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Jupiter's glowing moons to shine a light on exoplanets

Jenny Winder, News Writer
Jun 21, 2014, 15:30 UTC

Sen—Astronomers using the Subaru Telescope and Hubble Space Telescope have discovered that Jupiter's Galilean satellites, Io, Europa, Ganymede, and Callisto, glow slightly even when in the planet's shadow and not directly illuminated by the Sun.

The phenomenon sees them shine at up to one millionth of their normal brightness. The researchers found that the effect is particularly pronounced for Ganymede and Callisto.

It suggests that indirect forward scattering of sunlight by hazes in the upper Jovian atmosphere could be the reason for the illumination. This effect is similar to the one that causes Earth's moon to look red during a total lunar eclipse.

Images of Ganymede and Callisto while eclipsed by Jupiter. Image credit: NAOJ/JAXA/Tohoku University

Continuous observations of the Galilean satellites in eclipse made by the Japanese team of researchers at Tohoku University, Institute of Space and Astronautical Science/Japan Aerospace Exploration Agency (ISAS/JAXA), National Astronomical Observatory of Japan (NAOJ), provides a much better basis for studying the hazes in Jupiter's atmosphere and will provide new insights about the atmospheres of exoplanets.

"It is a serendipitous discovery made as a by-product of a cosmological study," said Dr. Kohji Tsumura (FRIS, Tohoku University) "It is very interesting that it provides us a new method to investigate the atmosphere of Jupiter and of exoplanets. I will keep studying from nearby space (the Solar System and exoplanets) out to the farthest universe through this project."

The new finding came when the team were trying to detect diffuse light from the most distant parts of the Universe. They planned to use the Galilean satellites in eclipse to occult, or block out, distant background emissions. This would allow an extremely accurate separation of the background light from the very bright foreground glow from dust in our Solar System (known as the zodiacal light).

The team assumed that the Galilean satellites would be "dark" while in Jupiter's shadow, and the difference in brightness between the dark satellite as an occulter and its surrounding sky would determine the still-unknown level of background emission from the distant universe.

Instead, they found that Ganymede and Callisto were still somewhat "bright" (illuminated) even when eclipsed, relative to the expected level of near-zero. Their eclipsed luminosity is low enough that this phenomenon has been undetected until now.

Schematic drawing of how to measure the background light. Image credit: NAOJ/JAXA/Tohoku University

The most plausible explanation is that the Galilean satellites are still illuminated during eclipse by sunlight that is scattered by hazes in the Jovian upper atmosphere. By comparison, the sunlight refracted in the atmosphere does not contribute to the illumination during the eclipse.

The origin of the cloud particles composing Jupiter's banded appearance are assumed to grow from tiny particles called aerosols or hazes that form somewhere in the upper part of Jupiter's atmosphere, which is very difficult to observe.

Schematic image shows that the Galilean satellites are illuminated by scattered sunlight  in the Jovian upper atmosphere. Image credit: NAOJ/JAXA/Tohoku University

This new method of studying the upper atmosphere of Jupiter via transmitted sunlight provides a basis for the study of other planetary systems. Atmospheres around some exoplanets have been investigated using "transit observations" (when the exoplanet passes between us and the host star, resulting in the star becoming slightly dimmer).

Some characteristics of the exoplanet's atmosphere are revealed as host starlight passes through it. This is the same situation seen with Jupiter and its Galilean satellites, and makes studies of transmitted sunlight of the planets in our Solar System essential for comparison.