Although the moon sports lakes of liquid hydrocarbons, such as methane and ethane, Titan’s ocean is probably mostly water.

“The subsurface ocean has to be made of water, or water mixed with a relatively small percentage of salts,” Iess said.

If the ocean were liquid hydrocarbons, the heavier surface ice would sink and Cassini would see a global hydrocarbon ocean on the surface, he added.

Scientists hope to refine gravity maps of Titan after additional Cassini flybys planned through 2017.

Cassini arrived at Saturn in 2004.

The research appears in this week’s issue of Science.