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Volcanoes and lakes in new images of Titan

Posted: August 06, 2009

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New ultra-high resolution radar images of the surface of Saturn’s moon Titan showing the effects of icy cryovolcanoes are being presented this week at the International Astronomical Union’s General Assembly in Rio de Janeiro.

Ammonia frosts have been detected evaporating on the frigid surface of Titan close to known cryovolcanoes in the Hotei regio area of the moon by the Visual and Infrared Mapping Spectrometer on NASA’s Cassini spacecraft, which can see through Titan’s thick atmosphere. Ammonia doesn’t survive long on the surface, but is believed to be pretty common deep underground. The hypothesis is that cryovolcanoes – which are volcanoes of ice rather than hot magma – belch out the ammonia and deposit it onto the surrounding plains, where it gradually evaporates.

What appears to be a dried up lake near the south pole on Titan. It appears to have a shoreline, bays, and channels running into it. A smaller lake can be seen to the south, which seems to have retained its liquid methane. The radar image is a mosaic made during several fly-bys of the moon by Cassini. The spacecraft has now imaged a third of TitanŐs surface with its radar. Image: NASA/JPL.

Volcanoes are not the only thing Titan has in common with Earth. Despite succumbing to an average temperature of –180 degrees Celsius, the surface of Saturn’s largest moon has been shown to be covered in river channels, dunes, lakes and mountain chains. But the rivers and lakes are made from liquid methane and ethane rather than water, and the dunes are ice crystals rather than sand. Even the towering mountains may be giant chunks of solid ice.

More new radar images show how the changing seasons on Titan affect the surface. Cassini’s radar instrument has detected what appears to be a dried up lake, or sea, in the southern hemisphere, which is currently experiencing ‘summer’. The feature, hundreds of kilometres across, does not appear as dark as other lakes seen on Titan (in radar imagery dark areas represent smoother surfaces like lakes that reflect radar back), but it does seem to have a shoreline to the north, and braided channels to the east and west that apparently feed the lake with liquid methane running down from the surrounding highlands, just like lakes and rivers on Earth. Over the coming years, as the seasons on Titan change again, summer will return to the northern hemisphere, the methane lakes there will begin to dry up, and then they will begin to pool again in the southern hemisphere once winter has returned there.

“Titan looks more like Earth than any other body in the Solar System, despite the huge differences in temperature and other environmental conditions,” says Dr Rosaly Lopes from the Jet Propulsion Laboratory (JPL).

Her colleague Dr Robert M Nelson, also of JPL, is willing to go further. “It has not escaped our attention that ammonia, in association with methane and nitrogen – the principal species of Titan’s atmosphere – closely replicates the environment at the time that life first emerged on Earth. One exciting question is whether Titan’s chemical processes today support a pre-biotic chemistry similar to that under which life evolved on Earth?”