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 of asteroids and comets strike other planets in the inner solar system, the greater densities of craters on them suggest that their surfaces are rather old compared with the Earth's. Geological activity on Venus generally ceased about half a billion years ago; Mars and Mercury seem to be older still.
The same principle can be applied to Jupiter's moons. Ganymede and especially Callisto -- which is literally saturated with craters -- are old compared with crater-free Io and compared with Europa. But just how old, in absolute terms? We cannot compare directly with the crater densities on terrestrial planets because Jupiter is struck by tens of times as many comets as it is struck by asteroids (asteroids dominate in the inner solar system. The gravity of Jupiter, which has scattered comets away from the inner solar system, has thus protected us from what might otherwise have been a continuing, life-threatening cometary bombardment.)
Gene and Carolyn Shoemaker, co-discoverers of the famous Shoemaker-Levy 9 comet that impacted Jupiter in July 1994 (Gene Shoemaker tragically died last year in an automobile crash on a lonely track crossing the Australian Outback while researching terrestrial craters), led the telescopic survey of comets in near-Jupiter space, defining the current impact rate. Other scientists, such as SwRI's Dr. Hal Levison, have calculated the dynamical evolution of comets, moving in toward Jupiter from their place of origin in the Kuiper Belt beyond the orbit of Neptune. Evidently, a crater 20 km diameter or larger is formed on Europa about every million years. So far there seem to be fewer than 10 such craters on Europa. If so, its surface averages just 10 million years old -- and much younger still in the nearly crater-free chaotic regions.
Only two years ago, Gene Shoemaker had concluded from the first Galileo images of Europa that Europa's surface might be a billion years old. Work at SwRI demonstrated, however, that the 5- to 10-km "pits" visible were really depressions formed locally where the i