Ice crystal tumbler on Titan

Titan’s largest highland region is sparkling with the reflected light of countless crystal balls of ice, washed down slopes by flash floods instigated by downpours of methane rain, judging by new analysis of radar observations that show they are the brightest features on Saturn’s largest moon.

A radar image of part of Xanadu, with some of the bright channels visible. The larger channels are as wide as the mouth of the River Thames, about five kilometres across. Image: NASA/JPL.

River channels on Titan’s largest ‘continent’, Xanadu, are reflecting microwaves from the Cassini spacecraft’s radar instrument back more strongly than their surroundings, to the extent that they are the brightest features on the moon ‘s surface, in radar maps at least. Scientists at NASA’s Jet Propulsion Laboratory (JPL) have judged that the channels are filled with small spheres of ice, ranging from centimetres up to a metre in size. According to JPL’s Alice Le Gall, such spheres are perfect for reflecting light, or indeed microwaves, back in the direction of the spacecraft, making such areas appear bright in radar maps.

Think of pebbles at the seaside, or on a river bank, that have been tumbled by particles of grit or sand as they tumble through the water; the ice crystals on Titan are forming in exactly the same way. “Bouncing downstream smoothes out the edges of the rocks... a lot like what creates polished river rocks on Earth,” says Le Gall.

Pebbles seen at the Huygens landing site (left) and on a river bank on Earth (right). Image: NASA/JPL/ESA/University of Arizona/S M Matheson.

The difference is that on Titan it’s not a flow of water that is doing the tumbling, but a flow of liquid hydrocarbons (molecules that combine hydrogen and carbon such as methane, ethane, acetylene and ethylene). On Titan it is so cold (–179 degrees Celsius) that the water cycle has been replaced with a methane cycle while water is frozen into the hard bedrock of the moon. When this concoction of organic chemistry rains out of Titan’s smoggy atmosphere, it can trigger landslides that pick up stones and boulders of ice, and Xanadu is the perfect environment for forming crystals. Its immense size, 3,400 kilometres across, and gentle south-facing slopes mean flows of methane moving at a metre per second pick up rocks and boulders of solid ice, tumbling them for hundreds of kilometres at a time.

This isn’t the first time that pebbles have been seen on Titan. In 2005 the European Space Agency’s Huygens probe landed in the middle of a floodplain on Titan to find a landscape littered with pebbles and icy boulders. However, the brilliance of the radar returns from Xanadu suggest that the channels and floodplains there are covered in many more, round ice crystals that are packed close together and line the bottom of the channels, awaiting the next flash flood to tumble them further downstream. Linda Spilker, Cassini Project Scientist at JPL, comments that “As the seasons change on Titan, maybe we’ll get a chance to see methane flow through some of the river channels.” As Cassini’s mission has been extended to 2017, there’s every chance that we could see these flash floods in action.