18 September 2015

Titan’s “bucket brigade” brings liquid to moon’s north pole

Posted by Nanci Bompey

By Larry O’Hanlon

This near-infrared, color mosaic from NASA's Cassini spacecraft shows the sun glinting off of Titan's north polar seas. A new study suggests a "bucket brigade" “bucket brigade” brings liquid to moon’s north pole. Credit: NASA/JPL-Caltech/University of Arizona/University of Idaho

This near-infrared, color mosaic from NASA’s Cassini spacecraft shows the sun glinting off of Titan’s north polar seas. A new study suggests a “bucket brigade” “bucket brigade” brings liquid to moon’s north pole.
Credit: NASA/JPL-Caltech/University of Arizona/University of Idaho

Researchers think they have found a veritable bucket brigade that has been slowly but surely drenching the north pole of Saturn’s moon Titan – the only world in the solar system, other than Earth, to have lakes, seas and rainy weather.

For about a decade – ever since the Cassini–Huygens probe peered through Titan’s smoggy atmosphere to reveal its surface – Titan’s extra wet northern region has puzzled scientists.

Because Titan only receives a hundredth of the energy that Earth gets from the Sun, and averages about 180 degrees below zero Celsius (292 degrees below zero Fahrenheit), the seas and stormy weather on Titan are made of liquid methane, which can exist at very low temperatures. The moon’s northern pole is full of these methane lakes and seas. By contrast, the southern hemisphere of Titan has only one significant body of liquid.

Scientists have resorted to models to try and reproduce the conditions that create the world’s wet northern region.

“We’ve been looking into different aspects of Titan’s climate for the last several years,” said Titan researcher Juan Lora of the University of California at Los Angeles. “We looked at different simulations to understand types of weather events in two dimensions and three dimensions. In two dimensions the atmosphere of Titan did not produce the asymmetrical liquid distribution. But we saw the asymmetry develop very quickly in 3D.”

Scientists thought the underlying cause of the differing amounts of surface liquids at the poles was the eccentric orbit of Saturn, and thus Titan. This orbit produces a shorter, more intense summer at the moon’s southern hemisphere and a longer, but less intense summer in the north. But how that translated into more surface liquid for the north had not been explained.

In a new study, Lora and his colleagues used three-dimensional models of Titan to show that methane, which quickly evaporates from the moon’s equator, also slowly circulates in the atmosphere from the south pole to the north pole.

That matches what has been observed on Titan, with its arid low latitudes, wet polar regions, and lots of clouds moving methane around the south pole during its summer, Lora explained.

More specifically, the three-dimensional model shows the methane being carried towards the equator from both poles in pulses, or eddies, in the atmospheric currents. These pulses form a “bucket brigade” that is more intense in the south, carrying more liquid away from that region to the north, according to the new study published online in Geophysical Research Letters, a journal of the American Geophysical Union.

“Over thousands to tens of thousands of years it builds up in the north,” said Lora, lead author of the new study. The result is the lopsided situation seen on Titan today, he said.

The new modeling work is hardly the last word on the subject, said Lora. Among the things it does not address are the complexities of the surface and subsurface of Titan. Including these features will make the model more realistic and allow scientists to understand how the surface and subsurface might affect the atmosphere, how the atmosphere interacts with the lakes and if there might be a methane table, like a water table, underground, Lora said.

Lora hopes that future models will be able to tie together these things and produce an even clearer picture of this cold, little cousin of Earth.

– Freelance science journalist Larry O’Hanlon acts as AGU’s blogs manager and social media coordinator.