Riddles on Titan.

Two puzzles have emerged from the Cassini spacecraft's first close flyby of Saturn's hydrocarbon-shrouded moon Titan (SN: 11/06/04, p. 291). Radar images from the Oct. 26 passage, which recorded just 1 percent of the moon's surface, show no obvious sign of craters. That's a surprise because Titan, the solar system's second-largest moon after Jupiter's Ganymede, is likely to have been pummeled by debris roaming the outer solar system.

One explanation is that new craters have been buried by hydrocarbon snowfall, suggests Cassini researcher Jonathan I. Lunine of the University of Arizona in Tucson. Eruptions of icy volcanoes might also have given Titan a facelift. Either way, he notes, the lack of obvious craters adds to the evidence that Titan today is a geologically active place.

Scientists are also intrigued by new measurements of Titan's atmosphere. Cassini's ion-and-neutral-mass spectrometer found that nitrogen's heavier stable isotope is considerably more abundant than the element's lighter one. Since lighter isotopes should escape into space from the top of Titan's nitrogen-rich atmosphere more easily than heavier ones do, this result makes sense. But carbon in Titan's atmosphere, as measured in methane, shows no such division between the lighter and heavier isotopes, notes Toby C. Owen of the University of Hawaii in Honolulu. To reconcile the disparity, Owen conjectures that some process, perhaps the evaporation of liquid or solid hydrocarbons on the surface, could be replenishing the carbon. --R.C