SAN FRANCISCO—Saturn’s moon Titan is a wet world, the only other place in the solar system that we know has flowing liquid on its surface. The colorful geomorphic map (above left) combines radar and topographic data of Titan’s north pole to show different features around a large sea called Ligeia Mare. The map, presented Dec. 15 here at the American Geophysical Union meeting, defines four different major regions according to colors: orange, dark green, and yellow for plains, pale green for small depressions, blue for seas, and pink for ridge and valley networks. Radar imagery of the same area is seen on the right.
Titan is a cold place, with surface temperatures averaging -300 degrees Fahrenheit. Its lakes and rivers and seas carry not water, which would be frozen hard as a rock on the surface, but liquid hydrocarbons like methane and ethane. Most of this liquid pools at the moon’s north pole, where enormous seas known as mare dominate the landscape. In contrast, the south pole is a relatively dry place, with a few small lakes and many giant basins, likely the remnants of ancient Titanean seas. Scientists think that long term cycles analogous to Earth’s Milankovitch cycles—where changes in our planet’s axial tilt have caused glaciers to advance and retreat—move large amounts of liquid from pole to pole roughly every 50,000 years.
Geologists like to know the features on a planet’s surface because it tells them something about its history and composition. Both poles show highland regions cut through with river channels that drain into wide basins. If you were standing at Titan’s poles, the view might look something like the U.S. Southwest, with rivers winding around steep-sided mesas and vast plains. At the north pole, there is evidence that catastrophic floods occurred when lakes broke their shorelines and spilled out into the plains. In the drier south pole, mountain ranges are more prevalent as well as long valley networks.
The data for this map comes from NASA’s Cassini mission, which has been flying around the ringed giant and its moons since 2004. During close flybys, Cassini shoots Titan with radar and analyzes the returning beams to figure out surface features. But radar data can’t tell researchers what the different terrains of Titan are made of. Water ice is likely to be the bulk of the material, which gets broken down into smaller particles that are the equivalent of earthly gravel and sand. But because scientists don’t know exactly how such materials erode and evaporate, much of Titan’s geologic history remains a mystery. Perhaps one day NASA will send a mission to the icy moon that can sample its surface and help identify its exact composition.
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