Word of the Week: Roche Limit

Hey, everyone! Earlier this week, I mentioned the Roche Limit as part of the post I did for the top eight moons list about Mars’s moon, Phobos. So, how about a quick post to check off this week’s Word of the Week? It’s going to be a quick one on account of the bottle of stout I have waiting and the ice cream cake melting across the room.

The Roche limit, named for the French mathematician, Edouard Roche, who first figured it out, is the distance from an object, a planet, say, closer than which a moon will be ripped to shreds by the bigger object’s gravity. This is also called the Roche radius.

Roche Limit
Roche Limit

Let’s use the example of Mars and Phobos. There, Phobos is being pulled inward by Mars’s gravity, which is quite a bit stronger than Phobos’s. If Phobos’s were stronger, well, it’d be Mars that gets pulled in. They’re both pulling on each other gravitationally, but Mars is winning. Eventually Phobos will reach the Roche limit and the pull of Mars’s gravity will overwhelm the much weaker pull that Phobos is able to muster to in order to hold itself together. From then on, Mars will have a gorgeous ring of formerly Phobos bits. This is probably what happened at the big outer planets. Things kept wandering too close and getting ringified. I think that’s a real science word.

Time for beer and cake. Clear skies, everyone!


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