ce 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 dilemma is whether we'll be able to reach Europa's oceans with modern technology: Is there near-surface access, or is this ocean tens of kilometers down? Solving that mystery, however, must await the next spacecraft mission to this remarkable Jovian moon.
1. Peale, S. J., P. Cassen, and R. T. Reynolds. Melting of Io by tidal dissipation. Science, 203:892-894, 1979.
2. Carr, M.H., M.J.S. Belton, C.R. Chapman and 19 others. Evidence for a subsurface ocean on Europa. Nature, 391:363, 1998.
Published in the Fall 1998 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.