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Cassini Observations Indicate Titan has a Subsurface Ocean

This artist’s concept shows a possible scenario for the internal structure of Titan, as suggested by data from NASA’s Cassini spacecraft. Scientists have been trying to determine what is under Titan’s organic-rich atmosphere and icy crust. Data from the radio science experiment make the strongest case yet for a global subsurface ocean, sitting above a subsurface layer of high-pressure ice and a water-infused silicate core. Image Credit: NASA/JPL-Caltech

Saturn’s largest moon Titan may have a liquid water ocean underneath its icy crust, according to a paper published in the journal Science.

Titan has long been an object of fascination to scientists. It is the only moon in the solar system with a thick atmosphere, even thicker than Earth’s. That atmosphere is also saturated in hydrocarbon molecules, another unique feature in the solar system.

Previous observations by Cassini showed “lakes” of liquid hydrocarbons on the surface, and moving sand dunes akin to those on Earth and Mars.

The existence of oceans or lakes of liquid methane on Saturn’s moon Titan was predicted more than 20 years ago. But with a dense haze preventing a closer look it has not been possible to confirm their presence. Cassini has changed all that. With every pass the spacecraft makes, this distant moon appears more and more similar to Earth with rain, lakes and now an ocean. Image Credit: NASA/JPL/USGS

A team led by Luciano Iess, of the Radio Science Laboratory at the Sapienza University of Rome, published the paper on June 28, 2012.

The team was able to make this discovery by measuring the tidal bulge raised by Saturn’s gravity on Titan. The moon’s orbit is not perfectly circular, and has an eccentricity of 0.028. This means that Titan is much closer to Saturn during its periapsis than during its apoapsis. When the moon is closest to Saturn, the giant planet pulls harder on its moon, raising a small bulge on the moon, much as Earth’s Moon raises tides on the Earth.

According to their paper, the researchers listened to signals sent by Cassini and noted the doppler shift of the signal. This allowed them to track changes in Cassini’s acceleration due to the gravity of Titan. They used tracking data from six Titan flybys from February 2006 to February 2011. The sensitivity of NASA’s Deep Space Network allowed the team to precisely measure the change in frequency of the probe’s signals to Earth.

They determined that Saturn raises a bulge of about 10 m (30 ft).

The team concluded that it was not possible for such a large deformation to be raised if Titan was made of solid rock and ice. Instead, there must be a liquid layer inside the moon.

The team considered three options to explain the data, a soft, low viscosity inner core, a sulfur-rich ocean deep under the surface, or an ocean about 100 km under Titan’s icy crust.

“This [the data] tells us there must be a liquid layer beneath the surface,” Iess said.

They concluded that the most likely explanation was a liquid water ocean about 100 km (62 mi) under the crust, noting that this model would also be consistent with observations made by the Huygens probe of Titan’s electrical field.

The team noted that a shallow ocean under the crust would also explain another of Titan’s mysteries: the large amount of methane in the moon’s atmosphere.

The colorful globe of Saturn’s largest moon, Titan, passes in front of the planet and its rings in this true color snapshot from NASA’s Cassini spacecraft. The north polar hood can be seen on Titan (3,200 miles or 5,150 kilometers across) and appears as a detached layer at the top of the moon. Image Credit: NASA/JPL-Caltech/Space Science Institute

Methane is unstable across geologic time, and the methane in Titan’s atmosphere is broken apart by the Sun’s ultraviolet rays, transforming it into complex hydrocarbon molecules that rain down onto the surface or remain suspended in a thick haze in the atmosphere. Since the methane is continuously being depleted, there must be a source replenishing it.

The team proposed that an ocean doped with ammonia might be the source. Bubbles of water and ammonia might push up into the surface ice layer and free additional methane trapped in the ice, allowing it to escape into the atmosphere where it replenishes the methane being depleted.

Jonathan Lunine, a team member from Cornell University of Ithaca, New York, noted, “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.”

This subsurface ocean puts Titan in the same class of moons including Jupiter’s moons Europa, Ganymede, and Callisto. Europa’s subsurface ocean is thought by many scientists to harbor life in its depths. But the team warns against suspecting life in Titan’s ocean.

Saturn’s retinue of natural satellites continue to surprise scientists and highlight how little is understood regarding the outer solar system. Image Credit: NASA/JPL-Caltech/Space Science Institute

They believe that there is another layer of ice underneath the ocean. On Europa, the ocean is in contact with the rocky core, which may vent nutrients necessary for life. Titan’s second layer of ice would make it very inhospitable for life.

According to JPL, Cassini’s next encounter with Titan will happen on July 24. The probe will come within 1012 km (629 mi). Three additional flybys are scheduled for the rest of the year.

Cassini’s is currently in its second mission extension, called the Cassini Solstice Mission, which is scheduled to last until 2017 at least.

With the addition of a possible subsurface ocean, Titan adds one more feature to its growing list of Earth-like features, However, this strange world is very different from our own and life is unlikely to have arisen there. Image Credit: NASA/JPL-Caltech/Space Science Institute

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