Saturn, like all of the outer planets, has many moons orbiting around it. Saturn also has an extensive set of icy moonlets forming its gorgeous rings. Although the age of Saturn's rings has been under debate, you might at least think that the moons formed with the rest of the solar system 4.5 billion years ago. Not so, and a new simulation shows how some of the moons formed much more recently.
Five of Saturn's 61 moons, Atlas, Janus, Pandora, Prometheus, and Epimetheus, may be as young as 10 million years old. That may seem really old to you, but astronomy of deals in billions of years. Three of these, Atlas, Prometheus, and Pandora, are shepherd moons, meaning they help shape and sculpt the rings that they are near, as shown in countless gorgeous Cassini images. Janus and Epimetheus orbit incredibly close to each other, and actually swap orbits every four years!
A few lines of evidence pointed to these moons being out of the ordinary. These rings moons are tiny and in a turbulent environment, and should have been destroyed by comets long ago. Also, Cassini probed the density of these moonlets, and found them not to be rocky and dense like asteroid, but less dense than water! Therefore, there may be another formation scenario to explain them.
The simulations reproduce the mass-distance distribution of the ring moons, shown here.
Alas, one has come along. In a simulation by astronomers at the Université Paris Diderot in Paris, France and at the University of Cambridge in the UK, some of the material in Saturn's rings outside of the Roche limit could begin to clump and aggregate and form little moons like the ones in question. The Roche limit is the minimum distance that a group of particles must be from a planet in order to clump together. Too close to the planet, and the tidal forces will ensure that a solid body never forms and the particles stretch out into a ring.
These baby moons may move from their birthplace to settle in a new region later. If such a process formed the rings moons we see today, then Saturn may be producing new moons right now. Lest you think that the Universe is stable and unchanging, don't forget that slow changes might be going on right under your nose.
Top Image: Prometheus creating streamers in a complicated gravitational dance with the F-ring. Credit: Cassini Imaging Team, ISS, JPL, ESA, NASA.
Bottom Image Credit: Cassini, NASA, JPL, SSI, Nature (doi:10.1038/nature09096)
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