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July 2, 2003: Blistering-hot volcanoes that belch snow. Moons bigger than planets. Icy worlds with vast underground oceans. All of these things can be found in the latest Harry Potter novel. And according to NASA space probes, they're all real.
It was late one night at Hogwarts when Harry, Ron and Hermione were doing their homework: "a long and difficult essay about Jupiter's moons," but Harry and Ron didn't have their facts straight.
Right: A family portrait of Jupiter and its largest moons, from top to bottom: Io, Europa, Ganymede, Callisto. [more]
"Harry, you must have misheard Professor Sinistra," says Hermione on page 300 of Harry Potter and the Order of the Phoenix. "Europa's covered in ice, not mice!"
Correct. Jupiter's moon Europa is way too cold for mice: 260° F below zero. Spacecraft have taken pictures of Europa's icy surface, and it looks totally lifeless.
Underground, however, might be a different matter. Some scientists think the ice on Europa hides the biggest ocean in the solar system -- bigger than the Pacific, Atlantic and Indian oceans combined. Here on Earth water and life seem to go together. Could there be life in the waters of Europa? Microbes? Alien fish? Swimming mice? No one knows -- not even Hermione.
"And it's Io that's got the volcanoes," she says on page 295, correcting Ron's essay.
Right again. Io is even weirder than Europa. Some people say Io, dotted with volcanoes, looks like a pepperoni pizza, and that's about right. Io has more pepperoni-colored volcanoes than Ron Weasley has freckles. At any given moment, dozens of them might be active, spewing the hottest lava in the solar system. The plumes rise 300 miles into space where it's so cold that volcanic ash freezes before it falls back to the ground--sulfurous snow. NASA's Galileo spacecraft has actually flown through some of these plumes and survived.
"Jupiter's biggest moon is Ganymede, not Callisto," Hermione adds, pointing over Ron's shoulder at another mistake.
Indeed, Ganymede is the largest moon in the whole solar system. It's slightly wider than the planet Mercury and more than three-quarters the size of the planet Mars. If it orbited the sun instead of Jupiter, Ganymede would surely be considered a planet. Heavily cratered Callisto is only a little smaller than Ganymede and, like Europa, might be hiding a subterranean ocean.
These four wonderful moons -- Io, Europa, Ganymede and Callisto -- are real. They were discovered by Galileo Galilei in 1610 when he looked at Jupiter through one of the first primitive telescopes. Galileo was amazed by the four little stars he saw near the giant planet, and even more amazed when they moved from night to night, orbiting Jupiter. Astronomers now call them the Galilean satellites.
Right: An excerpt from Galileo's 400-year-old manuscripts in which he sketched Jupiter's moons changing positions from night to night. [more]
Almost everything known about the Galilean satellites -- other than their number, four, and the basic shapes and sizes of their orbits -- comes from NASA spacecraft, especially the two Voyagers, which flew by Jupiter in 1979, and the Galileo space probe orbiting Jupiter now.
It's good to know that these missions have been closely followed at Hogwarts.