10:45 AM - 12:15 PM
[PPS07-P14] Mid-infrared imaging spectroscopy of solid SO2 simulating the environment of Jupiter's moon Io
Io is a moon of Jupiter, which together with Europa, Ganymede, and Callisto compose the Galilean moons, and orbits closest to Jupiter of the Galilean moons. The interaction of Jupiter with Europa and Ganymede generates significant tidal heat within Io, which is the source of Io's volcanic activity, the most active in the solar system. This volcanic activity produces a tenuous atmosphere of about 10-3 Pa-10-4 Pa, the main component of which is sulfur dioxide (SO2). Io's atmosphere condenses at nighttime and during Jupiter's eclipse to form SO2 frosts on Io’s surface. This is thought to be caused by the rapid decrease in the saturated vapor pressure as the surface temperature decreases from about 120 K to about 90 K from daytime to nighttime and during Jupiter's eclipse. On the other hand, SO2 frosts sublimate in the daytime due to heating by sunlight. Thus, with the day/night changes in surface temperature at Io, there is a gas-solid phase circulation of SO2 between the atmosphere and the surface.
The SO2 atmospheres and frosts at Io have been variously observed and experimented upon. For the SO2 atmospheres, submillimeter observations with the ALMA (Atacama Large Millimeter/Submillimeter Array) of Koga et al. (2021) suggest that SO2 gas condensation may have occurred in the plume during Jupiter's eclipse. However, in the radio wave region corresponding to the rotational transition, it is in principle impossible to obtain information on solids with constrained molecular rotational motion. On the other hand, in the infrared region corresponding to the vibrational-rotational transition, it is possible to obtain information on solids as well as gases at the same time. The Voyager I spacecraft observed the thermal infrared spectrum of the atmosphere in a warm volcanically active area and obtained a weak band of ν1 vibration (symmetric stretching vibration), the normal mode of SO2 molecules, at ∼7.4 μm and a strong band of ν3 vibration (asymmetric stretching vibration), also a its normal mode, at ∼8.7 μm. For the surface SO2 frosts, the Galileo spacecraft observed spectroscopically the near-infrared emitted from the Io surface and found that the size and density of the SO2 frosts varied with the location of there. In the laboratory, Nash and Betts (1995) measured the infrared diffuse reflectance absorption spectra of SO2 in various phase states simulating the Io's surface environment. They reported that the band shapes in the mid-infrared wavelength region differ significantly depending on the phase state. However, the correspondence between the growth and transformation mechanisms of SO2 frosts and the infrared spectra has not been elucidated.
In this study, mid-infrared spectra of SO2 frost were measured in-situ, simulating the Io's surface environment, in order to elucidate the microstructure, growth and transformation processes of SO2 frosts. Spectral imaging was performed using an imaging Fourier transform mid-infrared spectrometer (2D FT-MIR) based on near-common-path wavefront-division phase-shift interferometry (Qi et al. (2015)) to observe small regions of SO2 frosts. Deposition of SO2 frost was performed using a newly developed liquid nitrogen-cooled vacuum cryostat, and SO2 gas was jetted into the depressurized vacuum chamber using a pulse nozzle. SO2 frosts were deposited on a ZnSe substrate fixed in a 3 mm-diameter through-hole in an oxygen-free copper sample holder. Mid-infrared light was vertically irradiated there, and its transmission absorption imaging spectra were measured by 2D FT-MIR. Spectroscopic imaging results showed two bands at ∼7.4 μm (SO2 ν3 region) and ∼8.7 μm (SO2 ν1 region). In the spectrum acquired at 94 K, the band intensity in the ν1 region was observed to be relatively larger than that of the SO2 gas molecules, confirming the formation of SO2 frost. This may reflect the strength of the interaction between the SO2 molecules in the formed particles. Moreover, the line widths of both bands became broad when the temperature was increased to 176 K, suggesting the collapse of the solid structure (amorphization or lattice defects) due to sublimation.
The SO2 atmospheres and frosts at Io have been variously observed and experimented upon. For the SO2 atmospheres, submillimeter observations with the ALMA (Atacama Large Millimeter/Submillimeter Array) of Koga et al. (2021) suggest that SO2 gas condensation may have occurred in the plume during Jupiter's eclipse. However, in the radio wave region corresponding to the rotational transition, it is in principle impossible to obtain information on solids with constrained molecular rotational motion. On the other hand, in the infrared region corresponding to the vibrational-rotational transition, it is possible to obtain information on solids as well as gases at the same time. The Voyager I spacecraft observed the thermal infrared spectrum of the atmosphere in a warm volcanically active area and obtained a weak band of ν1 vibration (symmetric stretching vibration), the normal mode of SO2 molecules, at ∼7.4 μm and a strong band of ν3 vibration (asymmetric stretching vibration), also a its normal mode, at ∼8.7 μm. For the surface SO2 frosts, the Galileo spacecraft observed spectroscopically the near-infrared emitted from the Io surface and found that the size and density of the SO2 frosts varied with the location of there. In the laboratory, Nash and Betts (1995) measured the infrared diffuse reflectance absorption spectra of SO2 in various phase states simulating the Io's surface environment. They reported that the band shapes in the mid-infrared wavelength region differ significantly depending on the phase state. However, the correspondence between the growth and transformation mechanisms of SO2 frosts and the infrared spectra has not been elucidated.
In this study, mid-infrared spectra of SO2 frost were measured in-situ, simulating the Io's surface environment, in order to elucidate the microstructure, growth and transformation processes of SO2 frosts. Spectral imaging was performed using an imaging Fourier transform mid-infrared spectrometer (2D FT-MIR) based on near-common-path wavefront-division phase-shift interferometry (Qi et al. (2015)) to observe small regions of SO2 frosts. Deposition of SO2 frost was performed using a newly developed liquid nitrogen-cooled vacuum cryostat, and SO2 gas was jetted into the depressurized vacuum chamber using a pulse nozzle. SO2 frosts were deposited on a ZnSe substrate fixed in a 3 mm-diameter through-hole in an oxygen-free copper sample holder. Mid-infrared light was vertically irradiated there, and its transmission absorption imaging spectra were measured by 2D FT-MIR. Spectroscopic imaging results showed two bands at ∼7.4 μm (SO2 ν3 region) and ∼8.7 μm (SO2 ν1 region). In the spectrum acquired at 94 K, the band intensity in the ν1 region was observed to be relatively larger than that of the SO2 gas molecules, confirming the formation of SO2 frost. This may reflect the strength of the interaction between the SO2 molecules in the formed particles. Moreover, the line widths of both bands became broad when the temperature was increased to 176 K, suggesting the collapse of the solid structure (amorphization or lattice defects) due to sublimation.