“TITAN is shockingly Earth-like.” So says planetary scientist Paul Schenk of the Lunar and Planetary Institute, Houston, Texas, who has been amazed by data sent back by the Huygens probe, which landed on Titan earlier this month. “These could have been pictures from an alien probe landing along the Florida gulf coast.”
Titan is turning out to be a world in which alien materials form eerily Earth-like landscapes, as misfit streams of methane flow through rugged hills of ice. Even the mud appears familiar.
“We can now dream of sending rovers to the surface of Titan, or balloons to float around the moon”
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By combining different views from Huygens’s descent, the camera team led by Marty Tomasko of the University of Arizona has been able to produce 3D images of the moon’s surface. They show a ridge system with a peak 100 metres high. “These are water-ice ridges washed off by rainfall,” Tomasko says.
William McKinnon, an expert on the moons of the outer planets at Washington University in St Louis, Missouri, is more cautious. “Whether it actually rains or the terrain is simply covered with continuously accumulating dew, I do not know.”
But whatever the source of the liquid, it is clear that some kind of river system flows through the hills and out into the dark lowlands. Huygens’s images show that these plains are divided from the highlands by an abrupt “shoreline”. “Is this an effect of tides or wind-driven waves, or simply a tectonic boundary that has been filled in?” McKinnon asks.
Tomasko speculates that Titan might be like arid regions on Earth such as Arizona, where river beds and pools are occasionally filled by rainwater but are dry most of the time. There might even be a wet season once a year. If so, then Huygens is lying on an occasional beach. Then again, it might just have landed in a Titanian desert. Over the coming months and years, the Cassini probe now orbiting Saturn should be able to tell us which is more likely.
Huygens also found tentative evidence of volcanic activity in the form of the gas argon 40. This might have formed from the decay of radioactive potassium inside the moon and been released by volcanism. But the data is still being analysed.
Before Huygens, planetary scientists predicted that there would be liquid hydrocarbons on Titan, but they disagreed about what the liquid would be, suggesting ethane, methane and even propane. “We now know it is methane,” says Toby Owen of the University of Hawaii, a member of the Cassini-Huygens team. When Huygens hit, it sank 10 to 15 centimetres into the surface. Its heat evaporated methane in the mud, and gas was detected by an instrument on board. “We were fortunate to land in the goo,” says Owen.
As well as being wet, the “goo” resembles mud on Earth in other ways. The cameras on Huygens recorded the spectrum of light reflected off the surface and found a pattern that looks like water ice mixed with some darker substance. The dark stuff is thought to be a complex tarry mix of organic compounds created in the upper atmosphere when sunlight hits methane, then it slowly settles to the ground. It’s like humus in terrestrial soil. And the ice component could be like sand.
Settling these questions might need another visit to the surface, and there has already been speculation about what form that should take. Jean-Pierre Lebreton, Huygens’s mission manager, thinks rovers will be the key. “We can now dream of sending rovers to the surface of Titan, or balloons to float around the moon.”
