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Huygens' triumphant ten years on Titan

Kulvinder Singh, News reporter
Jan 17, 2015, 7:02 UTC

Sen—It was the most distant landing in the Solar System and took 147 minutes to descend to the surface. Once there it beamed 72 minutes of data to Earth before its batteries expired.

On Wednesday the European Space Agency’s (ESA) Huygens probe celebrated another milestone: 10 years on Titan—Saturn's largest moon.

And what a peculiar place it is. Titan has by far the thickest atmosphere of any moon in the Solar System. Thicker even than Earth's—at 1.45-times atmospheric pressure. Though composed of at least 95 per cent nitrogen, Huygens showed that the lower atmosphere contains a photochemical smog—much thicker than the kind that forms in polluted cities on Earth.

It also gave us our first-ever view of the surface when it landed in Titan's Xanadu Region. A chilly world of -180°C, Titan has dry, pebble-strewn river beds made of frozen hydrocarbons and water instead of rocks. These are likely submerged by liquid methane and ethane flows pouring into rivers from the deep drainage channels that Huygens observed.

Hitching a ride on NASA's Cassini spacecraft at launch, on 15 October 1997, it took seven years and 3.47 billion km for Huygens to reach its target (longer than Titan's 1.22 billion km-distance due to Cassini's planetary fly-bys). Despite lasting just over an hour on the surface, over 1160 scientific papers to date have been published using Huygens' data—the latest so far on 26 December 2014.

Professor John Zarnecki of the Open University is Principal Investigator of one of the science instruments on-board Huygens: the Science Surface Package. In his 10-year anniversary blog for Sen he articulates some of his worries at the time: “We had no idea what surface we would land on. Would it be solid ice, as hard as concrete? Or would it splash down and sink in the hypothesised sea of liquid methane? And maybe even worse, if Huygens did survive, would it find a bland, boring, wishy-washy place?”

Much to Zarnecki's relief, Huygens did survive and didn't sink into anything (though as Cassini discovered, Titan has two, large liquid hydrocarbon seas). And much to his delight, Titan wasn't boring at all. In fact the moon was more intriguing than first suspected.

Alongside the thick atmosphere, hydrocarbon seas, and river channels, Huygens' detected winds of 430 km per hour at an altitude of 120 km. This is 10 times Titans' own rotation speed and is called super-rotating wind.

Another aspect of the atmosphere is the presence of the gas argon-40 (40Ar), a by-product of the decay of potassium-40 (40K)—a radioactive isotope of potassium. The only place this could originate is from rocks deep within Titan's interior. One way 40Ar could escape into the atmosphere is through cryovolcanoes—volcanoes of ice instead of lava.

And the interior could have one, final, surprise to offer. An instrument on Huygens designed to detect lightning, instead discovered radio signals connected to electrical currents in Titan's ionosphere. These signals (know as Schumann resonances) are reflected by something below Titan's surface—something conductive. Possibly a water-ammonia ocean below the icy crust.

This amazing video uses data from Cassini and Huygens to reproduce the experience of approaching Titan and descending towards the surface.