Astronomers may have just witnessed the birth of a small, icy moon within the rings of Saturn. And since it could be Saturn’s last natural satellite, this is a particularly rare and precious opportunity for us to learn about the formation of the planet’s many moons, including the cloud-wrapped Titan and the ocean-holding Enceladus.
Exactly one year ago, a narrow angle camera on NASA's Cassini spacecraft accidentally recorded disturbances at the very edge of Saturn's A ring, the outermost of the rings. One of these disturbances is an arc -- about 1,200 kilometers long and 10 kilometers wide -- that was about 20 percent brighter than its surroundings. There’s also evidence of “unusual protuberances” in the usually smooth profile at the ring's edge.
Researchers think the arc and protuberances are caused by the gravitational effects of a nearby object. "We have not seen anything like this before," Carl Murray from Queen Mary University of London says in a news release. "We may be looking at the act of birth, where this object is just leaving the rings and heading off to be a moon in its own right."
Nicknamed Peggy after Murray’s mother-in-law, the icy object is probably no more than a kilometer in diameter, making it too small to be seen in images right now. What researchers have seen (and pictured above) is evidence of commotion at the ring’s edge believed to be caused by Peggy’s presence.
Saturn has 53 official moons and 9 provisional ones. They generally range in size depending on their proximity to the planet: The farther from the planet, the larger. And since many of the planet’s moons are composed primarily of ice -- like the particles that form the rings -- researchers believe that the icy moons formed from ring particles and then moved outward. The oldest moons formed when the rings were more substantial, coalescing and becoming larger as they merged with other moons on the way; these likely drifted into orbits farther away from the planet. “The theory holds that Saturn long ago had a much more massive ring system capable of giving birth to larger moons," Murray says. "As the moons formed near the edge, they depleted the rings and evolved, so the ones that formed earliest are the largest and the farthest out."
Younger moons, like this new one, tended to be smaller and stayed closer. It’s possible the process of moon formation in Saturn's rings ends with Peggy, since the rings are probably too depleted now to make more moons. The researchers don’t expect Peggy to grow any bigger, and actually, they think it’s falling apart.
Cassini’s orbit will move closer to the outer edge of the A ring in late 2016, giving researchers a closer look at the baby moon, and perhaps an opportunity to image it. The findings might also offer insight into how Earth and other planets formed and migrated away from the sun.
The observations were published in Icarus this week.