Best known for its stunning rings, Saturn also boasts a fleet of 62 moons—ranging from giant Titan, which is larger than the planet Mercury, to one as small as the ocean liner Titanic. Now astronomers may be witnessing something they have never seen before: the birth of a moon out of the same rings that make Saturn such a spectacle.
“The discovery was accidental,” says Carl Murray, a planetary scientist at Queen Mary, University of London. In April 2013 he was examining fresh images of Saturn's moons taken by the Cassini spacecraft when he noticed a bright feature more than 1,000 kilometers long at the edge of the A ring, the outermost of Saturn's three main rings.
The bright spot may signal the presence of a new moon struggling to arise, Murray and his colleagues speculate in the July 1 issue of the journal Icarus. The new moon is less than a kilometer across, too small to be seen—until something hit the moon last year and produced the flare-up that attracted his attention. The A ring is ripe for spawning new moons because the gravitational influence of a larger satellite, Janus, located beyond the ring causes particles at the ring's outer edge to congregate. If the particles form a big enough clump, their own gravity should then attract more material, eventually creating a moon. Murray thinks this one formed recently but does not know whether it was born just a few years ago or millions of years earlier.
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Jeff Cuzzi of the nasa Ames Research Center, who was not involved in the discovery, has little doubt that Murray has found a nascent moon. The more important question now, he says, regards the moon's fate: Will it successfully migrate out of the rings to take its place among the established moons, or will it disintegrate?
The fledgling moon still faces some challenges. Because it is presumably made of water ice, like Saturn's rings, impacts from meteoroids could pulverize it in the next few million years.
Murray hopes to catch a direct sighting in 2016, when Cassini skirts close enough to the A ring to capture a good image. Saturn's ring system resembles a young star's protoplanetary disk in that each is flat and revolves around a massive object. The moon's creation could yield insight into how planets are born. Saturn's rings may therefore offer a microcosm of newborn solar systems throughout the galaxy.