Flash! Lakes Confirmed in Titan's Northern Hemisphere!

NASA's Cassini spacecraft captured this image of sunlight
reflecting off of a lake on the surface of Saturn's moon Titan.
Credit: NASA/CassiniIn a literal flash of insight—well, "infrared sight," really--NASA's Cassini spacecraft has confirmed the existence of lakes of liquid in the Northern Hemisphere of Saturn's moon, Titan. That's really big news for scientists, and for those like me who absolutely love to envision the sweep of the landscape on alien worlds, be they science fiction or, as in this case, fact….

The evidence for the existence of lakes on Titan has been building since 2004, when the Cassini spacecraft, and its Titan-landing probe Huygens, arrived in the Saturn system and began collecting data.

First it was imagery from the Huygens camera as it descended through Titan's thick, hazy atmosphere: what looked like dark, flat, featureless regions defining apparent coastlines along solid terrain, as well as dentritic patterns like river channels draining into them.

Then it was imagery from Cassini's smog-penetrating infrared cameras showing numerous systems that looked for all the moon like giant lakes—on the order of size of Earth's Great Lakes. Cassini radar bouncing off the surfaces suggested that they were exceptionally flat, as one expects a watery surface to be.

Then there was confirmation of surface liquid in Titan's Southern Hemisphere—but not in the north, where the lake-like shapes were by far more numerous…an apparent land o' lakes.

Finally, what scientists hoped to see appeared, in a flash: sunlight, reflecting off of a Titanian lake called Kraken Mare, came shining through the moon's haze and was captured by Cassini's infrared camera. It's the same kind of reflection you see when sunlight blazes off the surface of the ocean before sunset. Astronauts see the same thing from Earth orbit, looking down at the ocean-lined limb of the Earth when the Sun is at the right position over it.

It required the right conditions; the Sun had to be in just the right position relative to the lake for the reflection to be seen. Ever seen the light of the setting Sun reflecting off the window of a distant house, though only for a moment when the geometry is just right? And only recently did the Sun rise over Kraken Mare's extreme northern latitude, after the 15-year dark of a Titanian arctic winter.

Titan is the only Solar System object known to have surface liquid, other than Earth. Jupiter's moon Europa is thought to harbor a vast ocean of liquid water, but under its outer crust of ice. Another of Saturn's moon's, Enceladus, also appears to hold liquid water inside, but its surface is as dry as the Earth's Moon. And yes, the Moon was recently confirmed to have surface water—but it's all frozen and mixed in with the lunar soil in a sort of dry cryo-mud.

The difference with Titan, however, is that its surface liquid is not water at all, but methane. It's too cold on Titan's surface for liquid water to exist there—but Titan's atmospheric pressure and temperature are right for methane to exist in its liquid state. So while on Earth we know methane as a greenhouse gas emitted by decomposing plants and the guts of cows—to name a couple of sources—on Titan it is the stuff of cloud, of rain, of river, and of lake.

What a wild world!

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