Cassini spots giant snowballs in Saturn's rings

Posted by Emma Woollacott

New pictures from NASA’s Cassini spacecraft show 12-mile-wide snowballs forming in one of Saturn's rings.

The images show the ring's icy particles clumping into giant snowballs in the F ring as the moon Prometheus swings by.

Scientists speculate that the process is probably similar to the way the planets may have formed from dust orbiting around the Sun.

"Astronomers have never seen objects actually form in space before," said Professor Carl Murray, a Cassini imaging team member from Queen Mary, University of London. "We now have direct evidence of that process and the rowdy dance between the moons and bits of space debris."

The gravitational pull of Prometheus, the larger of the F ring's  two 'shepherding' moons, sloshes ring particles around, creating waves that trigger the formation of giant snowballs. Because it cruises around Saturn slightly faster than the much smaller F ring particles, it stirs up particles in the same segment once in about every 68 days.

"Some of these objects will get ripped apart the next time Prometheus whips around - but some escape," Professor Murray said.

"Every time they survive an encounter, they can grow and become more and more stable eventually becoming dense enough to have what we call 'self-gravity'. That means they can attract more particles to themselves and, er, snowball as more ring particles join them."

"This new analysis fills in some blanks in our solar system’s history, giving us clues about how it transformed from floating bits of dust to dense bodies," said Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory in California. "The F ring peels back some of the mystery and continues to surprise us."