Why does Jupiter's moon Callisto have unusual jagged hills? This mystery came to light after the robot spacecraft Galileo , in orbit around Jupiter since 1995, swooped past the dark moon in May. The resulting pictures were the highest resolution yet obtained for a Jovian moon : objects as small as 3 meters across are discernable. The strange landscapes pictured above show areas rich in bright sharp mounds about 100 meters tall. A likely formation hypothesis holds that these hills are the result of ejecta thrown billions of years ago during a violent impact . The lower inset region apparently has undergone an epoch of relatively high ice- erosion , where dark rock has filled in some of the inter-hill regions. NASA has recently cleared Galileo to continue swooping Jupiter's moons until 2003, when it will end its journey with a spectacular dive into Jupiter's atmosphere .
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Credit: Galileo Project , Arizona State University , JPL , NASA