Imagine standing on the lush shores of a lake. Now imagine that it isn’t one on Earth. The vegetation disappears, the temperature drops to -179ºC, and the sky takes on a sickly orange hue. Despite a bone-chilling breeze, the lake’s dark surface lacks even the tiniest ripple. Welcome to Ligeia Mare on Titan.
Saturn’s largest satellite is an eerie world about the size of Mercury. It is the only moon in the solar system with a thick, hazy atmosphere and the only place aside from Earth that has large bodies of liquid on its surface. But there’s also one very big difference: on frigid Titan, the lakes and seas are filled with liquid hydrocarbons, such as methane. They also tend to be very smooth.
Earlier radar maps made by NASA’s Cassini probe had revealed that the largest southern lake, Ontario Lacus, is super-smooth, varying in height by less than 3 millimetres. The latest Cassini results, presented yesterday at the American Geophysical Union meeting in San Francisco, show that Ligeia Mare near the moon’s north pole is even flatter. Its surface height changes by no more than 1 millimetre.
“That’s a very interesting result, because we know there are beautiful 100-metre-high dunes near Titan’s equator, and in order to create dunes, you need wind,” says Alexander Hayes at Cornell University in Ithaca, New York. His team is now developing models to figure out why the winds are not making waves on Titan.
Flatness may be a seasonal trait, at least for Ligeia Mare, suggests Cassini project scientist Linda Spilker. A year on Titan corresponds to 30 Earth years, which means Cassini has been observing a long winter on the moon’s northern half. But soon northern spring should begin.
“In the next few years we are entering the most exciting time for Titan weather,” says Spilker. “What will these years bring as the north pole of Titan approaches summer and the sun is high in the sky? Will the lakes evaporate or fill with methane rain? Will the winds kick up, creating waves and little hurricanes on the lakes?”
Extreme flatness in the north wasn’t the only surprise from Cassini’s latest look at Titan. Previously, the liquid in Ligeia Mare was thought to be a chemical stew dominated by ethane. But the radar observations were able to peer all the way to the lake’s bottom, which suggests that the liquid is unexpectedly clear and pure. The results also show that it is mostly filled with methane, the main component in natural gas.
“Measurements indicate that the lake is 160 metres deep, and it alone contains by volume about 40 times more hydrocarbon liquid than Earth’s global oil reservoir,” says Hayes. “Together, all of Titan’s visible lakes and seas contain about 300 times the volume of Earth’s proven oil reserves.”
Could anything be alive inside these cold but carbon-rich seas?
“Titan has been an incredible mind-expander,” says Jeff Kargel at the University of Arizona in Tucson. If the moon does host life, he says, our imagination is the only limit to what it could be.