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Titan's icy crust could have deep roots

Elizabeth Howell, News Writer
Sep 4, 2013, 7:00 UTC

Sen—Saturn's moon Titan is likely covered in a firm ice shell with possible icy "roots" extending from the surface into an ocean underneath, according to a new study.

The finding, which was based on data from the Cassini spacecraft and published August 28 in Nature, could explain why areas of heavier topography have much less gravity than expected, the authors said.

In areas of high ridges, it's possible this is counteracted by an icy root that goes into the ocean underneath. This root would affect the gravity strength readings because ice is not as dense as water.

"Normally, if you fly over a mountain, you expect to see an increase in gravity due to the extra mass of the mountain," stated Francis Nimmo, a planetary scientist at the University of California, Santa Cruz who participated in the study. "On Titan, when you fly over a mountain, the gravity gets lower. That's a very odd observation." 

Titan / Huygens

A view of Titan's surface as captured by the Huygens probe in 2005. Credit: ESA/NASA/JPL/University of Arizona

"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," added lead author Douglas Hemingway, who is also at UC Santa Cruz. "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 finding is interesting in itself, but also stands in contrast with the belief that there could be icy volcanoes on Titan's surface. In 2010, observations by Cassini revealed evidence which, at the time, scientists said pointed to volcanoes. Pictures showed three large features, somewhat cone-shaped, with stuff littering the sides. The tallest of these cones was about 1,500 metres.

Icy volcanoes, the new study suggests, would be difficult to achieve as the crust would be so thick. It also would imply that plate tectonics would not be renewing Titan's crust, in contrast to how Earth's surface behaves.

The new study points to only one of several mysteries concerning Titan, which has intrigued scientists since the Voyager spacecraft went by the Saturn system in the 1980s. Researchers were astounded to see a moon socked in by cloud, and speculated for decades as to what could be on the surface.

In 2004, the Cassini orbital spacecraft arrived at Saturn's system bearing Huygens, a landing probe. Huygens successfully arrived on Titan's surface and lasted about 90 minutes, making discoveries such as detecting a hazier atmosphere than expected.

Cassini has made a wealth of discoveries since that time, including generating the first topographic map of Titan, finding an ocean of water and tropical lagoons, and tracking the changing seasons on the moon. A large unanswered question is whether the moon was or is hospitable to life, but that will likely take further missions to uncover.