Near-infrared Brightness of the Galilean Satellites Eclipsed in Jovian Shadow: A New Technique to Investigate Jovian Upper Atmosphere

TL;DR

This study reveals that Galilean satellites emit faint near-infrared light during eclipses, likely due to haze scattering in Jupiter's upper atmosphere, offering a novel method to study planetary atmospheres.

Contribution

It introduces a new observational technique using satellite brightness in eclipse to probe Jovian atmospheric properties and potential applications to exoplanet transit studies.

Findings

Europa and Ganymede are faintly luminous at 1.5 μm during eclipse.

Luminosity varies with wavelength, indicating atmospheric scattering effects.

Europa's brightness in eclipse is significantly less than Ganymede's, suggesting different scattering or atmospheric conditions.

Abstract

We have discovered that Europa, Ganymede and Callisto are bright around 1.5 {\mu}m even when not directly lit by sunlight, based on observations from the Hubble Space Telescope and the Subaru Telescope. The observations were conducted with non-sidereal tracking on Jupiter outside of the field of view to reduce the stray light subtraction uncertainty due to the close proximity of Jupiter. Their eclipsed luminosity was $10^{-6}$-$10^{-7}$ of their uneclipsed brightness, which is low enough that this phenomenon has been undiscovered until now. In addition, Europa in eclipse was <1/10 of the others at 1.5 {\mu}m, a potential clue to the origin of the source of luminosity. Likewise, Ganymede observations were attempted at 3.6 {\mu}m by the Spitzer Space Telescope but it was not detected, suggesting a significant wavelength dependence. The reason why they are luminous even when in the Jovian shadow is still unknown, but forward-scattered sunlight by haze in the Jovian upper atmosphere is proposed as the most plausible candidate. If this is the case, observations of these Galilean satellites while eclipsed by the Jovian shadow provide us a new technique to investigate Jovian atmospheric composition, and investigating the transmission spectrum of Jupiter by this method is important for investigating the atmosphere of extrasolar giant planets by transit spectroscopy.