The first sea discovered on any surface other than Earth's may have been found on Saturn’s moon Titan.
New radar images from the Cassini spacecraft, which made its eighth close approach to the moon on 7 September, have revealed what appears to be a very distinct shoreline, fed by meandering channels carved deeply in the surrounding terrain.
The dark, flat region next to the bright shoreline "is the area where liquid or a wet surface has most likely been present, now or in the recent past", says Steve Wall, Cassini radar team deputy leader from NASA-JPL.
And several long sinuous channels can be seen cutting through the bright region and ending at the shoreline, suggesting the existence of an Earth-like cycle of evaporation, rainfall and river systems to carry the liquid back to the sea. But instead of water, the liquid in this case is believed to be methane, kept liquid at Titan's -179°C surface temperature.
Seas of liquid methane, perhaps mixed with other hydrocarbons, had long been expected on Titan, the second-largest moon in the solar system. It is also the only moon with a thick atmosphere - thicker than Earth's.
Seas seemed necessary to explain the amount of methane seen in Titan's atmosphere. The fact that no clear evidence for such seas had been found was one of the big mysteries of the Cassini mission.
"We've been looking for evidence of oceans or seas on Titan for some time," Wall says. The quest was one of the main goals of the four-year Cassini mission. The discovery of the new features suggests Titan may indeed have periodic episodes of methane rainfall.
Ellen Stofan, another radar team scientist, says the network of bright channels indicate "that fluids, probably liquid hydrocarbons, have flowed across this region".
Some of the channels extend more than 100 kilometres, says Larry Soderblom of the US Geological Survey in Flagstaff, Arizona. The channels appear to be of two different types, some long and deep with few tributaries, others forming denser networks. "Some of them may have been fed by springs, while others are more complicated networks that were likely filled by rainfall," Soderblom said.
Titan’s abundance of hydrocarbons, thick atmosphere, and now evidence for the presence of large volumes of liquid, means it is considered a close analogue of the early Earth.
It may even host some of the early complex chemistry that on Earth led to the first living organisms, but whose traces have long been obliterated here. Cassini project scientist Dennis Matson has called Titan a "time machine" for studying how the early Earth "evolved into a life-bearing planet".
But exploring the new-found sea further will be tricky. Two coincidental glitches caused half of the data from the radar flyby to be lost. One glitch affected the spacecraft's data recording system, the other the receiving antenna in Goldstone, California, US.
Even worse, the region will not fall within the radar's view on any of Cassini’s 37 remaining Titan approaches. However, all the data from Cassini's cameras and spectroscopes was saved and may reveal more details of the features.
Cassini's next close flyby of Titan will take place on 26 October and will focus on the region where the Huygens landing probe hurtled down to Titan's surface in December 2004.
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