Saturn's exotic moon Titan, with its thick atmosphere and large, methane lakes, continues to beguile planetary scientists. Now, using data collected by an altimeter aboard NASA's Cassini spacecraft, scientists have found liquid methane flowing through deep gorges on the moon's surface.

Unlike Earth and its water cycle, Titan's hydrologic cycle is based on methane, the main component of natural gas. It exists as a liquid on the very cold surface of Titan, with an average temperature of about -180 degrees Celsius. Scientists have previously discovered a number of hydrocarbon seas—or mare—on Titan. However, by analyzing additional data from various Cassini flybys over the last decade, they are now finding other features of Titan's complex hydrologic cycle.

In a paper published in Geophysical Review Letters, an assessment of altimeter echoes has identified steep canyons carved into the icy surface of Titan as deep as 570 meters, with a reflection at their bottoms that indicates they are filled with liquid hydrocarbons. Studying the 400km-long Vid Flumina river that flows into Titan’s second-largest sea, Ligeia Mare, scientists identified eight ice-lined canyons branching off of this Nile River-like feature.

The scientists believe multiple geological factors led to the creation of the canyons, including changes in sea level and tectonic processes. "It's likely that a combination of these forces contributed to the formation of the deep canyons, but at present it's not clear to what degree each was involved. What is clear is that any description of Titan's geological evolution needs to be able to explain how the canyons got there," said Valerio Poggiali of the University of Rome, a Cassini radar team associate and lead author of the study.

Regardless of how they were formed, the features on Titan bear a striking resemblance to those on Earth. Our planet is rocky and warm, with rivers of water, whereas Titan is ice-covered and very cold, with rivers of methane. Moreover, although the fluvial valleys observed on Titan are comparable to glacially eroded fjords on Earth, they almost certainly couldn't have been caused by glaciers, which are difficult to imagine as existing on Titan.

Unfortunately, it will be some time before scientists can gather more information about Titan up close. Cassini is slated to end its mission next year. And while Congress has highlighted Titan as a world of high interest in its Ocean Worlds Exploration Program, it is behind Europa in the pecking order, and it is difficult to envision a mission launching to Titan before the late 2020s at the earliest.

Geophysical Review Letters, 2016. DOI: 10.1002/2016GL069679 (About DOIs).