15:00 30 November 2005
Cassini, Huygens’ mothership, imaged the many layers of Titan's upper atmosphere in this ultraviolet image, which has been adjusted to look like true colour (Image: NASA/JPL/SSI)
Titan's atmosphere is remarkably like Earth's, but even more complex and multilayered, according to results from the European Space Agency’s Huygens probe.
The lander also saw signs of lightning and found chemical clues to the source of Titan's methane, which probably bubbles up from deep inside Saturn’s giant moon.
Titan is the only satellite in the solar system to have any appreciable atmosphere. It is mainly nitrogen, like Earth’s air, but it is four times as dense as our terrestrial atmosphere. As a result, the parachute-braked descent of Huygens to the surface in January took a leisurely 2.5 hours, giving it ample time to sample the gases around it.
The atmosphere turns out to be an exotic layer-cake. It has a troposphere and stratosphere, as on Earth, divided by a boundary called a temperature inversion. But much further out – 500 kilometres above the surface and higher – Huygens’ Atmospheric Structure Instrument (HASI) detected many more such inversions, each defining their own narrow atmospheric layer.
That is unlike Earth, says Marcello Fulchignoni, head of the HASI team. But it comes as no great surprise, because Titan's atmosphere is so much thicker. Fulchignoni thinks that some of these upper layers may be quite stable and may undulate in great slow waves, driven by Saturn's gravity.
Down below, a thick layer of orange smog clings to the top of the stratosphere, 200 to 250 kilometres up, and there is a thinner layer of haze at an altitude of about 20 km.
In between the two, Huygens entered an unexpected zone. Mission scientists found that Titan's fierce winds die down rapidly below a height of 100 km. At 80 km there is almost a dead calm, but then as the probe fell further the winds rapidly strengthened to a bracing 140 km/hour. The team have no explanation yet for Titan's doldrums.
A little lower down, HASI picked up several bursts of electrical activity, possibly echoes of lightning elsewhere on Titan. The electrical waves had a frequency of about 36 hertz – a very low rumble. But Fulchignoni is cautious. "It could be an indication of lightning, but maybe the discharge is due to something inside the probe," he says.
Any lightning would probably be generated in clouds of methane, the gas that forms about 5% of Titan's atmosphere. It seems to play an analogous role to water on Earth, raining down to carve branching drainage patterns in the hills.
But the very presence of methane is a puzzle – it is destroyed in the stratosphere by sunlight, forming the orange smog of more complex organic molecules which eventually settles to the ground. This means the methane is being replenished from an unknown source.
A clue comes from another Huygens instrument – its Gas Chromatograph Mass Spectrometer (GCMS) – which has detected a telltale variety of argon. Argon-40 is generated by the radioactive decay of potassium-40, present in Titan's rocky core.
To reach Titan's atmosphere, the argon must have got through the ice crust, presumably in some kind of volcanic eruption – so the methane could be coming out that way too.
The GCMS team suggest two possibilities. Methane might be constantly generated by chemical reactions in the core. Or it might be stored in a great reservoir under the crust, as a methane-water ice.
The Huygens results are published in a series of papers in Nature.