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NASA’s Cassini may have witnessed the birth and death of a new Saturnian moon


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NASA’s Cassini satellite appears to have captured an incredibly rare photo that shows the birth of a new moon emerging from the rings of Saturn. The facts are a little hazy at the moment because we only have a handful of photos to work from, but there is some evidence to suggest that Cassini has actually spotted two new moons over the last couple of years, and that one of those moons has since been destroyed. Never in the history of humanity have we spotted the creation (or destruction) of a moon — and, more importantly (at least as far as science is concerned), this could tell us a lot about how Saturn’s larger moons formed over the last few billion years.

In the image above, captured by the Cassini orbiter in April last year, the actual target of the photo was Saturn’s moon Prometheus (the bright, elongated object in the middle). Upon further investigation, NASA astronomers spotted a bulge at the edge of Saturn’s A Ring (see enlarged image below). It is believed that this bulge and its tail is caused by the gravity of a small moon (which isn’t visible) pulling some of the ice particles out of the ring. [Research paper: doi: 10.1016/j.icarus.2014.03.024 - "The discovery and dynamical evolution of an object at the outer edge of Saturn’s A ring]


A bulge in Saturn’s A Ring, believed to be caused by a new moon called Peggy

This moon, informally called Peggy, probably formed from the icy particles of Saturn’s A Ring, and now occupies an orbit just outside the ring. That’s not all, though. Since Cassini has been orbiting Saturn for a few years, some astronomers have since gone back and looked at past photos to see exactly when Peggy formed. Phil Plait of Slate notes that the bulge is visible at least as early as January 2013 — and also, curiously, that for a few months in the middle of 2013, there were actually two bulges. Whether Saturn’s gravity or a collision cleaved Peggy in twain, or that two moons formed around the same time, we don’t know. Since the middle of 2013, though, the bulge caused by the second moon has disappeared, suggesting it has either disintegrated or moved too far away that its gravity no longer affects the icy particles (therefore making it invisible).


Saturn’s major rings

If you didn’t already know, Saturn has numerous rings, consisting mostly of small particles of ice and rock. The widest and densest rings are the A, B, and C Rings, with the other rings mostly being very diffuse and “dusty.” The gaps in the rings are believed to be caused by the gravity of Saturn’s moons — as they orbit, their gravity acts a bit like a “sweeper,” gathering up the icy particles, leaving an empty region of space. (Read: The Rose of Saturn: A massive hurricane that’s twice the width of Earth.)

Things are hotting up (figuratively) for the Solar System’s gas giants: Back in March we reported that NASA is being forced by the US government to plan a mission to Jupiter’s moon Europa, which many scientists believe to be the Solar Systems’ most likely location for extraterrestrial life. Then, only last week some scientists confirmed that Saturn’s sixth-largest moon Enceladus has a huge ocean of liquid water beneath its icy crust. And now we appear to be witnessing the birth of a new moon from Saturn’s rings.


One of Cassini’s most famous photos, taken from the shadow of Saturn. You should click to zoom in.

“The theory holds that Saturn long ago had a much more massive ring system capable of giving birth to larger moons,” says Carl Murray, who led the discovery of Saturn’s newest moonlet. ”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.” It is believed that the rings are now so depleted that Peggy, which is only around half a mile wide, was probably Saturn’s last and rather pathetic attempt at birthing a new moon. Oh how the giants fall.


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