Cassini sailed within 31 miles (50 km) of Enceladus's south pole on March 12, 2008. During that flyby, the probe sampled the plumes, and found that they were made of water laced with ammonia, methane, and carbon dioxide. More flybys were planned. By April 2014, it was clear that some of the geyser particles were salty and flavored with potassium and sodium, very similar to Earthly sea spray. Hydrogen gas was detected in the jets, as were silicate crystals, which could only have been formed in boiling water, scientists said. [Photos: Enceladus, Saturn's Cold, Bright Moon]
This evidence, taken together, suggests the presence of a salty ocean of liquid water under the ice, in contact with a hot, rocky seafloor through which mineral-laden hot water flows. It is easy to imagine hydrothermal vents there, very similar to the "black smokers" and "white smokers" found on the bottom of Earth's oceans, where they form oases for communities of life.
Enceladus seems to be begging us to return with instruments capable of detecting amino acids, fatty acids and long-chain carbon molecules — and perhaps even imaging equipment to photograph any possible microorganisms themselves.
Titan, which is 10 times wider than Enceladus, is also a world of liquids. But the fluids flowing on Titan's surface are hydrocarbons, not water. At Titan's surface temperature, water is rock-hard. But methane, ethane, and propane slosh around in all three states: solid, liquid, and gaseous. The surface of this moon is nearly impossible to see from space at visual wavelengths; thick, orange smog pervades the place. This hazy atmosphere is rich with organic compounds.
Fortunately, Cassini can see in infrared and radar wavelengths. And the Cassini orbiter carried a lander called Huygens, which touched down on Titan's surface in January 2005. Huygens carried cameras down through the atmosphere. And, upon soft-landing on the shore of a hydrocarbon lake, it sent close-ups of rounded pebbles, which were probably made of water. The landforms on Titan are eerily familiar; they look a lot like coastlines and river lands on Earth.
Titan, too, must be warm inside. This moon changes shape as it orbits Saturn once every 16 days. Cassini measured Titan's tidal bulging, finding the surface rising and falling on the order of 30 feet (9 meters). If Titan were completely solid, those tides would be at least 10 times less pronounced, scientists have said.