Saturn Double Shot 1/2: Less-Than-Ancient Moons

It’s always funny explaining astronomical time to a non-scientist.  I often get the craziest looks when I mention a million years as being a ‘blip on the radar.’  Perhaps there is some immortal alien race out there who would understand how nothing much happens on the scale of the universe in a million years.  To humanity and our ever-accelerating advancement, a million years is thrice the age of our entire species.  But I guess Einstein was right when he said that ‘it’s all relative.’

This brings us to Saturn, a planet as ancient as the solar system.  Moderately old in astronomical time, it’s 4.5 Billion years of history are slowly being pieced together by scientists.  But are the rings and Moons that give Saturn it’s character really that old?  We’re taking a second look.

The new paper finds that Saturn’s moon Rhea and all other moons and rings closer to Saturn may be only 100 million years old. Outer satellites (not pictured here), including Saturn’s largest moon Titan, are probably as old as the planet itself. Credit: NASA/JPL

For years the common belief has been that Saturn’s rings and it’s 62 Moons are as old as the mighty gas giant itself.  However, in a 2012 study by French astronomers, it was revealed that the rings are changing quickly through tidal interactions with the planet.  The particles that make up the ring systems are spiraling further from the planet, pointing to the conclusion that the rings and inner moons formed more recently than once thought.

The problem with having a lot of Moons is that there will inevitably be a lot of interactions.  It’s like a symphony of different orbits, shifting as they circle the planet, interacting in different ways.  One of the key interactions is called an orbital resonance, where the orbital period of one moon is a fraction of the orbital period of another moon.  During these resonances, the tidal interactions of the moons are amplified, and their orbits can change significantly.  With so many moons, this is likely to happen often.

“Moons are always changing their orbits. That’s inevitable,” says Matija Cuk, principal investigator at the SETI Institute. “But that fact allows us to use computer simulations to tease out the history of Saturn’s inner moons.” By comparing the orbits of Saturn’s Moons with predictions based on computer simulations, the researchers could analyze how much the orbits have changed.

For some of the closer Moons such as Dione, Tethys, and Rhea, the orbits are less altered than previously thought, meaning they haven’t had the time to go through many orbital resonances.  Long story short, they formed in orbits similar to their current configuration.  Compare this to the theorized strength of the tidal forces of Saturn, and you end up with how much time these forces would have needed to shift the Moons, an estimate of their age.  Surprisingly, this puts the Moons at about 100 Million years old, ancient by human standards, but only a few ‘blips on the radar’ in astronomical time.  Life on Earth would have been around for a few Billion years, and dinosaurs would have ruled the planet.

So what happened? Well if the ages are accurate, the likely scenario is one where a different set of Moons underwent a strong orbital resonance, causing their orbits to cross and producing epic collisions, creating the ring and Moon systems we see today.  This could explain why the Moon Dione has a heavily cratered trailing face.

If this is true, it could mean that we are incredibly lucky to be alive during the short lived life of the rings, which may only last for a few hundred million years.  Can other ring systems last longer? Are ring systems doomed to die after a short period of astronomical time? Hopefully it won’t take us an entire astronomical ‘blip on the radar’ to figure it out.


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