[P-CG19] Planetary Magnetosphere, Ionosphere, and Atmosphere

[PCG19-P07] Search for light metal elements in the Europa's atmosphere via visible and near-infrared observations using ground-based telescope

Tatsuhiro Tsurumi¹, *Jun Kimura¹, Seiko Takagi², Kizuku Hamada², Yuuki Sato², Shunsuke Ota³, Taro Matsuo³ (1.Osaka University, 2.Hokkaido University, 3.Nagoya University)

Jovian moon Europa has a surface mainly composed of solid H₂O ice and a global liquid water ocean is likely to exist under the ice shell. The icy surface has many endogenous geologic features that have been formed due to thermal effects from the interior. Evaluation of spectral profiles obtained by spacecrafts and ground-based telescopes suggests a presence of Na-, Mg-bearing salts, which are likely to be endogenic because their distribution is strongly correlated with the local geologic feature. On the other hand, distribution of sulfuric acid hydrate which has no correlation with surface geology indicates an exogenous origin. These materials form Europa’s tenuous atmosphere through sputtering due to high energetic particles from the Jovian magnetosphere and sublimation due to solar insolation. Thus, understanding atmospheric composition, distribution, and their temporal variation are important for clarifying composition of building material and evolutionary processes of moon, and material transport processes between moons as well. In addition, confirming the presence or absence of non-H₂O volatile such as methane and ammonia leads to constrain pressure and temperature conditions of moon’s formation. However, previous observational works are limited to highly localized area on flyby measurements of spacecrafts or only a few nights observations by ground-based telescopes in the visible and near-infrared wavelength. Important point for ground-based telescope observations is to remove sufficiently absorption effects by the Earth’s atmosphere.
In this study, imaging observations of Europa in visible and near-infrared wavelength (400 - 930 nm) were performed over 30 nights using the 1.6 m Pirka telescope at Hokkaido University. For data analysis, aperture photometry and spectroscopy were performed using Python/Astropy. In order to obtain highly accurate telluric corrections, we adopted two approaches: using photometric standard stars and using Earth atmospheric model based on radiative transfer calculations. In the telluric correction using photometric standard stars, absorption by oxygen molecules in the Earth’s atmosphere remains, which suggests that the telluric correction is insufficient due to difference in air mass between Europa and standard star. Thus, we improved the telluric correction by approximating photon number in case of same air mass between Europa and standard star, based on a relation between the air mass and the photon number of standard star. As a result, absorption due to telluric oxygen molecules was successfully reduced. Finally, reflectance spectra of full-disk Europa were calculated and emission and absorption lines were evaluated to detect materials. Emission lines due to neutral calcium at 445, 559, and 644 nm were detected with intensities exceeding 2σ which have never been reported in previous works. These detections were made when we observed a region between Europa’s leading hemisphere and the anti-Jovian hemi sphere. It is consistent to previous work that suggested a low electron density in the leading hemisphere.