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Science Forum Index » Astro Forum » The jet stream of Titan (Forwarded)
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| Andrew Yee |
 Posted: Wed Jan 24, 2007 11:52 am |
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ESA News
http://www.esa.int
24 January 2007
The jet stream of Titan
A pair of rare celestial alignments that occurred in November 2003 helped
an international team of astronomers investigate the far-off world of
Titan. In particular, the alignments helped validate the atmospheric model
used to design the entry trajectory for ESA's Huygens probe.
Now the unique results are helping to place the descent of Huygens in a
global context, and to investigate the upper layers of Titanıs atmosphere.
Occasionally Titan passes directly in front of a distant star. When it
does so, the light from the star is blocked out. Because Titan has a thick
atmosphere, the light does not 'turn off' straight away. Instead, it drops
gradually as the blankets of atmosphere slide in front of the star. The
way the light drops tells astronomers about the atmosphere of Titan.
By pure chance on 14 November 2003, fourteen months before Huygensı
historic descent through Titan's atmosphere, Titan passed in front of two
stars, just seven and a half hours apart. Bruno Sicardy, Observatoire de
Paris, France, organised expeditions to record the occultations, as such
events are called.
The first occultation was visible just after midnight from the Indian
Ocean and the southern half of Africa. The second could be seen from
Western Europe, the Atlantic Ocean, Northern and Central Americas. Teams
of astronomers set up along the occultation tracks.
Sicardy was looking for one observation in particular. "Titan's atmosphere
acts like a lens, so at the very middle of the occultation, a bright flash
occurs," explains Sicardy. If Titan's atmosphere were a perfectly uniform
layer, the central flash would be a pinprick of light, visible only at the
very centre of the planet's shadow. However, comparing the results from
many telescopes, Sicardy found that the central flash fell across the
Earth in a triangular shape.
"It is like the light falling through a glass of water and making bright
patterns on the table. The focused light is not perfectly round because
the glass is not a perfect lens," says Sicardy. Analysing the shape of the
flash showed that Titanıs atmosphere was flattened at the north pole. This
was because at the time of the occultation, Titan's south pole was tilted
towards the Sun. This warmed the atmosphere there, causing it to rise and
move towards the north of the moon, where the atmosphere cooled and sank
towards the surface.
There was one other key discovery that the occultation data allowed
Sicardy and his team to make. A fast moving, high altitude wind (above 200
kilometres) was blowing around Titan at latitude of 50 degrees north. They
estimated that it was moving at 200 metres per second (or 720 kilometres
per hour) and would encircle the planet in less than one terrestrial day.
"It is like the jet stream on Earth," says Sicardy, "Furthermore, we told
the Huygens team to expect some bumps near 510 kilometres altitude, due to
a narrow and sudden temperature variation."
Indeed, Huygens was jolted by exactly such a layer during its 14 January
2005 entry. "A temperature inversion was indeed detected by the
accelerometers during entry at this very altitude" says Jean-Pierre
Lebreton, Huygens project scientist.
The work does not stop there. Even though the Huygens descent took place
almost two years ago, the understanding of its data continues to provide
key insights into Titan.
Note to editors
The findings appear in the article "The two Titan stellar occultations of
14 November 2003", by Bruno Sicardy et al., published in Journal of
geophysical research (Vol. 111, E11S91, doi:10.1029/2005JE002624, 2006).
For more information:
Bruno Sicardy, Observatoire de Paris, France
Email: bruno.sicardy @ obspm.fr
Jean-Pierre Lebreton, ESA Cassini-Huygens Project Scientist
Email: jean-pierre.lebreton @ esa.int
[NOTE: Images and weblinks supporting this release are available at
http://www.esa.int/esaCP/SEMQO5SMTWE_index_1.html ] |
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