[P-CG21] Planetary Magnetosphere, Ionosphere, and Atmosphere

[PCG21-04] A search for water vapor plumes on Europa using SUBARU/IRCS

*Jun Kimura¹, Taro Matsuo², Hitomi Kobayashi³, Yuji Ikeda^4,5, Kazuo Yoshioka⁶, Seiko Takagi⁷, Shigeru Ida⁸ (1.Osaka University, 2.Nayoga University, 3.Kyoto Nijikoubou, LLP, 4.Photocoding, 5.Kyoto Sangyo University, 6.The University of Tokyo, 7.Hokkaido University, 8.Tokyo Institute of Technology, Earth-Life Science Institute)

Jovian moon Europa stands out as a primary target for astrobiological exploration in the solar system. The Galileo spacecraft's encounter with Europa revealed magnetic field fluctuations, suggesting the presence of a sub-surface global ocean containing electrolytes beneath the solid ice shell. Gravitational measurements and geomorphological interpretations suggest to host the ocean which has several tens of kilometers thickness, beneath the solid ice shell with few to few tens of kilometers thickness. The ocean is likely to be enriched in the materials and chemical energy necessary to sustain life. A report of a possible water eruption from UV observations with the Hubble Space Telescope suggests that the possibility that Europa's oceanic material could be ejected into space, providing directly access to the material without the drilling through the ice shell. The ejected material forms an atmosphere, which has been estimated (4.9 ∼ 8.2) x 10³¹ molecules in the southern hemisphere, production rate of (4.9 ∼ 8.2) x 10²⁸ molecules s⁻¹ assuming the residence time in the plume of ∼ 10³ seconds. On the other hand, Europa's atmosphere is also fed with materials stripped from its icy surface due to energy radiation from Jovian plasma and thermal desorption by solar radiation, and meteoritic impacts. As a result, water, oxygen and hydrogen molecules are estimated to be populated at a rate of about 10²⁶⁻²⁷ molecules s⁻¹. Therefore, estimated abundances of water vapor from the plume could be exceed from exogenic effects by factors 100–1000. However, such plume activity has not been confirmed by subsequent observations despite several attempts. Magnetic–field and plasmawave observations of the Galileo spacecraft during a close flyby of Europa has been interpreted as being caused by a plume. The reliability of direct imaging observations by the HST during Europa's transit in front of Jupiter, suggesting an upper limit of total water molecules of 1.3 x 10³², has been questioned, attributing potential statistical errors. Subsequent attempt with different approach has failed to confirm these findings. Infrared spectroscopy by the Keck telescope identified one tentative detection out of 17 dates suggesting an upper limit for total molecules of (7.0 ± 2.2) × 10³¹, and column density of (1.4 ± 0.4) x10¹⁹ molecules m^-2. Additionally, the James Webb Space Telescope observation of the leading hemisphere yielded no detection of water, the resulting 3σ upper limit for the total water molecules is 3.5 x 10³¹ and an average column density is 1.44 x 10¹⁹ m⁻² for the Europa's disk (1'' x 1''). Consequently, the existence of plume activity on Europa has not been convinced, and even if present, details regarding its temporal and spatial variations remain unknown.
To detect the direct evidence of water arising from Europa's plumes and to explore spatial variations in plume activity, we conducted high-dispersion near-infrared spectroscopic observations using the Subaru Telescope/IRCS on July 16, 2021. Employing high-spatial and high-spectral resolution spectroscopy (R ~ 20,000) in L-band, we performed a line-by-line analysis of water hot-band emissions and suppress contamination from sky background radiation. We utilized a 0.14'' x 6.69'' slit in the meridian direction, covering the entire disk of Europa in length (~1.06'') and ~15 degrees of longitude in width. Spectra were extracted from five distinct latitudinal regions on the leading hemisphere to investigate spatial variations in plume activity. As a result of careful reduction and analysis of the spectra, although none of the infrared water emissions were detected, we determined the 3σ upper limits of the total water molecules are < (2~9) x 10³¹ with a mean column density of (8~20) x 10²⁰ molecules m^-2 in each region. Additionally, considering the expected production rates using a simple plume model, we estimate a total water production rate of (2~10) x 10²⁹ molecules s^-1.