Lakes of methane could cover three-quarters of Saturn鈥檚 moon Titan, according to radio echoes from this cloud-shrouded world. If so, ESA鈥檚 Huygens spaceprobe could well be heading for a splashdown when it parachutes to the moon鈥檚 surface in January 2005.
A series of 25 radar observations, taken when Saturn made close approaches to Earth in 2001 and 2002, have provided by far the best evidence yet of methane lakes. Astronomers have suspected since the Voyager fly-bys of the 1980s that liquid hydrocarbons were present on Titan鈥檚 surface.
However, proving the presence of liquid hydrocarbons has been difficult because the moon鈥檚 surface is hidden by a nitrogen-rich atmosphere that blocks optical wavelengths.
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Infrared observations have provided rough maps of Titan鈥檚 surface but produced more evidence of water ice, rather than methane lakes. Other observations have shown methane clouds growing and shrinking in Titan鈥檚 atmosphere, suggesting a 鈥渕ethane cycle鈥� that includes methane rain and then evaporation back into the clouds.
Interplanetary radar
Now, Donald Campbell, at Cornell University, US, and colleagues have used the 305-metre Arecibo dish in Puerto Rico as an interplanetary radar to bounce hundreds of kilowatts of microwaves off Titan. Then, with either Arecibo, the world鈥檚 largest radio telescope, or the Green Bank 100-metre dish, they listened for the faint echo and analysed the results.
In three-quarters of their observations, as well as the diffuse echo from the entire hemisphere of Titan, they received a component that had been reflected from a flat surface on the moon, such as an ice sheet or an undisturbed pool of liquid.
Campbell told 快猫短视频: 鈥淭his was being produced by something that could only reflect about two per cent of the radio wave that hit it. Such a poorly reflecting surface is characteristic of liquid hydrocarbons.鈥� Ice would reflect much more.
The reflecting surfaces Campbell found are anything from a few tens to a few hundreds of kilometres in diameter. At that size, some researchers believe that these are methane lakes, sitting in impact craters.
Liquid landing
Ralph Lorenz, at the University of Arizona, says: 鈥淟iquid will collect in low places, and impacts are a good way of making low places.鈥� He estimates that Titan might reasonably be expected to possess 80 craters of 150 km in size and thousands of smaller ones.
The final answer on Titan鈥檚 methane will come 2005, when the European Space Agency鈥檚 Huygens spaceprobe will parachute to the surface of Titan.
Mindful of the possibility of a liquid landing, ESA have designed the probe to float and resist capsizing if necessary. It may be, thinks Campbell: 鈥淚f you take this data at face value, I think that the probability is high that Huygens will splash down.鈥�
Journal reference: Science (DOI: 10.1126/science.1088969)