Presentation Information
[O12-P88]Observations of Jupiter and the Galilean Moons Using the Hitomi Telescope: Potential for Io-Originating Na to Reach Ganymede's Orbit
*Fuu Ishibashi1, *Rei Arai2, *Keita Eba3, *Kei Murayama4 (1. SENDAI SHIRAYURI GAKUEN High School, 2. Tokyo Metropolitan Roppongi High School, 3. Tohoku Gakuin High School, 4. Sendai Nika High School)
Keywords:
jupiter,The Hitomi space telescope
Jupiter has the largest magnetic field among the planets in the solar system, and its magnetosphere is vast enough to encompass the orbits of the Galilean moons. Io, in particular, exhibits active volcanism due to strong tidal heating, emitting particles of sodium (Na) and sulfur (S). These particles are trapped by Jupiter's magnetosphere, forming the "Io plasma torus." On the other hand, Ganymede is the only moon in the solar system known to possess an intrinsic magnetic field, and its interaction with the external plasma creates a unique environment.
This study aims to investigate, via spectroscopic observation, how far the particles emitted from Io reach, with a particular focus on their spatial distribution around Ganymede. Specifically, we verify whether the Na emitted from Io reaches the vicinity of Ganymede's orbit and measure the intensity distribution of Na in the space between Io and Ganymede. Furthermore, we evaluate how the intensity variations around Ganymede relate to the moon's intrinsic magnetic field.
For the observations, we used the visible-light medium-dispersion spectrograph on the 1.3-meter "Hitomi Telescope" at the Sendai Astronomical Observatory to conduct spectroscopic measurements over a wavelength range of approximately 580–680 nm, which covers the emission lines of Na and S+. Two observations were conducted by placing Jupiter and the Galilean moons within the slit. First, taking advantage of the observational conditions where Jupiter, Io, and Ganymede were aligned almost in a straight line, we set the slit direction parallel to the orbital plane to obtain the spatial distribution from Io to Ganymede. Next, targeting Ganymede alone, we set the slit direction orthogonal to the orbital plane to prevent light leakage from Jupiter and Io, thereby obtaining the spatial distribution around Ganymede. As the sky was completely clear on the day of observation, we were able to acquire high-quality data. From the data acquired by the telescope (which had undergone dark subtraction, flat-fielding, and wavelength calibration), we obtained the wavelength spectrum for a given spatial region. However, this includes not only the emission lines from particles around the target but also absorption lines due to reflected sunlight (Fraunhofer lines). To remove this noise, we applied spectral normalization processing (such as dividing the satellite spectra by each other) to cancel out the solar absorption lines and search for faint emission lines.
As a result, the signal-to-noise (S/N) ratio for S+ was low in this observation, and no significant detection was achieved. On the other hand, the parallel slit data revealed that the sodium emission line appeared strongly in Io's spectrum compared to Ganymede's spectrum, and the sodium intensity decreased as the distance from Io increased. This supports the premise that Io is the primary source. Furthermore, a tendency for the sodium intensity to increase again was observed in the vicinity of Ganymede. This trend indicates the possibility that sodium originating from Io reaches the vicinity of Ganymede's orbit and accumulates around Ganymede. From the orthogonal slit data, the sodium emission line was detected near Ganymede. In particular, there is a tendency for the intensity to be higher in regions closer to the main body of Ganymede. This could be influenced by Ganymede's gravity, but the contribution of sodium emission due to sputtering from Ganymede's surface cannot be ruled out, and further observations are required to isolate the origin. In addition, a characteristic dip was observed around 620 nm. This is potentially due to methane absorption originating from Jupiter or absorption by molecular oxygen on Ganymede's surface, and is expected to be related to the surface materials or atmosphere of Ganymede.
