 "This July 10, 2016, image released by NASA was taken by the Juno spacecraft, five days after it arrived at Jupiter. The image shows Jupiter's Great Red Spot and three of its four largest moons.")
NASA's Cassini spacecraft has discovered steep-sided canyons, hundreds of meters deep, on Saturn's moon Titan that are flooded with liquid hydrocarbons.
The finding represents the first direct evidence of the presence of liquid-filled channels on Titan, as well as the first observation of the deep canyons, NASA said.
Scientists analysed Cassini data from a close pass the spacecraft made over Titan in May 2013. During the flyby, Cassini's radar instrument focused on channels that branch out from the large, northern sea Ligeia Mare.
The observations show that the channels - in particular, a network of them named Vid Flumina - are narrow canyons, generally less than a kilometre wide, with slopes steeper than 40 degrees. The canyons also are quite deep - those measured are 240 to 570 metres from top to bottom.
The branching channels appear dark in radar images, much like Titan's methane-rich seas.
This suggested that the channels might also be filled with liquid, but a direct detection had not been made until now.
Previously it was not clear if the dark material was liquid or merely saturated sediment - which at Titan's frigid temperatures would be made of ice, not rock.
Cassini's radar was used as an altimeter, sending pings of radio waves to the moon's surface to measure the height of features there.
The researchers combined the altimetry data with previous radar images of the region to make their discovery.
The radar instrument observed a glint, indicating an extremely smooth surface like that observed from Titan's hydrocarbon seas.
The timing of radar echoes, as they bounced off the canyons' edges and floors, provided a measure of their depths.
The presence of such deep cuts in the landscape indicates that the process that created them was active for a long time or eroded down faster than other areas on Titan's surface.
The researchers' proposed scenarios include uplift of the terrain and changes in sea level, or perhaps both.
"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," said Valerio Poggiali, Cassini radar team associate from the University of Rome and lead author of the study.
Earthly examples of both of these types of canyon-carving processes are found along the Colorado River in Arizona.
An example of uplift powering erosion is the Grand Canyon, where the terrain's rising altitude caused the river to cut deeply downward into the landscape over the course of several million years.
"Earth is warm and rocky, with rivers of water, while Titan is cold and icy, with rivers of methane. And yet it's remarkable that we find such similar features on both worlds," said Alex Hayes, from Cornell University.
The finding was published in the journal Geophysical Research Letters.
