Hints that Jupiter's moon Ganymede, the largest moon in the solar system, has a subsurface ocean date back more than 40 years, but it took the Hubble Space Telescope and some clever scientific detective work by to come up with the proof.
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Scientists made a pair of seven-hour long ultraviolet observations of Ganymede, with the goal of capturing the moon's brightly glowing aurora. The moon, which is larger than the planet Mercury, has been known to have a magnetic field, a discovery made by NASA's now-defunct Galileo spacecraft. The moon also is embedded in the much larger magnetic field of its parent planet, Jupiter.
With Hubble, scientists were able to watch how Ganymede's belts of aurora rocked to and fro as Jupiter, and its magnetic field, rotated. They discovered that another magnetic force was counteracting the impact of Jupiter's.
Computer models came up with a match: a subsurface, salt-water (hence, electrically conductive) ocean - and a big one at that, with more water than all the water on the surface of Earth. More than 100 other potential scenarios were ruled out, said lead researcher Joachim Saur, with the University of Cologne in Germany, who came up with the idea of using Hubble to learn more about the inner life of Ganymede.
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The discovery opens yet-another niche potentially friendly to life, and a new technique for using telescopes to peer inside planet.
"It's an astounding demonstration," Jim Green, NASA's planetary science division director, told reporters on a conference call Thursday.
Without an ocean, Ganymede's aurora would rock 6 degrees because of Jupiter's magnetic pull. Instead, it shifts by just 2 degrees.
Scientists extrapolated from the finding to estimate that the ocean is 60 miles thick - 10 times deeper than Earth's oceans - and buried under some 95 miles of ice.