Bad Astronomy

Don’t have gravity? Take your lumps.

It might seem like a tautology – and that’s because it is – but sometimes the only word you can use to describe an image from the Cassini Saturn probe is otherwordly:

[Click to engasgiantize.]

This otherworldy picture was taken on March 24, 2010. The big moon is Rhea, seen from 1.2 million kilometers (750,000 miles) away, and the little one below it is Epimetheus, from 1.6 million km (990,000 miles) away. Perspective makes them look right next to each other, but in reality the distance between them is the same as the Moon from the Earth! Saturn and its rings provide the backdrop for this stunning alien portrait. To me, the most striking thing about this picture is the difference between the two moons. Rhea is a ball, a sphere, while Epimetheus is clearly a lumpy rock. Rhea is also clearly a lot bigger, even accounting for perspective in the picture; it’s about 1520 km (940 miles) across, while Epimetheus is 144 x 108 x 98 km (86 x 64 x 58 miles) in size.

Why is Rhea round, and Epimetheus lumpy? Gravity. Rhea, being so much bigger, has a lot more mass, so its gravity is much stronger. Objects bigger than a few hundred kilometers across have enough mass that self-gravity becomes important in shaping them. A rock you might see lying on the ground is small and has very little gravity, so the important things that shape it are its chemistry, the crystal structure inside it, and its history (getting banged by another rock, erosion, and so on).

But as the mass increases, so does the influence of gravity. Eventually, gravity wins: it doesn’t matter what the composition is (metal, ice, rock) or the history (getting knocked around), because gravity is strong enough to shape the object into a sphere. Sure, other forces can be at play (for example, rotation can flatten an object out a bit), but gravity is the one with the biggest influence.

Gravity is an inward force, trying to draw everything into the center of the mass. That’s why big objects are spheres; anything large enough to stick up very far gets pulled down. Look at mountains on Earth: they can only get to a certain size before slumping. They can’t support their own weight! Olympus Mons on Mars is much bigger than any mountain could ever be on Earth, because Mars has less gravity.

So this is more than just a beautiful picture from Cassini; it’s an object lesson in gravity. And as science tells us over and again, size matters.

Credit: NASA/JPL/Space Science Institute

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The real Pandora, and two mooning brothers
Cassini eavesdrops on orbit-swapping moons