NASA's Saturn-orbiting Cassini spacecraft has imaged something peculiar on the outermost edge of the gas giant's A-ring. A bright knot, or arc, has been spotted, 20 percent brighter than the surrounding ring material and astronomers are interpreting it as a gravitational disturbance caused by a tiny moon.
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"We have not seen anything like this before," said Carl Murray of Queen Mary University of London. "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."
The feature in the ring's edge is approximately 750 miles (1,200 kilometers) long and 6 miles (10 kilometers) wide, but the possible baby moon is likely only half a mile across if it's confirmed to exist. The observation was made on April 15, 2013, using Cassini's narrow angle camera and the discovery was announced in a paper published today in the journal Icarus.
Saturn's moons are icy little worlds and many are thought to have been formed through an agglomeration of material from the extensive ring system that surrounds the planet to this day.
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Although there's the exciting possibility of this new object being the birth of a small satellite, it could also be signs of a small overlooked moon that's in the process of breaking up. But astronomers are hopeful that this might be the first time we've witnessed a baby moon in the process of forming in the solar system. The candidate moon is being nicknamed "Peggy."
"Witnessing the possible birth of a tiny moon is an exciting, unexpected event," said Cassini Project Scientist Linda Spilker, of NASA's Jet Propulsion Laboratory in Pasadena, Calif.
Generally, Saturn's moons get larger the further their orbits from the planet; over the system of satellites' history, moons likely migrated from close to the planet, scooping up smaller moons as they drifted further away. As a consequence, Saturn's rings are depleted of moon-forming material, potentially signalling that Peggy will be one of the last moons to form.
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"The theory holds that Saturn long ago had a much more massive ring system capable of giving birth to larger moons," said Murray. "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."
In 2016, Cassini's orbit will take is closer to the outside edge of the A-ring, hopefully giving us an opportunity to glimpse a tiny moon in the process of growing.