Saturn made news a few months ago when astronomers spotted what they believed was a new moon being born in the planet’s A Ring. As it turns out, the event was rare not because it featured the birth of a new moon, but because of its placement. Saturn’s F Ring, it’s outermost one, gives birth to moons fairly regularly, but those events are easy to miss because the moons die almost as quickly they appear.
To look at, the F Ring isn’t as bright as most of the inner ones, it’s made of chunky ice and is thinner than the others. But it does illustrate a principle called the Roche limit, which is the boundary beyond which moons can form and stay intact. Inside the Roche limit, gravity pulls at the newly formed moons on opposite sides with such force that it can tear them apart, particularly when they’re not particularly massive.