From Nature magazine

Data gathered by NASA’s Cassini probe as it repeatedly swept past Titan, Saturn’s largest moon, offers the best evidence yet that the smog-swaddled satellite has a substantial ocean of water sloshing beneath a thick icy crust.

During Titan’s 16-day orbit around Saturn, the distance between the moon and its planet ranges from slightly less than 1.19 million kilometres to almost 1.26 million km — a disparity that generates tides that flex the moon’s surface, says Luciano Iess, a planetary scientist at the Sapienza University of Rome. Estimates of the size of those tides and their effects can provide clues about the moon’s internal structure, he explains.

Since the Cassini probe began orbiting Saturn in July 2004, the craft has flown past Titan more than 80 times. For this study, Iess and his colleagues analysed how the moon's gravity caused Cassini to speed up as it approached Titan and then slow down as it receded during six of those flybys. Because Titan occupied different spots in its orbit during each flyby, the team of researchers could use the flyby data to discern subtle variations in the moon’s gravitational field as it moved through its orbit. These variations were brought about by changes in Titan's shape — which, in turn, were triggered by tidal flexing of the moon’s surface.

The team’s analyses suggest that the surface of the moon can rise and fall by up to 10 metres during each orbit, says Iess. That degree of warpage suggests that Titan’s interior is relatively deformable, the team reports today in Science. Several models of the moon’s internal structure suggest such flexibility — including a model in which the moon is solid but soft and squishy throughout. But the researchers contend that the most likely model of Titan is one in which an icy shell dozens of kilometres thick floats atop a global ocean. The team's findings, together with the results of previous studies, hint that Titan’s ocean may lie no more than 100 km below the moon’s surface.

Melting middles
“This is an exciting result that puts Titan firmly in the group of large satellites with oceans,” says Robert Pappalardo, a planetary scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California. Previously, scientists have inferred the presence of oceans beneath the icy surfaces of several satellites in the outer Solar System, including Enceladus, which is another Saturnian moon, and Europa, which orbits Jupiter.

Tidal flexing of Titan’s icy crust wouldn’t provide enough heat to keep the subsurface ocean liquid, says Jonathan Lunine, a planetary scientist at Cornell University in Ithaca, New York, and a co-author of the study. But the energy released by the decay of radioactive elements in the moon’s core, the chemical reactions that dehydrate many of the silicates there, and the small amounts of ammonia that may taint the ocean would help keep it from freezing, he notes.

However, such tidal flexing could serve as an explanation for why methane is present in Titan’s atmosphere, even though the gas is usually destroyed by chemical reactions driven by sunlight, says Lunine. Deposits of methane-rich ice in the upper portions of Titan’s crust could be warmed enough by the flexing to release the gas, thereby replenishing the moon’s atmospheric concentrations of the gas. This would then fall as rain into methane oceans and lakes on the surface.

“But that’s just one idea, because scientists haven’t measured near-surface concentrations of methane [on Titan] yet,” Lunine notes. “There’s no smoking gun for where it comes from.”

That evidence could soon be at hand. The Titan Mare Explorer (TiME) mission, one of three candidates now being considered by NASA for launch later this decade, would drop a floating, instrument-laden capsule into one of the large methane seas in Titan’s northern hemisphere to study the chemical and physical processes taking place there. "So far, we've only seen things during flybys," says Lunine.

This article is reproduced with permission from the magazine Nature. The article was first published on June 28, 2012.

6 Comments

1. 1. jonathanseer 10:18 PM 6/28/12

   Why is the assumption always that the ocean is global and continuous at its level?
   
   Wouldn't it make more sense if we modeled such subsurface oceans on the oceans of Earth, which are broken up by continents.
   
   On moons like Titan and Europa this would mean the oceanic layer would be like our surface turned upside down upon the mantles of those moons.
   
   While the liquid layer would dominate it would also feature sections where solid crust continues downward to the mantle without an intervening liquid layer.
   
   Conveniently this could also result in a liquid layer featuring spots and regions that would be much closer to the surface than other parts.

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2. 2. RonStrong in reply to jonathanseer 11:35 PM 6/28/12

   The earth's oceans are broken up by continents due to the continuing action of plate tectonics. Without that, the earth's surface would be one big ocean.
   
   Is there any evidence that Titan has sufficient internally generated heat to create convective flows in a mantle that pushes pieces of crust around the surface? If not, no land masses.

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3. 3. jonathanseer in reply to RonStrong 03:09 AM 6/29/12

   I'm not talking about the surface of Titan.
   
   I am talking about the liquid layer beneath the crust.
   
   

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4. 4. jtdwyer in reply to RonStrong 03:17 AM 6/29/12

   This article states:
   "Tidal flexing of Titan’s icy crust wouldn’t provide enough heat to keep the subsurface ocean liquid, says Jonathan Lunine, a planetary scientist at Cornell University in Ithaca, New York, and a co-author of the study. But the energy released by the decay of radioactive elements in the moon’s core, the chemical reactions that dehydrate many of the silicates there, and the small amounts of ammonia that may taint the ocean would help keep it from freezing, he notes."
   
   http://en.wikipedia.org/wiki/Titan_(moon)#Cryovolcanism_and_mountains
   indicates that the geology of Titan is thought to be analogous to Earth's, but with cryogenic liquid and solid states of water in place of solid and molten rock. Water and ammonia volcanoes erupt from its icy surface, and surface mountains and valleys have been detected. Extensive references are included.
   
   The Nature News article http://www.nature.com/news/tides-turn-on-titan-1.10917
   includes an illustration of Titan's geologic strata, as does the wikipedia entry. I think that, while measurement data may not yet be available, the water and ice of Titan comprise a larger percentage of Titan's composition than Earth's. Again, Titan's liquid and solid water is thought to behave similarly to Earth's solid crust and molten lava mantle. If the Earth's rocky surface was homogeneous and smooth, as I understand, water would comprise be a thin layer underneath Earth's atmosphere.
   
   Earth's continents are generally composed of older, lighter granite material that essentially rises above the newer, heavier basaltic rock of the ocean floor. Please see http://en.wikipedia.org/wiki/Continental_crust
   This contrast of surface material density is not present in Titan's surface ice and likely prevents a similar process to Earth's plate tectonics from developing.

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5. 5. jaimezal 03:08 PM 6/29/12

   this is one of the evidences that shows so important is to maintain space exploration

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6. 6. Mark652515163 12:56 PM 7/2/12

   
   After we colonize Mars (even Venus could perhaps also be terraformed with cyanobacteria, the Soviets pinpointed a species that could thrive there), surely the gas giant moons will be our next abode.
   
   Europa and Titan are the coolest spots in the Solar System, and I would not be surprised if Ganymede becomes an important urban center in a thousand years or so.
   
   Of course, we must first overcome overpopulation and bigotry alike if we are ever to have the economic surplus to enable all the fun stuff.
   
   

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