Saturn鈥檚 moon Titan is as dry as a bone over most of its surface, suggest new infrared images from Earth.
The work supports similar observations from the Cassini spacecraft, in orbit around the Ringed Planet. And it suggests previous radar studies from the ground 鈥� which hinted the giant moon was covered in liquid methane seas 鈥� were actually detecting signs of liquid that had long since vanished.
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NASA鈥檚 twin Voyager spacecraft discovered in 1980 and 1981 that methane formed a relatively large component of Titan鈥檚 thick atmosphere. That led to speculation that the gas was being constantly replenished from liquid methane seas, since ultraviolet radiation is thought to destroy the gas in 10 million years 鈥� a small fraction of Titan鈥檚 4.5 billion-year history.
Radar bounced off Titan using the Arecibo Observatory in Puerto Rico in 2003 strengthened this view. In 12 of the 16 sites surveyed, the surfaces appeared very smooth 鈥� on the scale of 13-centimetre radiowaves 鈥� with reflective properties that indicated those areas were covered by liquid.
Reflecting on findings
The Cassini spacecraft has also observed intriguing liquid-related features. It has detected dark, river-like channels since it neared the moon in 2004. And the Huygens probe, which was dropped down to the moon鈥檚 surface, sent back detailed photos of channels near its landing site.
But Cassini鈥檚 visible and infrared cameras have failed to find the reflections expected from surface liquid. These instruments measure wavelengths of light ranging from about 0.25 to 5 microns (or millionths of a metre) long.
Now, astronomers observing 2.1-micron-long infrared light at the Keck Observatory in Hawaii have reported similar findings. 鈥淸Before Cassini] we expected to see a large amount of liquid on the surface 鈥� perhaps 100 metres deep on average,鈥� says lead author Robert West, a planetary scientist at NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California, US. 鈥淵et we don鈥檛 see it. It鈥檚 hard to understand.鈥�
Evaporating oceans
The study offers several explanations for the conflicting observations of liquid on Titan. 鈥淎t one time, maybe a liquid water and ammonia mix flowed onto the surface and froze,鈥� says West. 鈥淭hat could be smooth on the scale of radar but rough on the scale we see.鈥�
Alternatively, the radar may have bounced off a relatively flat layer of organic material left over after liquid hydrocarbon rivers or seas evaporated. 鈥淭hat may explain the dark areas on Titan,鈥� West says.
A third possibility is that organic particles in Titan鈥檚 haze settled out of the atmosphere and were blown by wind into low-lying areas, forming smooth surfaces.
But the lack of widespread liquid methane raises questions about the source of methane gas in the moon鈥檚 atmosphere. West says subsurface geothermal activity may heat the frozen moon and allow the gas to leak continuously into the atmosphere.
He says another, less likely, scenario is that an 鈥渙utgassing event鈥� 鈥� possibly a large impact 鈥� warmed up Titan鈥檚 interior within the last 10 million years and freed methane trapped in the ice there. Perhaps even less likely again is that the methane is generated by life 鈥� Titan is exceedingly cold.
Journal reference: Nature (vol 436, p 670)
