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Saturn orbiter may have spotted the birth of a new moon

NASA's Cassini spacecraft, currently in orbit around Saturn, captured something interesting going on at the edge of the planet's primary ring, and scientists believe it could be the formation of a new moon.

The bright area circled in the above image, which is at the outer edge of Saturn's A-ring, was spotted in data collected by Cassini a year ago, on April 15, 2013, and reported on NASA's JPL website on Monday. This roughly 1,200 kilometre-long, 10 kilometre-wide 'disturbance' is thought to be a collection of icy objects from the ring that are being affected by the gravity of a larger object, perhaps 1 kilometre in diameter. Scientists have nicknamed this object Peggy.

"We have not seen anything like this before," said lead researcher Carl Murray, of Queen Mary University of London, according to the NASA press 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."

Saturn's other moons formed long before we even knew that Saturn was a planet that had moons, so if this is what we're seeing now, this discovery is a really special event. It would show that moon formation hasn't ended around Saturn, and it also would confirm some of the theoretical work on Saturn — specifically the idea that the planet's ring system was once much, much wider, extending out past the orbit of Titan, and that it held much larger objects, which went into forming the other moons. As those moons formed and swept up the material around them, it would have caused the rings to shrink to their current size, leaving the moons orbiting in the clear. This would explain why Saturn's larger moons all exist outside the ring system, with the largest of them tending to be further out.

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Since the size of the pixels in this Cassini image are roughly 7 km on a side, as it's just one small section of a much larger image taken by the spacecraft, there's no way to actually see Peggy right now. In fact, if Cassini hadn't caught this image of it, we may have completely missed it. Once it migrated beyond the outer edge of the ring, it would be far too small to pick up on imagery, and it may even be too small to cause much of a disturbance in the edge of the ring as it orbited (like the shepherd moons do). It might not even survive, as interactions with ring particles may end up pulling it apart.

Perhaps as NASA starts Cassini on the last phase of its mission, which will have the satellite making several orbits past Saturn's F-ring — just beyond the edge of the A-ring — they can pick up more images of Peggy, to see what it looks like. Prof. Murray and his colleagues are particularly interested in confirming whether or not the older, bigger moons nearby play a roll in the birth of new moons.

Prof. Murray presented his findings at the American Geophysical Union conference in Dec. 2013, and the research on this discovery was published in this week's print edition of the journal Icarus.

(Images courtesy: NASA/JPL-Caltech/Space Science Institute)

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