It looks like a planet, it sounds like a planet...
December 12, 1996
Web posted at: 11:45 p.m. EST
(CNN) -- Jupiter's huge moon Ganymede doesn't just look like a planet, it also sounds like one, scientists revealed Thursday.
Scientists played audio recordings of electromagnetic activity as the Galileo spacecraft twice passed through a region of charged particles surrounding Ganymede.
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The waves were picked up by instruments on Galileo, digitized, then transmitted back to Earth, where they are played back at one-tenth speed to make it audible.
The soaring whistle and hissing static provide further evidence that Ganymede -- the largest moon in the solar system -- has its own planet-like, magnetic cocoon called a magnetosphere.
Such a region of hot, ionized gases and highly charged particles has never before been found around a moon. The noise released Thursday is consistent with magnetic fields scientists have detected around Earth, Saturn and Jupiter.
"This is the first confirmed magnetic field of a moon in the solar system," said UCLA scientist Margaret Kivelson.
Over the past seven months, Galileo has explored each of Jupiter's four largest moons. According to its findings, Io and Europa are made of rock, while Ganymede and Callisto are icy -- 60 percent rock and 40 percent ice.
But as scientists understand better what's inside these moons, Ganymede looks more like Io. They both have iron cores.
So while on the surface Io and Ganymede are very different objects, their interiors are quite similar.
Ganymede, three-quarters the size of Mars at 3,269 miles across, has ridges, icy grooves and craters that hint at an Earthlike crust that pulls apart and fills in with flowing rock.
NASA also is preparing for next Thursday's encounter with Jupiter's moon Europa, suspected of hiding a frozen ocean and possibly life beneath its fractured, icy crust.
The unmanned Galileo spacecraft, now making a two-year tour of Jupiter and its largest moons, will come within just 433 miles of Europa, close enough to take ultra-high resolution pictures.
Correspondent Ann Kellan contributed to this report
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