While many of his colleagues have been struggling to understand Mars, Larry Esposito of the University of Colorado has been pondering another planetary enigma: Saturn and its spectacular system of rings. Astronomers have calculated that the rings are less than a billion years old, mysteriously young compared to the 4.5-billion-year age of the solar system. The seeming implication is that the rings exist only because of a relatively recent cataclysm. Now, a computer model designed by Esposito shows that the age discrepancy may be the result of a cycling process that keeps renewing the rings.
Saturn’s rings consist of small chunks of ice and rock that constantly collide, breaking into dust that is light enough to drift away. Esposito’s model, which draws on work by former graduate students Joshua Colwell, Robin Canup, and John Barbara, takes into account tiny moonlets within the rings that have enough of a gravitational attraction to hold on to the particles, keeping them in the system. “A one-centimeter piece of ice can’t combine with another one-centimeter piece of ice, but a one-kilometer moon can easily pick up a one-centimeter piece of ice,” Esposito says. Collisions between the moonlets and smaller bits of debris in the rings knock new material loose from the moonlets, keeping the recycling process going, he concludes: “Individual rings survive only a few million years, but the system lasts indefinitely.”
Eposito hopes to confirm this theory when the Cassini probe arrives at Saturn in July and scrutinizes the planet’s narrow, outlying F ring. He expects to find small moons inside or between the rings; according to his model, those bodies should be balls of rubble, not solid objects. If they show up, that will bolster the case for a similar cycle that maintains thin rings around Jupiter, Uranus, and Neptune—and quite possibly around the planets of many other stars as well.
