[Intro]

If you're anything like us, you're probably a fan of Saturn; with its seven groups of gorgeous rings, sixty-two main moons, and hundreds of smaller moonlets.

And it also has one tiny artificial satellite, Cassini, which just finished its last close fly-by of one of Saturn's moons, Dione, on August 17 (2015).

Cassini's been exploring the Saturn system since 2004, and it's made a lot of fly-bys of moons since then. But there are still unanswered questions, especially about Dione.

At first glance, Dione might look like your typical moon— pockmarked with craters— but it turns out that those craters are actually on the trailing side of the moon; not the one that would have collided with other bodies.

So it must have spun around at some point. Probably because of some of those crater forming impacts, but scientists still aren't sure how it could have rotated exactly a hundred and eighty degrees.

And the wispy white lines on its surface were originally thought to be frost deposits, but they're actually giant ice canyon walls up to hundreds of kilometers long. They must have been formed by some kind of geologic activity that shifted Dione's crust around, but no one knows exactly what.

There's been speculation that Dione could have a subsurface ocean and maybe some geysers, but so far we haven't found strong direct evidence for them.

With this last fly-by, Cassini was looking for signs of that geologic activity by sniffing for dust particles, and mapping warmer, heat-trapping areas of the moon in the infrared, among other things.

But we're going to have to say good-bye to Dione now, for a good long while. This was our last close look at the moon, until we send some other high-powered probe to Saturn. And this year is going to be full of final moon fly-bys for Cassini.

After eleven years of impeccable service, the probe is leaving Saturn's equatorial plane near the orbits of mini-moons and shifting to a path that will take it just outside Saturn's outermost group of rings: the F ring.

There will start a series of measurements, that will finally give us a better estimate of the masses of Saturn's rings; before what mission scientists are calling the Grand Finale: a sweep just four thousand kilometers from the planet, and a final dive into the atmosphere.

But that's not the only reason the F ring has been in the news lately. It's a mysterious little band, at a hundred and forty kilometers from Saturn— a little more that half the distance from Earth to our moon— and only a hundred kilometers wide. Its so hard to spot that we didn't even know about it, until the Pioneer 11 probe flew past Saturn in 1979.

Scientists have never quite been sure how the F ring formed, but according to a study published in Nature this week, it might have had to do with two tiny moonlets crashing together, and in a very specific way.

Those moonlets, called Prometheus and Pandora, are known as shepherd moons, because their gravitational forces keep the particles in the F ring bound into a ring that spans the distance between them. But how those particles got stuck between them in the first place has been an open question for a long time.

At first astronomers thought they might have come from a collision between the two moonlets. But when they simulated the impact, they found that both the satellites would have been completely destroyed. In the new study a team of Japanese researchers decided to simulate the collision in a different way.

In previous simulations, astronomers had assumed that the moonlets were so-called rubble piles— a bunch of icy particles, bound together in a loose ball— and that the collision would have been head on. But there's evidence that these moonlets might actually have a solid, denser core, and instead of plowing straight into each other, they could have sort of grazed past.

In that case, the researchers found that Prometheus and Pandora would have only partially disintegrated; with some of the particles knocked off by the crash forming a narrow band between them. Exactly what we're seeing in the real life F ring.

And the team points out that something similar might have happened around Uranus, which as its own ring system with similar shepherd moons.

This may also help us understand the formation of giant planets outside our solar system, which we found lots of. It'll be a long time before we send probes to get a better look at those planets, but in the meantime we have data much closer to home, like from Cassini, to help us learn what those far off systems might look like.

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[Outro]