The Cassini spacecraft has spotted what could be the longest river system seen beyond Earth.I’m not normally a planetary science groupie, but this new Cassini image* of a river on Saturn’s moon Titan grabbed my attention. Rivers on Titan aren’t a new discovery — since NASA’s Cassini spacecraft dropped the tag-along ESA Huygens probe onto the haze-swathed moon in 2005, Titan has made surprising scientists its day job. Seasonal lakes grow and wane at the poles, while the largest dunes in the solar system gird the moon’s waist. There are river deltas, canyons, and even a sea as large as Lake Superior in North America. These bodies are filled not by liquid water, but by liquid methane and ethane.
This particular river is more than 400 kilometers (250 miles) long. That’s about the length of the Thames in England, and less than 10 percent the length of the Nile. In terms of the ratio between a river’s length and the diameter of its parent body, the Titan river only rises to Rhine status: the alien river is about 8 percent of Titan’s diameter, compared with the Nile, which stretches nearly 6,700 km, or roughly half Earth’s diameter (don’t forget, the river curves). So the “mini Nile” description in the press announcement is a bit of a stretch — “micro Nile” might be better — but considering only a few percent of Titan's total surface is covered with lakes and seas, a little hyperbole is merited.
Liquid bodies on Titan look black in radar images like this one because radar reveals landscape by bouncing off the surface. When radar photons hit a smooth river or sea, two things can happen. Either they will reflect off the surface as though it were a flat mirror (which won't point them back at the spacecraft), or they'll pass into it. If a lake is deep enough, they’ll attenuate before hitting bottom and never boing back up to the radar detector.
Researchers have been able to catch radar reflections off a Titanian lake bed: radar bounced off the bottom of the south pole's Ontario Lacus allowed researchers to measure changes in lake level.
You can read more about the river and Titan's landscape in the ESA's Cassini press release.
*There should be a long black-and-white image on this page. But occasionally we’re struck by our image gremlin. To dodge his dastardly tactics, put a ? at the end of this page’s URL and refresh the page. If that doesn’t work, put 1=1. Math scares gremlins even more than questions.