17 December 2015

Scientists Map Titan’s Lakes, Revealing Clues to their Origins

Posted by lhwang


A map of Titan’s North Pole, including its lakes, sediments and complex terrain.
Credit: NASA/JPL-Caltech/Space Science Institute.

by Ramin Skibba

As Saturn’s largest moon, Titan earns its name. It’s also the only known body other than Earth with seas, numerous surface lakes, and even rainy weather. Now scientists have mapped out Titan’s polar lakes for the first time, revealing information about the moon’s climate and surface evolution. They found that the lakes formed differently than had been previously thought—and differently than any lakes on Earth.

A collaboration of scientists led by Alexander Hayes of Cornell University presented their findings at the 2015 American Geophysical Union Fall Meeting. They used NASA’s Cassini spacecraft to penetrate Titan’s smoggy atmosphere and probe the complex lake systems below.

Titan’s seas and giant lakes, which are larger than the Caspian Sea and Great Lakes, appear unique in the solar system, the study found. They consist of mostly liquid hydrocarbons like methane and ethane, possibly making them a promising location to search for building blocks of carbon-based extraterrestrial life. Because Titan is tilted with respect to its orbit, it also experiences seasons, which drive these lakes toward its North Pole. But Saturn’s eccentric orbit makes the lakes shift from pole to pole, Hayes explained.

By combining Cassini RADAR mapper observations with other data, Hayes and his colleagues compiled detailed information about Titan’s lake systems and topography, allowing scientists to test ideas for how these lakes developed.

“Topography in geology is the key because it drives the evolution of landscapes,” said Samuel Birch, lead author of one of the Titan studies and a Ph.D. student at Cornell.

Hayes and his collaborators found that the lakes have floors at similar elevations and appear to be regionally linked by subsurface channels. Scientists had thought that the lake depressions form as the moon’s surface dissolves, like the formation process of sinkholes on Earth. However, with their new observations, Hayes saw that the lakes have raised rims and sharply rising walls hundreds of meters high, refuting that hypothesis. The lakes were “made from the undulating plain as if with a cookie cutter,” Hayes said.

In a separate presentation, Birch and colleagues showed topographical maps of Titan’s North and South Pole terrains, including a diversity of lakes and canyons. In the south, they saw unfilled lake depressions, long since emptied of their contents except for layers of sediments left behind.

Finally, Cornell researcher Marco Mastrogiuseppe, along with Hayes and Birch, described their third set of results. They focused on the composition and depth of Titan’s major lakes and seas, such as Ligeia Mare and Ontario Lacus. These turned out to be deeper and more methane-rich than scientists previously thought. The abundance of methane, which has a biological origin on Earth, remains unexplained on Titan.

Together, these observations and topographical maps provide planetary scientists with constraints on the formation of Saturn’s unusual moon. Hayes looks forward to Cassini’s final flyby of Titan in 2017, giving them one more chance to examine its complex and dynamic surface.

– Ramin Skibba is a freelance science writer and a graduate student in the UC Santa Cruz science communication program. You can follow him on twitter at @raminskibba