Planetary scientists going over new data from Saturn's largest moon, Titan, have discovered an unusual 'reversed' relationship between elevation and gravity on the moon's surface, leading them to the conclusion that the moon's icy crust is far more rigid, and far less geologically active, than previously thought.
Speculation about Titan so far has been that its thick icy crust is very active, being constantly recycled, similar to how Earth's surface is constantly recycled by plate tectonics. Also, it's thought that higher elevations seen on the moon are due to ice volcanoes cracking apart the relatively fragile crust to spew out methane and other materials from the moon's interior. All of this ice likely rides over a subsurface ocean of liquid water, with another layer of ice below that, before reaching Titan's rocky core.
However, a recent study by scientists at the University of California Santa Cruz, Stanford University and the Università La Sapienza in Rome, looked at data Cassini gathered about Titan's gravity, and it showed an usual pattern that doesn't mesh with those previous ideas.
"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," Francis Nimmo, a professor of Earth and planetary sciences at UC Santa Cruz, said in a press release.
In their search for an explanation for this puzzle, Nimmo and his colleagues came up with an idea that's quite different.
Instead of the higher elevations being built up onto the surface by ice volcanoes, they may be produced by the ice being pushed upwards from below. This would be due to ice 'roots' that extend from the base of the crust down into the subsurface ocean. Since ice is less dense than water, these roots would cause that areas of the crust above them to be pushed upwards. However, this wouldn't necessarily explain the lower gravity under these higher elevations, unless the icy crust was particularly rigid.
With a rigid crust, the ice roots would push the surface up a small amount, creating the higher elevations, but they would mostly displace the water underneath the crust to the sides.
"Because ice is lower density than water, you get less gravity when you have a big chunk of ice there than when you have water," Nimmo said.
As to what causes these ice roots, it could be due to tidal heating of the interior of Titan. Similar conditions cause the water ice jets on Enceladus, and it could cause different thicknesses of the ice on Titan.
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This study gives a really interesting view of what Titan might be like, but knowing for sure will probably require a more dedicated mission: either an orbiter or something to land on the surface and drill beneath the ice.
Until then, though, Cassini continues to provide us with fascinating new information about Saturn, its rings, and the incredible moons orbiting around the planet.
(Images courtesy: NASA/JPL-Caltech, D. Hemingway/UC Santa Cruz)
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