Hubble telescope spots ocean on Jupiter moon Ganymede

Traci Watson, Special for USA TODAY 1:32 p.m. EDT March 12, 2015

Photo of Ganymede, taken from NASA's Galileo spacecraft.(Photo: NASA)

The biggest moon in the solar system harbors a salty ocean beneath its frozen surface, according to a study that examined the moon's flickering auroras to probe its interior.

A number of worlds in our solar system are thought to have oceans. But this is the first clear-cut data of its kind to suggest that a sea lies hidden under the icy shell of Jupiter's moon Ganymede, which is 50% bigger than our own moon. Scientific models predicted an ocean on Ganymede, and when NASA's Galileo spacecraft visited Ganymede in the 1990s, it collected data that hinted at an ocean. But new images from the Hubble Space Telescope offer strong confirmation of a liquid body of water inside Ganymede, scientists say.

Galileo's observations "provide inconclusive evidence for the ocean," says study co-author Joachim Saur of the University of Cologne. "The Hubble data require an ocean."

Finding an ocean on a celestial body hundreds of millions of miles from Earth is no easy feat. Saur and his team turned to the space-going Hubble, which trained its keen eyes on Ganymede in 2010 and again in 2011. The Hubble focused on Ganymede's two auroras, shimmering patterns in the sky similar to the earthly phenomenon known as the Northern Lights. A person standing on Ganymede's surface and looking up would see a red glow, Saur says.

Ganymede has two auroras, one around its north pole and one around its south pole, both created in part by the moon's own magnetic field. These auroras don't stay fixed in place. Instead, they wander slightly across Ganymede's face. With the help of supercomputers, the scientists calculated how much Ganymede's auroras would shift if the moon had a salty sea. A layer of salty water could carry electrical current, generating another magnetic field that would affect the auroras.

Illustration of the interior of Ganymede, Jupiter's largest moon.(Photo: NASA)

The researchers found that the aurora shift witnessed by Hubble nicely matched the prediction of what should happen if Ganymede has an ocean. Just as importantly, the Hubble data did not match the prediction for an ocean-less Ganymede, the scientists reported online last month in the Journal of Geophysical Research.

Ganymede's ocean is sandwiched between two layers of ice. That's not particularly hospitable to life, says planetary scientist William McKinnon of Washington University in St. Louis, who didn't work on the new study. But Saur says it's still possible that Ganymede's waters are habitable.

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Hubble telescope spots ocean on Jupiter moon Ganymede