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Astronomie - Der Winter kommt ... am Südpol von Saturn-Mond Titan

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Titan has seasons just like Earth, only each season lasts over seven years instead of three months courtesy of NASA
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Gigantic polar clouds of hydrogen cyanide roughly four times the area of the UK are part of the impressive atmospheric diversity of Titan, the largest moon of Saturn, a new study led by Leiden Observatory, the Netherlands Institute for Space Research and the University of Bristol has found. The research is published today in Nature.
Titan is unique in our solar system because of its dense nitrogen-methane atmosphere, which is very similar to Earth's in some ways, but very different in others.  For example, air temperatures are around 200 degrees colder and, in contrast to the warm salt water seas of Earth, frigid hydrocarbon lakes populate Titan's surface.
Titan has seasons just like Earth, only each season lasts over seven years instead of three months due to its ponderous orbit around the Sun. After equinox in 2009, Titan's south pole entered the perpetual darkness of polar winter.  Soon after, instruments on NASA's Cassini spacecraft observed the development of a gigantic polar cloud covering over one million square kilometres – roughly four times the area of the UK.
Bristol researcher and co-author Dr Nick Teanby said: "The cloud was first seen in images from Cassini's cameras taken in 2012.  It started off quite small but soon grew to cover the entire south polar region.  This was totally unexpected and set us puzzling over what the cloud could be made of. Unfortunately, while the images showed that the cloud was very high up, at over 250km above the surface, they did not allow us to figure out what the cloud was actually made of or why it was there."
For the next two years Cassini gathered more data including infrared spectra of the cloudy region.  
Lead author Remco de Kok said: "When we looked at the spectra, we saw two large peaks that weren't present in spectra of other places on Titan.  These peaks coincided exactly with the peaks you'd expect from ice particles of hydrogen cyanide, or 'blauwzuur' (blue acid) as it's known in the Netherlands, which is highly toxic.  This was very surprising to us, since we did not expect HCN ice to be able to form so high in Titan's atmosphere."
This new research suggests that Titan's south pole must be extremely cold to allow hydrogen cyanide to condense.  In fact, the upper atmosphere must have cooled by over 50 degrees in less than a year to reach a blisteringly cold -150C.  
Remco de Kok concluded: "This is a very rapid change given Titan's long annual cycle and is much colder than previously thought possible.  It suggests that once the pole is in shadow the upper atmosphere acts as a very efficient radiator of heat, perhaps due to the high abundance of exotic hydrocarbon and nitrogen based compounds, which emit strongly in the infrared.  
"Cassini is set to continue observing Titan until it takes a dive into Saturn at the end of its mission in 2017.  It will be fascinating to see how the cloud will develop."
Quelle: University of Bristol
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TITAN’S SWIRLING POLAR CLOUD IS COLD AND TOXIC
Spectral map of Titan
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The international Cassini mission has revealed that a giant, toxic cloud is hovering over the south pole of Saturn’s largest moon, Titan, after the atmosphere has cooled in a dramatic fashion.
Scientists analysing data from the mission found that this giant polar vortex contains frozen particles of the toxic compound hydrogen cyanide.
“The discovery suggests that the atmosphere of Titan’s southern hemisphere is cooling much faster than we expected,” says Remco de Kok of Leiden Observatory and SRON Netherlands Institute for Space Research, lead author of the study published in the journal Nature.
Unlike any other moon in the Solar System, Titan is shrouded by a dense atmosphere dominated by nitrogen, with small amounts of methane and other trace gases. Almost 10 times further from the Sun than Earth, Titan is very cold, allowing methane and other hydrocarbons to rain onto its surface to form rivers and lakes.
Like Earth, Titan experiences seasons as it makes its 29-year orbit around the Sun along with Saturn. Each of the four seasons lasts about seven Earth years and the most recent seasonal switch occurred in 2009, when summer transitioned to autumn in the southern hemisphere.
In May 2012, images from Cassini revealed a huge swirling cloud, several hundred kilometres across, taking shape at the south pole.
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Artist’s impression of the change in observed atmospheric effects before, during and after equinox in 2009. The Titan globes also provide an impression of the detached haze layer that extends all around the moon (blue). 
During the first years of Cassini’s exploration of the Saturnian system, Titan sported a ‘hood’ of dense gaseous haze (white) in a vortex above its north pole, along with a high-altitude ‘hot spot’ (red). During this time the north pole was pointed away from the Sun. 
At equinox both hemispheres received equal heating from the Sun. Afterwards, the north pole tilted towards the Sun, signalling the arrival of spring, while the southern hemisphere tilted away from the Sun and moved into autumn. 
After equinox and until 2011 there was still a significant build up of trace gases over the north pole, but the vortex and hot spot had almost disappeared. Instead, similar features began developing at the south pole, which are still present today. 
These observations are interpreted as a large-scale reversal in the single pole-to-pole atmospheric circulation cell of Titan immediately after equinox, with an upwelling of gases in the summer hemisphere and a corresponding downwelling in the winter hemisphere.
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This polar vortex appears to be an effect of the change of season, with large amounts of air being heated by sunlight during the northern spring and flowing towards the southern hemisphere.
A puzzling detail about this swirling cloud is its altitude, some 300 km above Titan's surface, where scientists thought it was too warm for clouds to form.
“We really didn’t expect to see such a massive cloud so high in the atmosphere,” says Dr de Kok.
Keen to understand what could give rise to this mysterious cloud, the scientists turned to the rich data from Cassini. After careful scrutiny, they found an important clue in the spectrum of sunlight reflected by Titan’s atmosphere.
A spectrum splits the light from a celestial body into its constituent colours, revealing signatures of the elements and molecules that are present. The Visual and Infrared Mapping Spectrometer on Cassini takes spectra at many different points on Titan, mapping the distribution of the chemical compounds in its atmosphere and on its surface.
“The light coming from the polar vortex showed a remarkable difference with respect to other portions of Titan’s atmosphere,” says Dr de Kok. “We could clearly see a signature of frozen hydrogen cyanide molecules – HCN.”
Quelle: ESA
 
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