The (Not So) Sad Story of Peggy, Saturn’s Newest Moon

[Updated December 14, 2016]

Peggy, the small Saturn moon caught in the act of being born, is still alive.

First observed in 2013, Peggy is about 1.2 miles wide and lives near the edge of Saturn’s A ring. The tiny moonlet’s fate was an open question two years ago, when observations in 2014 did not reveal the same big, bright blip in the rings that originally betrayed Peggy’s presence. At the time, planetary scientist Carl Murray surmised that Peggy had either been gravitationally booted into the void, or had broken apart in a collision.

But in Cassini observations from 2015 and 2016, Peggy is back. It does appear as though the moonlet is not entirely intact, as a new chunk of something is now orbiting Saturn nearby. Murray suspects that Peggy did, in fact, collide with an unknown object in early 2015: The encounter pushed the tiny moon deeper into the disk of the A ring, created the other chunk (adorably called Peggy B), and produced a storm of icy particles that orbits in tandem with Peggy.

“Collision events have happened before—probably the discovery of Peggy itself was facilitated by a collision which produced a large glow,” Murray said on December 14 during a presentation at the American Geophysical Union’s meeting in San Francisco. “So this is not unheard of, but we’re getting these two very clear trends at the moment, and we’re trying to monitor them.”

[Original story, posted January 2015]

Often, the time frames on which celestial objects operate don’t conveniently fit into a human lifetime.

So, people were pretty excited last year when scientists announced they might have caught a new moon in the act of forming. Named Peggy, the newbie hiding in Saturn’s A ring had been spotted in images taken by NASA’s Cassini spacecraft in April 2013.

“You couldn’t miss it. It was a very bright, kind of extended object, somewhere near the edge of the ring,” says astronomer Carl Murray, of Queen Mary University of London. Murray first spotted Peggy, and he ended up naming it after his mother-in-law, who was celebrating her 88th birthday on the day of the discovery.

When astronomers pulled images from the previous year, they found hints of Peggy there as well. During a period of less than a year, Peggy appeared to be spiraling outward toward the edge of the A ring, and orbital projections suggested emergence might be imminent.

At the time, Peggy was just an anomalously bright, elongated smear near the ring’s edge, a blur presumably caused by a moonish clump embedded in the ring. No one knew what Peggy actually looked like or what would become of the fledgling, but scientists hoped Cassini might be able to see the roughly 1-kilometer-wide object during a 2016 observing campaign.

Then, in a presentation at the American Geophysical Union meeting last month, Cornell University’s Matthew Tiscareno casually mentioned that the situation with Peggy was “not as clear as you would like.”

Put bluntly, Peggy had most likely broken apart. Images shot near the middle of 2013 showed not just one moon-betraying smear in the rings, but two. And, as expected of recently fractured objects, the two smears were moving independently of one another.

By the end of 2013, one of the smears was missing. The remaining blur in the rings wasn’t nearly as bright as the original, and it was no longer moving outward. Instead, it was moving decidedly inward. Orbital migration within the rings can be random, Murray says, but this is also what would  happen if the missing object headed outward, thanks to the conservation of angular momentum.

Instead of catching a moon in the act of forming, scientists may have glimpsed a moon in the act of dying. “It may be that we saw the split of the object itself. That’s why it was so bright,” Murray says, referring to the original detection. Objects breaking apart in the ring plane will generate a lot of dust, shine more brightly, and be easier to spot.

Is it possible Peggy is still alive? Yes, but the observations showing two discrete clumps strongly suggest that Peggy no longer exists in its original form. Whether Peggy fledged and left Son of Peggy behind, or whether Peggy chucked a moonlet out into the big bad world is a mystery. It’s possible we’ll never know.

“It may be that the original Peggy has left the rings, in which case we’ll be hard-pressed to see it again. Unless something hits it,” Murray says.

Regardless, Murray and his colleagues will continue to keep an eye on the area and track the blur’s movement through the ring. Maybe, when Cassini swoops in for a close look at the A ring in a few years, the team will be able to write the rest of Peggy’s story.