(Every day until Christmas, I’ll be posting a science-related image.)

Day 12

I will not apologize for the love I’m giving Saturn’s moons this month. Thanks to the Cassini probe, we are learning so much about these worlds, and none has been revealed to be more amazing than Titan. Saturn’s largest satellite, Titan is also the only Solar System moon with a substantial atmosphere, and the only world other than Earth known to have liquid seas. Yesterday (December 12), the Cassini team announced they had also discovered a river flowing into one of those seas, which looks remarkably like rivers on Earth. (Mars has riverbeds, but they are now dry.)

Titan is much colder than Earth, though: the liquid in the rain, rivers, and seas is methane and other hydrocarbons, not water. Methane vaporizes at Earth temperatures, while water can stay liquid over much of the surface. On the other hand, much of Titan’s surface and interior is ice, with perhaps a hidden subsurface ocean of liquid water. Since Titan’s atmosphere is much thicker than Earth’s, Cassini uses radar to image the surface, gauging the terrain by the patterns of radio waves that come back. In the case of the seas and rivers, the radar doesn’t bounce back at all, leaving dark regions such as the ones you see in the image at right.

The river, which Cassini researchers are dubbing “Titan’s Nile”, is about 400 kilometers long (roughly 250 miles). Its valley is fairly straight by Earth-river standards, leading the team to suspect it follows a rift in the moon’s crust. Since Titan doesn’t have tectonic plates like Earth has, these rifts are possibly fractures from Saturn’s tidal forces, or they are cracks left from the period when the moon cooled after formation.

With its thick atmosphere, liquid seas, rain, and now rivers, Titan can sometimes seem like Earth, viewed through a distorting lens. Perhaps it resembles the early Earth in some respects; perhaps it shows an alternate path toward habitability. No matter what, each world we explore reveals something new, something that teaches us about our Solar System home.