seas and lakes of Titan
Figure 1. Cassini radar image (left) of the largest body of liquid found on Titan's north pole, compared to Lake Superior (right). Image credit: NASA/JPL/GSFC.
Figure 2. Titan north polar lakes.
The Cassini spacecraft, in orbit around Saturn, has returned remarkable pictures, using both radar and infrared imagery, of large bodies of liquid on Saturn's largest moon, Titan. These images reveal what appear to be lakes and seas of liquid methane and ethane, most notably in Titan's north polar region. The Cassini image above shows a feature on Titan that covers at least 100,000 square kilometers (39,000 square miles), which is greater in extent than Lake Superior (82,000 square kilometers or 32,000 square miles) – one of Earth's largest lakes (see Figure 1). The feature covers a greater fraction of Titan than the largest terrestrial inland sea, the Black Sea. Whereas the Black Sea covers 0.085 percent of Earth's surface, the observed body on Titan covers at least 0.12 percent of Titan's surface. Because of its size, scientists have referred to it as a sea.
Figure 2 is an image constructed from radar data supplied by Cassini, shows numerous dark regions in the vicinity of Titan's north pole that appear to be lakes of various sizes together with features commonly associated with lakes on Earth, including islands, bays, inlets, and channels. For more than two decades, scientists have debated whether liquids on Titan exist, and if so, where they would be located. Pre-Cassini observations from the 1980s indicated that something on Titan's surface must be re-supplying the methane to its atmosphere. A global ocean was once hypothesized. Subsequently, disconnected lakes or seas were predicted. The discovery of numerous lakes near Titan's north pole by the Cassini radar instrument in July 2006 has confirmed the latter idea, and indicates an apparent preference during the current season for liquids to be located near the north pole.
While there is no definitive proof yet that the dark, lake- and sea-like features on Titan contain liquid, their shape, their dark appearance in radar that indicates smoothness, and their other properties point to the presence of liquids. The liquids are probably a combination of methane and ethane, given the conditions on Titan and the abundance of methane and ethane gases and clouds in Titan's atmosphere.
Titan is the only world in the Solar System, other Earth, on which large bodies of liquid are though to exist on the surface, although several worlds, including Europa and possibly Titan itself, are suspected of harboring subterranean reservoirs of liquid.