A new analysis of data from the Cassini spacecraft indicates that Saturn's highly cratered moon Hyperion is truly spongelike, filled with small pores that may take up 40 percent or more of its volume. Repeated Cassini flybys allowed researchers to gauge the moon's density and composition, which they report is probably a mix of ice and carbon-based molecules.

High porosity would explain the profusion of craters on Hyperion, the largest of Saturn's irregularly shaped moons, says planetary scientist Peter Thomas of Cornell University, co-author of one of two papers describing the findings online today in Nature."Impacting into more porous targets means there's less debris flying around to cover them up," he says of the craters.

High-resolution images of Hyperion, snapped by Cassini from a distance of about 500 kilometers (311 miles) in September 2005, documented a richly pockmarked surface in contrast to other Saturnian moons such as Phoebe, which is similar in size but has craters more like those found on Earth's moon.

To figure out the reason for all the craters, Thomas and his colleagues from NASA and other institutions combined the snapshots with visual, ultraviolet and infrared spectra as well as other data captured during three more distant flybys in 2005 and early 2006.

One team calculated Hyperion's volume by comparing views from different angles and gauged its mass from the drag Cassini experienced as it flew past, resulting in a measure of the moon's density. The high reflectivity of the moon's surface is consistent with ice, according to a second team, whereas smaller craters show darker spots that may represent the presence of carbon dioxide and larger carbon-rich molecules.

If made of ice, which is a major component of Saturn's rings and some of its satellites, Hyperion would be about 40 percent porous, Thomas says. "It's just a reminder," he adds, "that you can get odd-looking things by combinations of what are probably ordinary processes."