SAN FRANCISCO — Using data from NASA's Cassini spacecraft, scientists have created this beautiful mosaic mapping the northern hemisphere of Saturn's moon Titan, which is full of rivers, lakes, and seas.

"Titan is a very alien place that looks very Earth-like," said planetary scientist Stephen Wall, leader of Cassini's radar team, during a press conference here at the American Geophysical Union conference.

The material filling Titan's lakes is not water but rather hydrocarbons such as methane and ethane, which are typically gases on Earth but remain liquid at Titan's average temperature of −180 degrees Celsius. Ever since Cassini started radar mapping the frozen moon in 2004, researchers have seen that Titan is a weird and wet world. But Cassini's scans missed the true extent of some seas, including the biggest, Kraken Mare. This new map fills in almost all the area of Titan's north pole and provides scientists with important answers to some of their questions.

While the northern hemisphere is dotted all over with hundreds of tiny lakes, the large seas seem confined to a specific area, mainly on the lower right side of the image above. As geophysicist Randolph Kirk of the USGS pointed out during the briefing, you could almost draw a rectangular box around this area, suggesting that geological processes are at play. The team thinks that Titan's crust has fractured here when active tectonics created almost straight lines of parallel mountain chains. The low-lying areas are what gets filled with liquid, creating Kraken Mare and its smaller neighbor, Ligeia Mare. The scientists think the process may be analogous to flooding 12,000 years ago of similar geology in Nevada that likely created large bodies of water.

Other tectonic processes are probably behind the smaller dotted lakes, though scientists don't yet know precisely what. Some of the lakes could be infilled calderas of former active volcanoes on Titan (which would spew molten water instead of lava). But there isn't enough volcanic activity on the moon to account for all of them. Instead, many were probably created when liquid hydrocarbons dissolved the frozen ice, in the same way that water on Earth dissolves limestone to create features like the Bottomless Lakes in New Mexico.

"This creates a kind of exciting prospect that under the northern pole of Titan is a network of caves," said Kirk. Caves on Earth are often filled with life, so perhaps Titan's caves could be as well.

Other radar data has shown the depth and volume of one of Titan's seas, Ligeia Mare. Because the radar can penetrate below the liquid surface it reaches the bottom of Ligeia, which has a maximum depth of about 170 meters, on the same order as a body like Lake Michigan. Cassini has measured the surface area of Ligeia Mare, and scientists can calculate that it has a volume of about twice that of Lake Michigan as well.

Below is a simulated flyover of Titan's north pole made using stereo data from Cassini's radar maps.