NASA | JPL-Caltech | Cassini Solstice Mission | 2012 June 28
Data from NASA's Cassini spacecraft have revealed Saturn's moon Titan likely harbors a layer of liquid water under its ice shell.
- Inside Titan (Author's Concept) - Credit: NASA/JPL-Caltech/A. Tavani
Researchers saw a large amount of squeezing and stretching as the moon orbited Saturn. They deduced that if Titan were composed entirely of stiff rock, the gravitational attraction of Saturn would cause bulges, or solid "tides," on the moon only 3 feet (1 meter) in height. Spacecraft data show Saturn creates solid tides approximately 30 feet (10 meters) in height, which suggests Titan is not made entirely of solid rocky material. The finding appears in today's edition of the journal Science.
"Cassini's detection of large tides on Titan leads to the almost inescapable conclusion that there is a hidden ocean at depth," said Luciano Iess, the paper's lead author and a Cassini team member at the Sapienza University of Rome, Italy. "The search for water is an important goal in solar system exploration, and now we've spotted another place where it is abundant."
Titan takes only 16 days to orbit Saturn, and scientists were able to study the moon's shape at different parts of its orbit. Because Titan is not spherical, but slightly elongated like a football, its long axis grew when it was closer to Saturn. Eight days later, when Titan was farther from Saturn, it became less elongated and more nearly round. Cassini measured the gravitational effect of that squeeze and pull.
Scientists were not sure Cassini would be able to detect the bulges caused by Saturn's pull on Titan. By studying six close flybys of Titan from Feb. 27, 2006, to Feb. 18, 2011, researchers were able to determine the moon's internal structure by measuring variations in the gravitational pull of Titan using data returned to NASA's Deep Space Network (DSN).
"We were making ultrasensitive measurements, and thankfully Cassini and the DSN were able to maintain a very stable link," said Sami Asmar, a Cassini team member at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "The tides on Titan pulled up by Saturn aren't huge compared to the pull the biggest planet, Jupiter, has on some of its moons. But, short of being able to drill on Titan's surface, the gravity measurements provide the best data we have of Titan's internal structure."
An ocean layer does not have to be huge or deep to create these tides. A liquid layer between the external, deformable shell and a solid mantle would enable Titan to bulge and compress as it orbits Saturn. Because Titan's surface is mostly made of water ice, which is abundant in moons of the outer solar system, scientists infer Titan's ocean is likely mostly liquid water.
On Earth, tides result from the gravitational attraction of the moon and sun pulling on our surface oceans. In the open oceans, those can be as high as two feet (60 centimeters). While water is easier to move, the gravitational pulling by the sun and moon also causes Earth's crust to bulge in solid tides of about 20 inches (50 centimeters).
The presence of a subsurface layer of liquid water at Titan is not itself an indicator for life. Scientists think life is more likely to arise when liquid water is in contact with rock, and these measurements cannot tell whether the ocean bottom is made up of rock or ice. The results have a bigger implication for the mystery of methane replenishment on Titan.
"The presence of a liquid water layer in Titan is important because we want to understand how methane is stored in Titan's interior and how it may outgas to the surface," said Jonathan Lunine, a Cassini team member at Cornell University, Ithaca, N.Y. "This is important because everything that is unique about Titan derives from the presence of abundant methane, yet the methane in the atmosphere is unstable and will be destroyed on geologically short timescales."
A liquid water ocean, "salted" with ammonia, could produce buoyant ammonia-water liquids that bubble up through the crust and liberate methane from the ice. Such an ocean could serve also as a deep reservoir for storing methane.
Titan’s tides point to hidden ocean
ESA Space Science | 2012 June 28
Nothing like it has been seen before beyond our own planet: large tides have been found on Saturn’s moon Titan that point to a liquid ocean – most likely water – swirling around below the surface.
On Earth, we are familiar with the combined gravitational effects of the Moon and Sun creating the twice-daily tidal rise and fall of our oceans. Less obvious are the tides of a few tens of centimetres in our planet’s crust and underlying mantle, which floats on a liquid core.
But now the international Cassini mission to Saturn has found that Titan experiences large tides in its surface.
“The important implication of the large tides is that there is a highly deformable layer inside Titan, very likely water, able to distort Titan’s surface by more than 10 metres,” says Luciano Iess of the Università La Sapienza in Rome, lead of author of the paper published in Science magazine.
If the moon were rigid all the way through, then tides of only one metre would be expected.
The tides were discovered by carefully tracking Cassini’s path as the probe made six close flybys of Saturn’s largest moon between 2006 and 2011.
Titan orbits Saturn in an elliptical path once every 16 days, changing shape with the varying pull of its parent’s gravity – at its closest point, it is stretched into a rugby-ball shape.
Titan’s gravity pulls on Cassini and the moon’s changing shape affects its trajectory slightly differently on each visit, revealed by tiny differences in the frequency of radio signals received from the spacecraft back at Earth.
“We know from other Cassini instruments that the surface of Titan is made of water ice mostly covered with a layer of organic molecules – the water ocean may also be doped with other ingredients, including ammonia or ammonium sulphate,” underlines Dr Iess.
“Although our measurements do not tell anything about the depth of the ocean, models suggest that it may be up to 250 km deep beneath an ice shell some 50 km thick.”
This also goes some way to explaining the puzzle of why Titan has so much methane in its atmosphere, which given its naturally short lifetime must be replenished somehow.
“We know that the reservoirs of methane in Titan’s surface hydrocarbon lakes are not enough to explain the large quantities in the atmosphere, but an ocean could act as a deep reservoir,” explains Dr Iess.
“This is the first time Cassini has shown the presence of an ocean below Titan’s surface, providing an important clue as to how Titan ‘works’, while also pointing to another place in the Solar System where liquid water is abundant,” says Nicolas Altobelli, ESA’s Cassini project scientist.
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The Tides of Titan - Luciano Iess et al