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Solar system: Saturn moons could be young.

Planetary scientists suspect that some of Saturn's moons may be only 100 million years old.

Saturn waltzes through space surrounded by an entourage of 62 moons. Alongside its lone gallant, Titan, orbit an assortment of icy, midsize moons, their diameters ranging from Mimas's 400 km (250 miles) to Rhea's 1,525 km (948 miles). Together, they have a mere V20 as much mass as Titan does. Interspersed are a vast collection of moonlets.

In terms of numbers, Saturn's retinue matches that of the king of the planets, Jupiter, whose current satellite tally is 67. Yet Jupiter boasts a more glamorous court with its four large Galilean moons, and it doesn't have any midsize moons.

Planetary scientists have long wondered why Jupiter has come out so much the winner when it comes to big moons. In 2013, for example, Erik Asphaug (Arizona State University) and Andreas Reufer (then at University of Bern, Switzerland) suggested that Saturn started with a Galilean system of its own, but the moons crashed into and obliterated one another. The rubble then coalesced into Titan and the midsize moons.

A new paper explores the collision idea further. Matija Cuk (SETI Institute) and his colleagues turned back time by simulating the icy moons' orbital evolution and figuring out when and how they could have interacted, given their current locations and orbits. The team found that the moons can't have migrated much from where they first formed. But that doesn't make sense if they've been orbiting the planet from the solar system's early days: Saturn's tides are just too tenacious--the moon Enceladus and its tidal-triggered geysers confirm that.

Instead, the team argues, the midsize moons can only be about 100 million years old. The moons formed from a ring of debris, born from the collisions of large precursor moons, the team suggests in the April 1st Astrophysical Journal. (The researchers exclude Titan from the list of phoenix moons.)

Cuk's team offers a test: craters. If the midsize moons are indeed young, they won't have had time to build up a perfectly homogenous peppering of pockmarks. Instead, craters on moons within Titan's orbit (all the midsize moons but Iapetus) would concentrate around those satellites' equators, because the moons would have been exposed to a lot of debris in the plane of their birth ring but not nearly as much from other angles. Many of the moons are heavily scarred, so it'll take dedicated work to tease out whether the moons have these crater girdles.

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Title Annotation:News Notes
Author:Carlisle, Camille M.
Publication:Sky & Telescope
Date:Aug 1, 2016
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