[P-PS07] Planetary Sciences

[PPS07-05] A search for emission lines in the atmosphere of the Galilean moons with ground-based telescope

*Seiko Takagi¹, Taro Matsuo², Jun Kimura³, Kazuo Yoshioka⁴, Tatsuhiro Tsurumi³ (1.Hokkaido University, Graduate School of Science, 2.Nagoya University, 3.Osaka University, 4.The University of Tokyo)

Keywords:Galilean moon, ground-based observation, atmosphere

For Europa, numerous sets of observations exist of atomic sodium and potassium [Brown and Chaffee, 1974; Brown, 2001]. It has been suggested that these atmospheric components are related to the materials present on the ice crust. Spectrographic evidence on the surface acquired from the Galileo spacecraft suggests that hydrated salt minerals, such as magnesium and sodium sulfates, sodium carbonate and their mixtures, are concentrated at the linear features and in the chaotic terrains [McCord et al., 1999]. Furthermore, recent visible spectroscopic observations by the Hubble Space Telescope suggests the presence of sodium chloride [Trumbo et al., 2019]. The following mechanisms of the origin of Europa’s tenuous atmosphere have been proposed: (1) sputtering of materials from the surface by energetic particles in the Jupiter’s magnetosphere, (2) the impact supply from meteorites and dusts, (3) contamination due to volcanism from nearby Io. However, sodium-to-potassium ratio observed around Europa does not agree with estimates from chemical models of the Europa’s ocean measurement of meteorites. In addition, although an emission signal from magnesium atoms (285.2 nm) has been expected, no positive signal has been reported [Horst and Brown, 2013].
The search for such materials is crucial not only to understand the chemical information in the present Galilean moons, but also to evaluate the primordial environment, formation process and evolution of the moon. We search minor species on the icy Galilean moons with the ground-based telescope of Hokkaido University (Pirka telescope, the primary mirror is 1.6 m in diameter). The Pirka telescope and its on-board observation device has started full-scale operation in 2011 with priority to observe planetary bodies in the Solar System. The multi spectral imaging device [Watanabe et al., 2012] has been installed at the Cassegrain focus of the telescope and it allow us to obtain surface spectral data with wavelength between 350 and 1050 nm. We observed the Galilean moons in 2018-2021 and searched for emission lines in the atmospheres. Here we will report the investigation.