Saturn’s largest moon, Titan, is an intriguing place. With its surface temperature averaging -180°C, more than a hundred degrees colder than an Antarctic winter, any visitor would expect to find a solid frozen wasteland. Yet when the Cassini spacecraft first arrived in 2004 and began peering through its thick cloud layers with radar, scientists found something completely unexpected. Titan has a climate very much like Earth’s, with rain, rivers, and even oceans with waves crashing on pebbly beaches. The crucial difference, though, is the temperature. On Titan, the weather is so cold that water is just another rock in the ground, while ethane and methane gas condense from the atmosphere into rain which fills rivers that feed a system of seas and lakes.
Cassini mission controllers have steered the spacecraft in recent months to perform a series of close fly-bys of Titan so that they can map its surface in more details. Titan is covered with a thick hazy atmosphere full of clouds and smog, so special techniques are needed to see all the way down to the ground. In the image above, Cassini used an infra-red filter on its wide-field camera to penetrate the haze. But for the most accurate data, mission scientists used the on-board radar to record surface features and map the sea beds. They paid the most attention to the region around Titan’s two largest seas, known as Kraken Mare and Ligeia Mare, since they’re especially interested in comparing the hydrology of Earth with that of another world.
“Learning about surface features like lakes and seas helps us to understand how Titan’s liquids, solids and gases interact to make it so Earth-like,” said Steve Wall, acting radar team lead at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “While these two worlds aren’t exactly the same, it shows us more and more Earth-like processes as we get new views.”
Surprisingly, they found that the free liquids on Titan are concentrated on one small area of the moon’s surface. Fully 97% of all surface liquid on Titan falls within an area measuring 900 by 1800 kilometres. “Scientists have been wondering why Titan’s lakes are where they are. These images show us that the bedrock and geology must be creating a particularly inviting environment for lakes in this box,” said Randolph Kirk, a Cassini radar team member at the U.S. Geological Survey in Flagstaff, Ariz. “We think it may be something like the formation of the prehistoric lake called Lake Lahontan near Lake Tahoe in Nevada and California, where deformation of the crust created fissures that could be filled up with liquid.”
Further study of radar data, using techniques originally developed to examine Mars, allowed planetary scientists to look beneath the surfaces of the lakes. By a stroke of good luck, the fluid in the lakes and seas turned out to be remarkably pure, allowing the radar signals to penetrate all the way to the lake beds, and confirming that they are filled mostly with liquid methane. The lakes are remarkably calm, with smooth surfaces, and are deeper than expected – Ligeia Mare bottoms out at 170 meters deep. “Ligeia Mare turned out to be just the right depth for radar to detect a signal back from the sea floor, which is a signal we didn’t think we’d be able to get,” said Marco Mastrogiuseppe, a Cassini radar team associate at Sapienza University of Rome. “The measurement we made shows Ligeia to be deeper in at least one place than the average depth of Lake Michigan.”
Thanks to these measurements, we now have an estimate of just how much liquid is present on the surface of Titan. Alexander Hayes, of Cornell University has calculated that there are 9000 cubic kilometres of liquid methane – some forty times the volume of all proven oil reserves on Earth. But questions remain: Where did all these hydrocarbons come from, and why are they so concentrated in just one region? Hopefully as summer approaches in the northern hemisphere of Titan, and weather patterns adapt, we’ll find the key to these mysteries.