Titan's turbulence surprises scientists (Forwarded)

ESA News
http://www.esa.int

30 November 2005

Titan's turbulence surprises scientists

Strong turbulence in the upper atmosphere, a second ionospheric layer
and possible lightning were among the surprises found by the Huygens
Atmospheric Structure Instrument (HASI) during the descent to Titan's
surface.

HASI provided measurements from an altitude of 1400 km down to the
surface of the physical characteristics of the atmosphere and surface,
such as temperature and density profiles, electrical conductivity, and
surface structure. The Huygens SSP made measurements just above and on
the surface of Titan.

High-altitude atmospheric structure had been inferred from earlier solar
occultation measurements by Voyager, but the middle atmosphere (200–600
km) was not well determined, although telescopic observations indicated
a complex vertical structure.

Very little was known about the surface of Titan because it is hidden by
a thick 'haze' -- initial speculation was that the surface was covered
by a deep hydrocarbon ocean, but infrared and radar measurements showed
definite albedo contrasts -- possibly consistent with lakes, but not
with a global ocean.

Earlier observations showed that the surface pressure on Titan was
comparable to that on Earth, and that methane formed a plausible
counterpart to terrestrial water for cloud and rain formation. There was
also speculation on the possibility of lightning occurring in Titan's
atmosphere that could affect the chemical composition of the atmosphere.

HASI found that in the upper part of the atmosphere, the temperature and
density were both higher than expected. The temperature structure shows
strong wave-like variations of 10-20 K about a mean of about 170 K.
This, together with other evidence, indicates that Titan's atmosphere
has many different layers.

Models of Titan's ionosphere predicted that galactic cosmic rays would
produce an ionospheric layer with a maximum concentration of electrons
between 70 and 90 km altitude. HASI also surprised the Huygens team by
finding a second lower ionospheric layer, between 140 km and 40 km, with
electrical conductivity peaking near 60 km.

HASI may also have seen the signature of lightning. Several electrical
field impulse events were observed during the descent, caused by
possible lightning activity in the spherical waveguide formed by the
surface of Titan and the inner boundary of its ionosphere.

The vertical resolution of the temperature measurement was sufficient to
resolve the structure of the planetary boundary layer. This boundary
layer had a thickness of about 300 m at the place and time of landing.
The surface temperature was accurately measured at 93.65 +/- 0.25 K and
the pressure 1467 +/- 1 hPa (very close to measurements made earlier by
Voyager, about 95K and 1400 hPa).

Notes to editors:

This summary is based on a paper which appears on line in Nature, on 30
November 2005.

For more information:

Marcello Fulchignoni, PI for Huygens Atmospheric Structure Instrument
Univ. de Paris/Observatoire de Paris-Meudon
E-mail: marcello.fulchignoni @ obspm.fr

Jean-Pierre Lebreton, ESA Huygens Mission Manager
E-mail: jplebret @ rssd.esa.int

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* Huygens 3D animation of Titan's surface
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* Huygens landing site animation
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Related links

* Mars Express instruments
http://www.esa.int/SPECIALS/Mars_Exp...C75V9ED_0.html
* Huygens instruments
http://www.esa.int/SPECIALS/Cassini-...W82VQUD_0.html
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[NOTE: Images supporting this release are available at
http://www.esa.int/SPECIALS/Results_...KRULWFE_1.html
]