Moment of Science: Planetary Rings

“The scientific theory I like best is that the rings of Saturn are composed entirely of lost airline luggage.” -Mark Russell
We're back out in space for this week's Moment of Science, discovering how we get rings around some planets, but not others.
Published: May. 10, 2022 at 5:02 PM EDT|Updated: May. 10, 2022 at 5:03 PM EDT
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Noted philosopher Beyoncé once said, “If you liked it, then you should’ve put a ring on it.” We’re back out in space this week, discovering how we get rings around some planets, but not others.

* Chances are you just thought of the planet Saturn. Galileo discovered those rings in 1610... and until the 1970s, we thought it was the only planet with rings. It turns out the other gas giants -- Jupiter, Uranus, and Neptune -- also have them... but why? In our gravity episode, we explored how every mass is attracted to every other mass in the universe to some degree, though it goes down a lot with distance. That includes one side of a planet being closer to a celestial body than the other side... and runs the risk of being torn apart by tidal forces, like entering black holes but a lot slower.

* Here’s where distance really comes into play: Back in the 1840s, a French astronomer named Edouard Roche calculated the distance at which masses would either get torn apart and form rings around a planet, or stick to each other and start forming their own larger mass farther away. This “Roche limit” is about two and a half times the radius of the larger body... usually a planet. That also factors in density: If you have a bunch of loose rocks all scattered about, it’s more likely to get ripped apart than a more solid object. Earth’s moon is a great example of the latter, and it’s theorized it was formed when Earth was hit by what amounts to a rock the size of Mars. Some chunks of that debris formed temporary rings around Earth, but a lot of it decided to have a little get-together on our orbit far enough away from Earth, and became more concentrated and well-rounded over time.

* Lucky for us, the Moon is well outside its Roche limit to shear itself apart all over again... in fact, it’s getting gradually farther from us. The earth used to spin a lot faster than it does today, and the Moon served to slow down our spin over the years, drifting farther away from us to compensate for conserving that momentum (4cm/yr). That conservation is also why the Moon is “tidally locked”, and us Earthlings have only ever seen one side of it from our home.

* So... why do the gas giants have rings and not us? The inner planets of the solar system -- us included -- may be a bit shielded from more extreme impacts, so there’s less debris to form rings... or, more moons around the gas giants may mean more collisions... or, a lot of the icy rings simply melted being closer to the Sun. Either way, even Mars will have rings of its own in due time! Its biggest moon, Phobos, is moving faster around Mars than the planet is spinning... so, it’s getting closer to reaching that Roche limit. In tens of millions of years, it’ll shear itself apart... and whatever doesn’t fall to the Red Planet will just keep on spinning around it.

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