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 ice had collapsed due to internal geological activity. They were not impact craters, after all, and so should not be counted in calculating surface age. These inferences have been verified by subsequent, closer pictures of Europa's amazing complex of internally caused pits, domes, and moats.
Real impact craters are rare on Europa. One of the largest is named Pwyll. About 26 km across, it is surrounded by dark, reddish-brown materials dredged up from below when the comet struck. A dramatic pattern of bright radial rays emanating from Pwyll stretches across many hundreds of kilometers, reminiscent of rays (visible through good binoculars) surrounding the lunar crater Tycho. One Pwyll ray is draped across part of the Conamara Chaos region. Evidently Pwyll, which is the youngest of the large Europan craters and thus may have been formed roughly a million years ago, postdates much of Conamara Chaos. Yet other parts of the Chaos may encroach on the ray. Perhaps the broken, floating blocks of ice were active only a million years ago.
Large craters like Pwyll are far too sparse to use for relative age-dating of the restricted localities of most recent activity on Europa. So we wish to count the more frequent, much smaller craters. But our studies at SwRI have shown that most of the small craters are, actually, not due to impacts of small comets on Europa but are instead "secondary craters" created by material ejected from the few larger craters. Like the pits, they can't be counted as primary comet impacts. It appears that small comets must be quite rare. And the most recent activity we've seen on Europa happened only a few hundred thousand years ago. Regions not yet imaged by Galileo may be younger still.
Indeed, Europa's geological activity may be continuously, if episodically, ongoing: somewhere Europan ice is cracking even as you read this article. Far from being a frozen tableau, Europa is still warmed by Jupiter's tides and remains active. Very likely, it has a watery ocean and perhaps life today. The remaining dile