Planets and moons are big and heavy. Even so, if they get a little top-heavy, they may just tip over.
Scientists had already found evidence of this tipping business on Mars and on Enceladus, one of the moons of Saturn. And now they've also seen it on Europa, one of the moons of Jupiter.
In all three cases, scientists suspect that lots of ice built up at the poles. The extra mass made the worlds unstable, so they tipped over on their sides. Yet they didn't spin on their sides -- the poles moved, too. It would be like Earth tipping so that the ice caps were along the equator, with the poles in the middle of the Pacific and Atlantic oceans.
On Europa, scientists found evidence of such a tip-over in pictures of large curved basins on the moon's surface. These depressions are near the equator today, but in the past they might have aligned with the poles. They suggest that when the ice at the poles grew thick enough, Europa's axis shifted by about 80 degrees -- about a quarter of the way around the big moon.
Jupiter appears near our own Moon tonight. It looks like a brilliant cream-colored star that's a little to the upper right of the Moon at nightfall. They scuttle low across the south during the night. Even through the lunar glare, binoculars should reveal Europa and Jupiter's three other big moons. They look like tiny stars lined up quite close to the planet -- worlds with their own intriguing stories.
Script by Damond Benningfield, Copyright 2008
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