Comet Tempel 1, the target of NASA's Deep Impact probe back in July, has offered scientists a stunning view of cometary topography. The first published results from the mission team will appear in the September 9 issue of Science and have been released at the Division of Planetary Sciences meeting at Cambridge. Among other things, Tempel 1 is the first comet to demonstrate impact craters, an indication of collisions with various space debris over the aeons. The craters range from 40 to 400 meters across, but are they common to all comets? Those we've had good views of, including Borelly and Wild 2, show significant differences in topography and shape. Of Tempel 1, we can only say that its lifetime has been complex. "This comet is a geologic wonder," said Peter Schultz, a professor of geological sciences at Brown University and a co-investigator on the mission team. "There are smooth surfaces, filled-in craters, ridges, cliffs. Tempel 1 also features an area marked by innumerable...

Saturn's moon Enceladus is back in the news at the Division of Planetary Sciences meeting at Cambridge University this week. Not that it has ever quite left the spotlight since 1981; that's when Voyager 2 photographs told scientists that parts of the moon had been geologically active as recently as 100 million years ago. The moon's smooth terrain was hard to explain -- how does an object 314 miles across get hot enough to melt? Then Cassini came and Enceladus' wonders increased. We now know that the moon has an atmosphere of water vapor, nitrogen, carbon dioxide and other organic (i.e., carbon-based) molecules concentrated at its south pole. Moreover, that polar region is hotter than expected, -183 degrees Celsius vs. -203 Celsius as predicted by the models, and is marked by 80-mile long parallel cracks that vent vapor and ice particles. Some of this material may have crystallized on the surface as recently as the past decade. At the Cambridge meeting, Robert H. Brown (University of...