A small moon's dash by the outer ring of Saturn is creating a trail of giant snowballs in its wake, the first time scientists have been able to watch celestial objects form.
The gravitational hands of Prometheus and partner moon Pandora sculpt the edges of the planet's outermost discernible ring, known as the F-ring.
Prometheus, larger and closer to Saturn, cruises a bit faster than the particles in the ring and is slightly inclined relative to the ring's plane. About every 68 days, Prometheus collides with the diffuse, inner edge of the F-ring where the moon's gravity is strong enough to pull streamers of particles from the ring, creating channels.
Over time, the disrupted particles — mostly dense, sticky ice — can take on a life of their own, clumping together under their own growing gravitational force.
Images from NASA's Saturn-orbiting Cassini spacecraft show shadows cast by Prometheus-sired objects as big as about 12 miles in diameter.
"We've never actually seen this before. You can see a real cause and effect. These objects didn't exist before Prometheus passed," lead researcher Carl Murray, with Queen Mary, University of London, told Discovery News.
Future visits by Prometheus could stoke a snowball's growth — or it could destroy it.
"Once Prometheus passes by, it gives these objects a kick. They have about 68 days to sort of get things together before Prometheus comes around again," Murray said.
"Whether at the end of the day you end up with a fully formed satellite we don't know. That's something we want to look at. Maybe these objects are precursors of fully formed satellites," he said.
The complicated physics in Saturn's ring — particularly the F-ring where the planet's tidal forces wane and objects can begin to form under their own gravitational sway — serve as a living laboratory for understanding how planets form, said Cassini project scientist Linda Spilker, with NASA's Jet Propulsion Laboratory in California.
"You can think of Saturn's rings as miniature versions of the disks where planets form. The same physical processes are occurring," Spiker told Discovery News.
The findings were presented this week at the Committee on Space Research meeting in Bremen, Germany. The research also is published online by the Astrophysical Journal Letters.