erfully wrenched. Days before Voyager reached Jupiter, Science published the predictions of Peale et al. of prolific volcanism on Io. Indeed, the first close-ups of Io showed its colorful, crater-free landscape dominated by sulfurous deposits from unending eruptions of numerous volcanoes.
Almost lost amid public excitement about Io's volcanic plumes were Voyager's mysterious views of Europa. Europa was least favorably situated for close-up imaging, so even the best pictures barely exceeded our own Moon as viewed through powerful binoculars. The smooth, ice-covered world was criss-crossed by enigmatic lines -- most of them straight, but some forming strange curlicue shapes. Europa's secrets would have to await Galileo's much higher-resolution investigations, some 14 years later.
Meanwhile, researchers continued to speculate about Europa. Though the tidal pulling and tugging on Europa's interior is much less than for Io, the rate of heating is hardly trivial. Given other evidence that Europa contains abundant water, tidal heat just might keep the water liquid. Beneath its icy crust, Europa could have an ocean. So, unexpectedly, the solar system has a second potentially habitable zone -- out near Jupiter.
Europa reveals her mysteries
As Galileo approached Jupiter in 1995, some researchers were expecting a Europan "ocean;" some even had the temerity to discuss possible life ("fish") in the Europan seas. Others calculated that the tidal heat would be lost too rapidly to sustain high enough temperatures for liquid water. We simply knew too little about Europa, and the processes within it, to decide. Galileo's engineers and scientists had to make tough choices about what to study during the spacecraft's orbital tour, because the main antenna's failure to deploy left a shrunken radio "pipeline" for sending data back to Earth. Europa was given priority.
What we have seen of Europa during the first 15 Galileo orbits (11 in the prime orbital tour) has been stunning, if only to exemplify the artistry of nature. Like Jack Frost's windowpane creations writ large on Europa's surface, an amazing variety of shapes, forms, and patterns bedazzles the eye (see accompanying pictures). How the topographic shapes were actually formed, however, tests the interpretive powers of geomorphologists. And implications for the geophysics of Europa's interior are yet more difficult to fathom. Just as the tastes and textures of an exotic fruit are unknowable from the form of its skin, so scientists cannot know what lies beneath the intricate, low-relief topography of Europa. But our curiosity is whetted, so we try.
Galileo's low-sun pictures exaggerate the apparent relief. Actual ridge heights rarely exceed a couple hundred meters. Most of Europa's topography is more like Kansas than the Appalachians. Polar researchers look at Europa and perceive familiar arctic landscapes of shifting ice plates. Some analogies break down in detail, but many Galileo geologists believe that we are observing a thin ice crust floating on top of liquid water2. Indeed, water may have reached the surface locally, forming a once-again-frozen "pond." Analysis of small cracks adjacent to large topographic loads implies a layer of brittle ice, less than 1 kilometer thick, above Europa's supposed ocean.
The ice is much thinner where it has obviously foundered. Nowhere is the crustal disruption more dramatic than in the chaotic zones of Conamara Chaos, first imaged during Galileo's sixth Jupiter orbit. Here, ice blocks several kilometers across, bearing the ubiquitous ridges characteristic of Europa, have broken off and floated; they seem to have twisted, tipped, and even foundered in a rough matrix, perhaps the once-again-frozen ocean that was temporarily exposed at Europa's surface.
Yet the conclusion that Europa does indeed have an ocean close to the surface is not entirely secure. It is notoriously difficult to judge rates from images of static geological formations. (Consider glaciers flowing viscously down high mountain valleys on Earth.) Does the brittle ice rest atop liquid 