Why Do Jupiter's Moons Still Shine When Eclipsed?

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Jupiter's largest moons don't go completely dark when the giant planet blocks their sunlight, astronomers have found.

Imagine a geyser hundreds of kilometers high. Sound crazy? Well, on Jupiter's moon they exist, and what these super geysers eject could give us invaluable insight into this unique, ice-covered planet.

The discovery could reveal more about Jupiter's mysterious upper atmosphere, which the researchers suspect is responsible for keeping the moons lit when they are not directly illuminated by the sun. This research could also help scientists better understand the atmospheres of alien planets, study team members said.

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Jupiter, the largest planet in the solar system, has 67 known moons — more than any other planet. Jupiter's four largest moons — Io, Europa, Ganymede and Callisto — are known as the Galilean satellites, after their discoverer, famed astronomer Galileo Galilei. [Photos: The Galilean Moons of Jupiter]

The researchers made their discovery about the Galilean moons accidentally. Their original plan was to detect the diffuse light from the most distant parts of the universe. They wanted to find dark objects in space that could block this far-off light. The difference in brightness between those dark objects and the surrounding sky could then help them determine how bright the diffuse and distant light is.

The researchers assumed the Galilean satellites would be dark while immersed in Jupiter's shadow. As such, they "planned to use the Galilean satellites in eclipse as 'occulters' to block distant background emissions," said lead study author Kohji Tsumura, an astronomer at Tohoku University in Japan.

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Instead, using the Subaru Telescope and Hubble Space Telescope, the researchers found an unexpected surprise: The Galilean satellites were still slightly bright even when eclipsed. This effect is especially pronounced for Ganymede and Callisto.

"This is a serendipitous discovery, which makes us surprised and excited," Tsumura told Space.com.

Making this discovery was very challenging because the Galilean satellites are extremely faint while eclipsed, and the incredibly bright face of Jupiter near them can blind attempts to see them. Furthermore, the eclipses only take place at specific times, and Jupiter and its moons are continuously in motion, which makes observations very complex, the researchers said.

All in all, when eclipsed, the luminosity of these moons was one-millionth to one-ten-millionth of their uneclipsed brightness — dim enough for the phenomenon to remain undetected until now, even though researchers have observed the Galilean moons in eclipse for centuries.

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