And when scientists looked at the results, they were baffled.
"Normally, if you fly over a mountain, you expect to see an increase in gravity due to the extra mass of the mountain. On Titan, when you fly over a mountain the gravity gets lower. That's a very odd observation," said Francis Nimmo, a professor of Earth and planetary sciences at the University of California at Santa Cruz (UCSC) and one of the lead scientists on the team, in a press release.
Wait a minute, you say. Isn't gravity constant? Drop an apple, it falls, end of story. Right?
Gravity depends on the mass of nearby objects. If you wander around Earth's surface with a "gravimeter," an instrument that measures the precise tug of gravity in any location, you'll discover that gravity pulls a bit more strongly over a mountain than it does over nearby lowlands, because of the huge volume of rock that makes up a mountain.
In fact, mountains are even bigger than you may have realized. We usually think of mountains as rising up from the surrounding plains, but that's only seeing the top sliver of the story. The rest is underground, called the "root," which, at least on Earth, is less dense than the underlying rock. Just like icebergs have more mass underwater than above the surface, mountains have more mass in their "roots" than above ground.
You can prove it to yourself: Pour a glass of water and drop an ice cube in it. See how little of the cube pokes above the water, and how much is in your drink? That above-to-below ratio will stay constant, even as the ice cube melts. Mountains have to follow the same law of physics, which scientists call "isostatic equilibrium," but which I'll call "the ice cube principle."
Mountains, volcanoes, icebergs, ice cubes – they all follow the same rule. Some above, more below. Even on Titan.