Titan is tricky. The enormous moon of Saturn has topography like Earth's, but it's made of ice instead of rock. Throw in an opaque (and poisonous) atmosphere and a space probe that only occasionally passes by to pick up a new swath of data, and you have a truly mysterious planet. Er, moon.
"Titan has been a hard planet to study," says Chuck Wood, a member of the Cassini science team based at the Planetary Science Institute in Tucson, adding, "It's bigger than Mercury; that's why we call it a planet."
The opaque atmosphere has defeated Cassini's impressive cameras, leaving scientists relying on low-resolution radar and other instruments to interpret Titan's mysteries. "It's almost like seeing the moon with telescopes from Earth," says Dr. Wood. "It's hard to have enough resolution to be confident in what we're seeing."
Despite the challenges, scientists have been announcing new and bizarre findings from Titan, Saturn's biggest (and arguably weirdest) moon ever since NASA's Cassini-Huygens mission arrived at Saturn in 2004.
Titan is a "deranged version of Earth!" scientists said. Titan has lakes! When scientists dropped the Huygens probe through Titan's thick atmosphere, they saw shorelines! Rivers! Evidence of "exotic and exciting" methane-ice-sludge volcanoes! Five years later, a methane rainstorm made more headlines, followed by Titan's potential for bizarre and smelly lifeforms, mile-wide dunes shaped by 'backward' winds, and theories about Titan-esque DNA.
Last year, the liquid story got more involved, with the discovery of a humongous methane lake in the (comparatively warm) tropics and growing evidence for a subsurface ocean underneath a frozen crust.
With that model, Titan is like an egg: liquid on the inside, thin and brittle on the outside. And there may or may not be a solid (or solid-ish) "yolk" somewhere deep inside.
Now, a team of scientists from the University of California at Santa Cruz are throwing a new wrinkle into the story, published Wednesday in Nature. What if Titan's crust isn't thin and brittle like an eggshell, but thick and firm, with humongous mountains floating over its subsurface ocean? What does that do to the implications for lakes, subsurface seas, and life?
It started when Cassini had made enough passes by Titan to generate a reasonably complete topography and gravity map of the moon's surface. Since Cassini is orbiting Saturn, and only occasionally swinging near Titan, it has taken the better part of a decade to brush past Titan close enough, often enough, to gather the necessary data.
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.
Dr. Nimmo and his team used the ice cube principle to create a model of Titan's outer layer that could explain why Titan's mountains have less gravity than the surrounding areas, in defiance of expectations.
To explain the low gravity, they suggested that each bump on the surface of Titan has a truly enormous "root" – big enough to overwhelm the gravitational effect of the bump on the surface, and much bigger than the ice cube principle would predict. "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 explained.
So how are they getting around the ice cube principle? They suggest that something – maybe methane rain – has eroded away part of the mountains. Taking a piece off the top should mean that the rest of the mountain rises up, just like knocking a chip out of the top of your ice cube would make it float a little higher.
But on Titan, they argue, the ice crust is so thick and rigid that it fights against the mountains' buoyancy. "It's like a big beach ball under the ice sheet pushing up on it, and the only way to keep it submerged is if the ice sheet is strong," said Doug Hemingway, a doctoral candidate in planetary geophysics at UCSC and lead author of the paper.
To fit the massive root model, the crust would need to be at least 25 miles thick, the team calculated, and ruthlessly inflexible.
"The fact that you can 'unload' the mountain and it doesn't spring back straightaway does suggest the crust is rigid," agrees Ralph D. Lorenz, a Titan expert at the Johns Hopkins University Applied Physics Lab, who was not involved in the study.
"That's already weird," he says, because other data from Cassini's gravimeter suggested that Titan was flexing back and forth under the effects of Saturn's gravity – an argument for a flexible crust, not a rigid one. "How do we explain this? Titan deforms on orbital timescales – short timescales – yet it's stiff enough to hold mountains up that aren't being held up by [the ice cube principle]. So it's kind of a challenge. A bit of a paradox," says Dr. Lorenz.
"That's the challenge of science that they never tell you when you're learning science in high school or undergrad," says Lorenz. "You get the impression it's like a crossword puzzle: There's a right answer, and when you get the right answer, you'll know. That it's just a case of fitting the facts together. But it's really a lot more like a detective novel. You get all these pieces of information, but somebody out there is lying, and you have to figure out who that is."
The ambiguity of the data only adds to Titan's mysteries. Is Titan really flexing with the tides? Is it rigidly supporting floating mountains? Could both be true?
A rigid crust has serious implications for understanding the planet-sized moon. For example, on the one hand a thick, rigid ice shell makes it very difficult to produce ice volcanoes, which some scientists have proposed to explain certain surface features. On the other hand, Earth has thick, rigid continental crust that magma succeeds in forcing its way through. In fact, the challenging journey through rigid crust ultimately makes the eruptions more explosive, and explains many of the differences between a Mount St. Helens-style eruption and a gently oozing volcano like Hawaii's Kilauea, which rises through the thinner, more flexible ocean crust.
Studies like this one are important, says the Planetary Institute's Dr. Wood, because "anything about the crust that gives us information about whether the crust would restrict the flow of possible heated material to the surface is important.... It's frustrating, with the evidence we have, to really understand how Titan works. Is Titan geologically active today, with volcanism and tectonism happening recently? Or is everything dominated and made to happen by the atmosphere?" he asks. "On Titan, we just don't have clarity."