Cassini reveals oddities of Titan’s icy shell, leaving scientists baffled

Cassini reveals oddities of Titan’s icy shell, leaving scientists baffled

Titan’s shell begins to reveal its secrets.


Titan, the largest moon of Saturn, is an inscrutable world. Its thick murky atmosphere is all but impenetrable to telescopes. With an equatorial radius of 1,600 miles, it is the second largest moon in the Solar System after Jupiter’s Ganymede and is even larger than the planet Mercury. Its surface is unimaginably cold, -289 degrees Fahrenheit.

Yet even the cloud-enveloped Titan is slowly yielding its secrets. NASA’s Cassini spacecraft has been orbiting Saturn since 2004, and Titan has been an object of constant interest. In January 2005, Cassini released the Huygens probe into Titan’s atmosphere; the probe landed and took the first photographs of Titan’s surface before falling silent. Cassini has continued to examine Titan with cloud-penetrating radar and other instruments, and has made a new discovery concerning the moon’s shell of ice.

The Cassini data were analyzed by Douglas Hemingway and Francis Nimmo, planetary scientists at the University of California, Santa Cruz. The fresh data concerned Titan’s gravity and topography, and revealed a counterintuitive relationship between the two properties.

“Normally, if you fly over a mountain, you expect to see an increase in gravity due to the extra mass of the mountain,” Nimmo explained in a Jet Propulsion Laboratory press release. “On Titan, when you fly over a mountain, the gravity gets lower. That’s a very odd observation.”

Hemingway, Nimmo, and colleagues developed a new model to explain these anomalous readings. They propose that each relatively small rise in Titan’s topography is offset by a deeper, iceberg-like root massive enough to overwhelm the gravitational signature of the bump on the surface, yielding the “odd observation” of the relationship between Titan’s topography and gravity. Cassini would detect less gravity because water ice is less dense than water.

“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,” Hemingway explained. “If large roots under the ice shell are the explanation, this means that Titan’s ice shell must have a very thick rigid layer.” The ice shell would be at least 40 kilometers thick according to the model.

It is not yet clear what processes formed Titan’s topographical features and their deep roots. Titan’s surface is distorted as it orbits Saturn, generating tidal heating, which could give rise to variations in the thickness of the ice shell.

The new findings were published in the August 28 issue of the journal Nature.

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