Saturn's large moon Titan, already Earth-like with its thick atmosphere and rich organic stew of chemicals, also harbors a liquid water ocean beneath its crust, new findings from NASA's Cassini spacecraft show.
Gravity maps painstakingly pieced together from data collected over five years revealed Titan's shape changes by about 10 meters (about 33 feet) due to Saturn's gravitational tugs, a squishiness that is best explained by a liquid body of water relatively close to the surface.
Scientists don't know if the subterranean ocean is feeding lakes spotted on the moon's surface. Any ocean, however, would most likely be made of water, which is heavier than the methane and ethane that dominate the pools of surface liquids.
The evidence for an ocean on Titan isn't set in stone, however. The moon's rocky core could be dehydrating, or it could be filled with warm ice.
But the most likely explanation, said Luciano Iess, with Rome's Sapienza University and colleagues, is that Titan has a liquid ocean between 50 and 100 kilometers (31 to 62 miles) beneath the surface.
"The evidence for an ocean on Titan is nearly as good as the evidence for an ocean on Europa," planetary scientist Jonathan Lunine, with Cornell University, told Discovery News.
"That puts Titan in an elite class of objects that are uniquely qualified to possibly host life," Lunine added.
One of the biggest questions about Titan's suitability for life is if the ocean touches rock, a source of minerals and a pathway for heat. Unlike Europa's ocean, which is believed to sit on a rock floor, Titan's water may be sandwiched between layers of ice, leaving it without access to minerals and temperature variations to spark life.
Scientists measured Titan's gravity as Cassini flew by the moon six times between 2006 and 2011. The measurements were made by tracking minute changes in the pitch of radio signals traveling between the spacecraft and Earth during the flybys.
They found that as Titan circles Saturn every 16 days, the moon gets squeezed. A similar phenomenon occurs on Earth by the gravitational pull of the moon. The most visible effects here are the ocean tides, but the moon also distorts Earth's crust by 50 centimeters (about 20 inches.).
Saturn's pull on Titan causes a 10-meter change. If the moon were solid it wouldn't be as squishy. Iess and colleagues at NASA's Jet Propulsion Laboratory and elsewhere then turned to computer models to understand what was happening. The best fit for the data is that Titan contains a liquid layer of water a couple of hundred kilometers thick buried beneath 100 kilometers of surface ice.
Scientist hope to refine the gravity maps with additional data from Cassini and additional computer modeling. The spacecraft's next pass by Titan is on July 22.
The research is published in this week's journal Science.