It's official: Saturn's largest moon, Titan, has canyons that are flooded with liquid. Well, liquid hydrocarbons, anyway: Scientists analyzed images from a flyby of the Ligeia Mare sea of methane that the spacecraft did in 2013 and found channels branching out from it. Some of these were narrow and deep but it wasn't until further examination that they confirmed the steep troughs were carved out by liquid.
In previous flybys, NASA scientists used Cassini's radar as an imager to peer through the thick haze covering the surface of Titan. But in 2013, they used it as an altimeter to measure the depth of geological features around Ligeia Mare. The spacecraft's signals reflected off the canyon floors in unique ways, glinting like the bottom of the moon's methane seas, indicating a surface smoothed by liquid. Scientists counted the seconds it took radar pings (demonstrated in the GIF below) to get back to the spacecraft and estimated the channels' depths to range from 790 to 1,870 feet below sea level.
Such deep cuts in the surface imply either a long-running process or that the canyon geography was worn down much more quickly than others on Titan. In a paper published in Geophysical Research Letters studying the image analysis, researchers gave Earthly comparisons to the competing theories about what cut the Saturn moon's channels. In an uplift powering erosion, rising terrain altitude sent water punching deeply down, creating the Grand Canyon. Variations in the water level, like those creating Lake Powell, increase the river's rate of erosion.
Though Cassini is currently making its last pass around Titan before it concludes its 20-year journey in Saturn's atmosphere in September 2017, researchers will likely apply this imagery-and-depth radar combination to analyze other channels leading out of the Ligeia Mare sea.