Jupiter's large moon Europa is a captivating enigma: smooth as a billiard ball, almost as white as snow, criss-crossed by a peculiar network of brown fractures. A set of images from the Galileo spacecraft now orbiting Jupiter add to Europa's allure. They depict a world of complex ice geology and--most exciting of all--bolster the evidence that the surface ice may cover a global ocean tens of kilometers deep. Some scientists have even proposed that Europa's buried ocean could harbor life--though such ideas remain wildly speculative at this point.
The most dramatic of the new images (above) shows a wide swath of Europa's equatorial region. The cluttered pattern of streaks and cracks looks like nothing so much as a huge Jackson Pollock painting (although, as yet, nobody has suggested that it is the work of alien abstract artists!). Scientists believe we are looking at fractures that divided Europa's frozen crust into vast plates of ice, as large as 30 kilometers across; the voids between the plates filled with darker water or slush from below and then refroze. These formations indicate that Europa once had a warm interior, but they do not prove that a global ocean still lurks beneath the ice.
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Further clues about Europa's inner nature come from another Galileo image which shows a large impact crater--one of the few large ones visible on Europa's smooth surface. The crater seems to have thrown a layer of bright, fresh ice onto a darkened region. A dark band in this image displays a light streak down the middle, possibly produced by ancient geysers.
The Galileo imaging team has assembled a mosaic image of Europa that gives a sense of the extraordinary diversity of the satellite's terrain. Some circular depressions, probably impact craters, can be seen close to the day-night line, or terminator. The overall lack of such craters implies that Europa's surface is young and active; ongoing eruptions of slush or water volcanoes may be renewing the landscape.
Impressive though they are, these images offer but a taste of things to come. The smallest details visible here are about two kilometers across. During three subsequent encounters, Galileo will approach so close to Europa that it will be able to spot features a mere 11 meters across. Will we find definitive evidence of a warm (and, just possibly, life-bearing) ocean under the convoluted ice floes? Stay tuned: the next close approach is on December 19.
