A River on Titan

If viewed in black and white, it could be Earth,

with river deltas, shores and sculpted rock.

But sands in endless dunes round half its girth,

are ever-frozen grains of ice which flock,

enslaved by Saturn’s tidal-driven winds.

Revealed in filtered amber twilight haze,

the similarity to Earth rescinds.

- Diane Hine, On Saturn’s Moon (16 April 2012)

Titan is the only other world in the solar system that has stable bodies of liquid on its surface. With a surface temperature of minus 180 degrees Centigrade, Titan is so cold that water is frozen solid and as hard as a rock. Instead, the liquid that fills its lakes and flows in its rivers is a mixture of ethane and methane. It is a challenge to image Titan because its surface is shrouded under a thick and opaque atmosphere. As a result, NASA’s Cassini spacecraft uses radar to map Titan’s surface by bouncing pulses of microwave energy off the surface of Titan and measuring the time it takes for the pulses to return to the spacecraft. Essentially, the radar instrument on Cassini “sees” the surface of Titan using microwaves instead of visible light.

Figure 1: The colourful globe of Titan passes in front of Saturn and its rings in this true colour snapshot from NASA’s Cassini spacecraft. The image was obtained with the Cassini spacecraft narrow-angle camera on 21 May 2011, at a distance of approximately 2.3 million kilometres from Titan. Credit: NASA/JPL-Caltech/Space Science Institute.

On 26 September 2012, a swat of radar imagery of Titan’s surface was acquired by Cassini. The image shows a river system which stretches more than 400 kilometres in length and empties into a large sea known as Ligeia Mare in Titan’s north polar region. It appears dark along its entire length and this indicates a smooth surface, consistant with it being filled by some form of liquid. The liquid is probably a mixture of ethane and methane. This radar image is a good snapshot of a “hydrological” cycle on another world. Here on Titan, liquid ethane and methane falls as rain, and rivers transport the liquid into lakes and seas where evaporation kicks off the cycle all over again.

Figure 2: Radar image of a river on Saturn’s moon Titan. This “Nile-like” river stretches more than 400 km from its ‘headwaters’ into Ligeia Mare - one of the three great seas in the high northern latitudes of the moon. The image was acquired on 26 September 2012, on Cassini’s 87th close flyby of Titan. Credit: NASA/JPL-Caltech/ASI.

The formation of this river system probably involved some faulting process since part of the river flows along fault lines. A fault line is basically a fracture or discontinuity in a volume of rock. “Though there are some short, local meanders, the relative straightness of the river valley suggests it follows the trace of at least one fault, similar to other large rivers running into the southern margin of this same Titan sea,” says Jani Radebaugh, a Cassini radar team associate at Brigham Young University, Providence, Utah.