In the future, we hope to reveal the three-dimensional structure of the Na distribution around Ganymede by combining multi-directional spectroscopy and Doppler spectroscopy. It is desirable to directly verify the interaction with the intrinsic magnetic field through observations of Ganymede's auroras (especially in the ultraviolet). We intend to advance the investigation of observation conditions aimed at identifying the nature of the 620 nm absorption and detecting species sensitive to magnetic fields, such as sulfur ions. Since S+ was not visible in this visible-light observation and its relationship with the magnetic field could not be discussed, we plan to proceed with the analysis of UV observation data (such as from the Hisaki satellite, the Hubble Space Telescope, and JWST) next.
This study aims to investigate, via spectroscopic observation, how far the particles emitted from Io reach, with a particular focus on their spatial distribution around Ganymede. Specifically, we verify whether the Na emitted from Io reaches the vicinity of Ganymede's orbit and measure the intensity distribution of Na in the space between Io and Ganymede. Furthermore, we evaluate how the intensity variations around Ganymede relate to the moon's intrinsic magnetic field.
For the observations, we used the visible-light medium-dispersion spectrograph on the 1.3-meter "Hitomi Telescope" at the Sendai Astronomical Observatory to conduct spectroscopic measurements over a wavelength range of approximately 580–680 nm, which covers the emission lines of Na and S+. Two observations were conducted by placing Jupiter and the Galilean moons within the slit. First, taking advantage of the observational conditions where Jupiter, Io, and Ganymede were aligned almost in a straight line, we set the slit direction parallel to the orbital plane to obtain the spatial distribution from Io to Ganymede. Next, targeting Ganymede alone, we set the slit direction orthogonal to the orbital plane to prevent light leakage from Jupiter and Io, thereby obtaining the spatial distribution around Ganymede. As the sky was completely clear on the day of observation, we were able to acquire high-quality data. From the data acquired by the telescope (which had undergone dark subtraction, flat-fielding, and wavelength calibration), we obtained the wavelength spectrum for a given spatial region. However, this includes not only the emission lines from particles around the target but also absorption lines due to reflected sunlight (Fraunhofer lines). To remove this noise, we applied spectral normalization processing (such as dividing the satellite spectra by each other) to cancel out the solar absorption lines and search for faint emission lines.
As a result, the signal-to-noise (S/N) ratio for S+ was low in this observation, and no significant detection was achieved. On the other hand, the parallel slit data revealed that the sodium emission line appeared strongly in Io's spectrum compared to Ganymede's spectrum, and the sodium intensity decreased as the distance from Io increased. This supports the premise that Io is the primary source. Furthermore, a tendency for the sodium intensity to increase again was observed in the vicinity of Ganymede. This trend indicates the possibility that sodium originating from Io reaches the vicinity of Ganymede's orbit and accumulates around Ganymede. From the orthogonal slit data, the sodium emission line was detected near Ganymede. In particular, there is a tendency for the intensity to be higher in regions closer to the main body of Ganymede. This could be influenced by Ganymede's gravity, but the contribution of sodium emission due to sputtering from Ganymede's surface cannot be ruled out, and further observations are required to isolate the origin. In addition, a characteristic dip was observed around 620 nm. This is potentially due to methane absorption originating from Jupiter or absorption by molecular oxygen on Ganymede's surface, and is expected to be related to the surface materials or atmosphere of Ganymede.
In the future, we hope to reveal the three-dimensional structure of the Na distribution around Ganymede by combining multi-directional spectroscopy and Doppler spectroscopy. It is desirable to directly verify the interaction with the intrinsic magnetic field through observations of Ganymede's auroras (especially in the ultraviolet). We intend to advance the investigation of observation conditions aimed at identifying the nature of the 620 nm absorption and detecting species sensitive to magnetic fields, such as sulfur ions. Since S+ was not visible in this visible-light observation and its relationship with the magnetic field could not be discussed, we plan to proceed with the analysis of UV observation data (such as from the Hisaki satellite, the Hubble Space Telescope, and JWST) next.