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.
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.
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.
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.