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

Abstract

Based on observations from the Hubble Space Telescope and the Subaru Telescope, we have discovered that Europa, Ganymede, and Callisto are bright around 1.5 μm even when not directly lit by sunlight. 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 μm, a potential clue to the origin of the source of luminosity. Likewise, Ganymede observations were attempted at 3.6 μm by the Spitzer Space Telescope, but it was not detected, suggesting a significant wavelength dependence. It is still unknown why they are luminous even when in the Jovian shadow, but forward-scattered sunlight by hazes 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 with a new technique to investigate the Jovian atmospheric composition. Investigating the transmission spectrum of Jupiter by this method is important for investigating the atmosphere of extrasolar giant planets by transit spectroscopy.

High Spatial and Spectral Resolution Observations of the Forbidden 1.707 μm Rovibronic SO Emissions on Io: Evidence for Widespread Stealth Volcanism*

“…• On 15 December 2015 we observed Ganymede in eclipse before Io went into eclipse. The observed glow from Ganymede is most likely caused by sunlight scattered in the forward direction by hazes in Jupiter's upper atmosphere, as originally proposed by Tsumura et al (2014). No SO emissions were detected.…”

“…Hence the flux density received on Earth from Ganymede due to Io-shine, assuming a perfect 100% reflectivity, will only be 2.3 x 10 -15 ergs s -1 cm -2 µm -1 , i.e., roughly a factor of 3000 less than was received. Such large discrepancies between expectations and observations were also seen and discussed by Tsumura et al (2014). They attributed Ganymede's (and other Galilean satellites') glow during an eclipse to forward scattered sunlight by hazes in Jupiter's upper atmosphere.…”

Earth’s atmosphere’s lowest layers probed during a lunar eclipse

“…Alternatively we calculate z min for both umbra and penumbra with use of the 4th order Runge Kutta method following Tsumura et al (2014).…”

Extragalactic background light: inventory of light throughout the cosmic history

“…Besides in [31,57] the dark-cloud method was used also in [58] at 400 nm and in [59] in the near-IR. In an innovative attempt the Jovian satellites during eclipse phase have been utilized, instead of a dark cloud, as opaque foreground screens [60].…”