic hot spots, cools to ~90 K during nighttime, causing SO2 gas to condense and deposit as solid SO2. During daytime, solar irradiation raises the surface temperature to ~120 K, leading to partial sublimation of the solid SO2. The surface SO2 is undergone thermal cycling over Io's ~42.5-hour orbital period and is subjected to photochemical processes induced by ultraviolet (UV) irradiation from space. However, the mechanisms underlying the alteration of solid SO2 and its corresponding spectral variations remain unclear. We have developed a cryostat cooled by liquid nitrogen, capable of simulating low-temperature and low-pressure planetary environments, and established an infrared spectroscopic imaging system for solid particles (Koga et al., 2024). In this study, we deposited solid SO2 under simulated Io surface conditions and investigated its spectral evolution using in-situ infrared spectroscopic imaging while reproducing day-night temperature variations and UV irradiation effects.
Our experiment focused on the mid-infrared (MIR) spectral region, where strong absorptions corresponding to symmetric (ν1) and asymmetric (ν3) stretching vibrations of SO2 molecules are observed. We employed a two-dimensional Fourier-transform MIR spectrometer (2D FT-MIR) based on near-common path wavefront-division phase-shift interferometry (Qi et al., 2015), enabling spectral measurements over the 7 µm–14 µm range with a spectral resolution of ~0.7 µm and a spatial resolution of ~36 µm. The experimental procedure included: (1) Heating and dehydrating the vacuum chamber, followed by evacuation to ~10-3 Pa using a dry pump. (2) Cooling a zinc selenide (ZnSe) infrared transmission substrate to ~90 K using a liquid nitrogen-cooled oxygen-free copper sample holder. (3) Pulsed jetting of SO2 gas into the vacuum chamber to form solid SO2 on the substrate, followed by spectral imaging over a ~3 mmφ region. (4) Spectral imaging of the solid SO2 during temperature cycling between ~90 K and 120 K (annealing). (5) Spectral imaging of solid SO2 under UV irradiation (115 nm–400 nm) from a deuterium lamp.
Experiment (3) confirmed the presence of a strong absorption band at ~7.5 µm (SO2 ν3 region) and a weaker band at ~8.7 µm (SO2 ν1 region) (JpGU 2024, PPS07-19). In experiment (4), a newly implemented precision temperature control mechanism (accuracy≦1 K) attached to the sample holder was used to regulate the heating and cooling rates, thereby inducing changes in the crystal structure. The spectral differences compared to experiment (3) indicated an irreversible enhancement of the ν1 band intensity with temperature. Additionally, experiment (5) revealed the emergence of a new absorption band near 7.2 µm, attributed to the degenerate S=O stretching vibration (ν3) of sulfur trioxide (SO3). The formation of SO3 is interpreted as resulting from the competition between the photodissociation-recombination reactions of SO2 molecules and the concerted reaction associated with the B←X electronic transition (Ito et al., 2023). In this presentation, we plan to report on the relationship between the wavelength of irradiated UV light and the progression of the disproportionation reaction process associated with charge transfer on the surface of solid SO2, using a UV bandpass filter.