The way the wind blows on Titan (Forwarded)

ESA News
http://www.esa.int

1 June 2007

The way the wind blows on Titan

A simulation of the winds encountered by Huygens has lead planetary
scientists to believe that it's entire atmosphere is circulating around on
a conveyor belt. This huge system of moving gas transports warm air from
the southern hemisphere to Titan's north pole and back again.

As on any body with an atmosphere, the direction and speed of the wind
encountered at a single point can be related to the general atmospheric
circulation. So by reproducing the winds encountered by Huygens during its
parachute descent to the surface, planetary scientists have been able to
improve their ideas about Titan.

At first, all they had to go on with was the way in which Earth's
atmosphere behaved. "Some of the early computer models were actually based
on Earth's circulation," says Jean-Pierre Lebreton, ESA Huygens Project
Scientist. To this they added temperature measurements of Titan's
atmosphere taken by NASA's Voyager spacecraft and the bulk properties of
Titan such as the mass of the world, its rate of rotation, the amount of
heating from the Sun and the tides experienced by the gravitational pull
of Saturn.

Now, they have the wind profile for the entire descent through the
atmosphere to add into the mix -- and it is making a world of a
difference. "Our knowledge of the low-level atmospheric circulation was
virtually absent prior to the Huygens mission," says Tetsuya Tokano,
Institut für Geophysik und Meteorologie, University of Köln, Germany, who
has been spearheading the new effort at modelling the atmospheric
circulation on Titan.

Huygens encountered its maximum wind speed about ten minutes after
beginning its descent. The speed was roughly 120 metres per second (or 432
kilometres per hour) and was measured at an altitude of about 120
kilometres. As the probe dropped below 60 kilometres, the wind speed
dropped too. During the final seven kilometres of the descent, Huygens
encountered wind speeds of just a few metres per second, allowing it to
drop in an almost straight line. At the surface of Titan, there was
nothing but a gentle breeze of just 0.3 metres per second.

During its descent, Huygens found that the winds were flowing in the
direction of Titan's rotation, from west to east. The winds reversed
direction twice. The first reversal was at six kilometres and the second
occurred just 700 metres above the ground. These points have turned out to
be vital in understanding the general circulation of Titan's atmosphere.

Tokano's model suggests the upper reversal is caused by temperature
differences between the north and south. The lower reversal happens at the
boundary between the upper and lower portion of a huge circulating pocket
of air, known as a Hadley cell. This extraordinarily large 'cell' of
rotating atmosphere circulates from the south pole to the north pole and
back again and is the principal way in which Titan's atmosphere
distributes its warmth.

On Titan, the southern hemisphere is currently facing the Sun, making it
southern summer on the moon. Warm southern 'air' rises and flows towards
the colder northern hemisphere, forcing colder air from the north down
towards the south. This cooler air is less buoyant and hence flows at
lower altitudes.

"At the time of the landing of Huygens, the model suggests that Titan must
have been warmer at 10 degrees south than it was at the equator," says
Tokano. Southern summer on Titan will last until 2010, when Saturn's
orbit, which governs the moon's motion, will tilt the northern hemisphere
towards the Sun. Such a large Hadley cell is only possible on a slowly
rotating world, such as Titan where one day equals 16 Earth days.

So, even though Titan may superficially look Earth-like, by feeding the
details of the winds encountered by Huygens into a computer model of the
moon's atmospheric circulation, planetary scientists have shown that first
impressions can sometimes be deceptive. The wind systems on Titan are
unlike those on the Earth, even though they are governed by the same
physical principles.

This is good news, for it provides a whole new climatological system for
planetary scientists to study.

Note for editors

This article is based on two papers to appear in a special issue of the
Planetary and Space Science magazine dedicated to Huygens results:
'Near-surface winds at the Huygens site on Titan: interpretation by means
of a general circulation model', by T. Tokano, and 'Titan atmosphere
profiles from Huygens engineering (Temperature and acceleration) sensors',
by R. D. Lorenz.

Cassini-Huygens is a joint mission between NASA, ESA and the Italian Space
Agency (ASI).

For more information

Tetsuya Tokano
Institut fuer Geophysik und Metorolgie, Univ. of Köln, Germany
Email: Tokano @ geo.uni-koeln.de

Ralph D. Lorenz
John Hopkins Univ. Applied Physics Lab., MD, USA
Email: Ralph.Lorenz @ jhuapl.edu

Jean-Pierre Lebreton, ESA Huygens Project Scientist
Email: Jean.Pierre.Lebreton @ esa.int

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