Just how old are Saturn's rings?

Contributed by
Sep 23, 2008

Something that has bugged me lately is the idea that Saturn's rings are young, like maybe 100 million years old. I talked about this in a recent video I made about Saturn, in fact.

You can't just count them to get the age, y'know.

It bugs me because Saturn's rings are gaudy and obvious, spectacular and awesome. It seems really unlikely that they are so young, compared to the fantastic age of the solar system (4.55 billion years). It's too much of a coincidence to think that they happened to exist just when we happen to be around to see them.

Astronomers don't like coincidences. It's usually more likely that we're missing something.

In this case, that might very well be right: we are missing something. Or, more accurately, our models of the rings are missing something: like, 2/3 of the mass of the rings.

A new study from the University of Colorado indicates that the way we've been thinking about the rings may be slightly in error. Usually, people think of them as countless individual chunks of ice. However, in the new CU study the chunks tended to cluster together, forming aggregate clumps of particles.

Ring particles may like to stick together.

Oddly, this makes a huge difference in our results. For one thing, if this is correct it means the mass of the rings has been underestimated. The rings may be two to three times as massive as previously thought. The mass has been determined by measuring the amount of light absorbed by the rings when a star passes behind them. More absorption means more material. But this also depends on the distribution of the particles; if they clump together, they can be far more massive in total and still absorb light the same amount as individual particles do.

So the rings may be 3x as massive as we used to think. But that also affects how old we think the rings are! The age of the rings is estimated by how reflective they are; the idea is that meteorite impacts grind up the ice particles, and also leave meteoric dust covering the particles. Over time, the dust drops the reflectivity of the particles. By measuring how reflective the rings are, you can estimate the age. The rings are shiny and bright, so it's been assumed they are young. But the new models show that the more massive ring clumps can be old and still look shiny, despite being pelted by meteorites.

This study is pretty new, and may or may not pan out (astronomers are still comparing this model to observations of the rings to see if everything hangs together). But like the previous post I wrote about Mercury's spider, this new model does explain some things about Saturn's rings that are currently uncomfortable. I use that word on purpose: it's entirely possible that the rings are young (if 100,000 millennia can be considered young). But that seems unlikely, and that discomfort can spur on more research that yields better insights and results closer to truth. I don't necessarily trust those kinds of instincts or feelings, but it does pay to investigate them!

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