st'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 water, or does it instead cover slush or simply warmer ice? Some Galileo geologists believe that the surface topography reflects subsurface solid-state convection. Yet the heat that warms and mobilizes the ice -- simulating water -- implies that liquid water itself might exist still farther below, down perhaps 20 kilometers. That would devastate any hopes for near-term sampling of Europa's ocean. Conceivably the technology might be developed to break through tens or hundreds of meters of ice in a future mission to Europa, but 20 kilometers seems formidable. Soon, a new mission to Europa may fly sounding experiments to determine how close the water -- if indeed it exists -- really is to the surface. Perhaps thin spots exist, even if the ice is generally very thick. So far, Galileo has taken sharp pictures of only a tiny fraction of Europa's surface.
How old is Europa's surface?
Possibly Europa was once the thin-ice, watery world -- even teeming with life -- that we imagine or wish it to be, but then it subsequently cooled and froze and has remained unchanged for eons. In this view, Europa presents a tableau of a once-fascinating world, now dormant and long dead. Can we tell if Europa is still alive?
To answer this question SwRI researchers have focused on Europa's craters, or rather its virtual lack of them. Comets and asteroids crash onto planetary surfaces, leaving impact craters. Impact craters are rare only on those few worlds (like the Earth and Io) where rapid geological forces erode, cover up, or otherwise destroy craters soon after they form. Terrestrial craters have been dated, like the 50,000-year-old Meteor Crater in Arizona and the Chicxulub Crater in the Yucatan formed by the impact that rendered dinosaurs (and most other species, too) extinct 65 million years ago. Some lunar craters were dated through analysis of Apollo moon rocks. These crater ages agree with estimates, from telescopic surveys of Earth-approaching asteroids and comets, of the current bombardment rate. Since roughly the same numbers 