The unexpected temperature profile of Venus's atmosphere (Forwarded)

ESA News
http://www.esa.int

28 November 2007

The unexpected temperature profile of Venus's atmosphere

Venus has a rich and complicated atmosphere -- the densest of all the
rocky planets -- which is the key to understanding the planet itself.
Venus Express, designed to perform an extensive investigation of the
atmosphere, has revealed surprising details about its temperature
structure.

Venus has thick clouds of sulphuric acid that extend between altitudes of
40 to 60 km. Above this, the region between 60 to 100 km is known as the
mesosphere, and is a transition region between the lower winds, which whip
the cloud tops around the planet in four days, and the circulation of the
upper atmosphere, which is driven by the influx of solar radiation. Having
absorbed solar radiation, the hot upper atmosphere rises still further,
circulating to the night side of the planet where it cools and sinks back
to the level of the cloud tops.

Jean-Loup Bertaux, Service d'Aéronomie du CNRS, France, Ann-Carine
Vandaele, Institut d'Aéronomie Spatiale de Belgique, and colleagues have
now used Venus Express to discover an unexpectedly warm layer of air on
the planet's night-side.

It sits between the altitudes of 90­120 km, a region that is generally so
cold at night that scientists often refer to it as Venus's cryosphere. The
new measurements show that the temperature excess ranges from 30 to 70 C
and peaks at an altitude of 100 km.

This region of Venus's atmosphere contains light hazes of various aerosol
particles, composed chiefly of sulphuric acid and water. These are highly
variable and can often be seen as bright regions of the planet.
Nevertheless, it is a poorly studied region of the planet because earlier
spaceprobes that descended through the atmosphere only began their
measurements at or below 60 km.

Venus Express made the measurements using its SPICAV/SOIR instrument
(SPectroscopy for the Investigation of the Characteristics of the
Atmosphere of Venus/Solar Occultation in the InfraRed). This watches
stars, or even the Sun, as they set behind Venus, a technique known as an
occultation.

The instrument measures the amount of light absorbed by the atmosphere at
different wavelengths, and by doing so it identifies both the chemicals
and the temperature in the different layers of the atmosphere.

The 'temperature inversion', as the layer of warm air is called, was
detected in several stellar occultations performed on the night-time side
of the planet. The only thing that can heat the atmosphere here is when
pockets of gas sink back down into the denser atmosphere. The increased
air pressure squeezes the pockets, raising the temperature of the gas
inside (similar to what happens when you activate a bicycle pump).

Another instrument on Venus Express has been probing the temperatures in
the atmosphere, from the rarefied reaches of Venus's ionosphere at heights
of 500-100 km, down to around 50 km above the surface. This is the Venus
Express Radio Science (VeRa) experiment, that works a bit like
SPICAV/SOIR, except that in this case the spacecraft is the 'star' that
astronomers on Earth watch set behind Venus.

VeRa emits ultra-stable radio waves that travel through Venus's atmosphere
on their way to Earth and that can be detected by radio telescopes on
Earth. The time delay in these radio waves can be translated into
information about the atmosphere of Venus.

Martin Pätzold, Universität zu Köln, Germany, and colleagues have
determined the fine structure in temperatures at Venus's upper cloud-deck,
detected distinct day-to-night temperature differences in the southern
middle atmosphere, and tracked day-to-day changes in Venus's ionosphere
(the upper atmospheric layer).

So far, measurements have been obtained from only a limited number of
latitudes and local solar times. Should the Venus Express mission be
extended, the coverage will be improved and a global characterisation of
the temperature in three dimensions will be possible.

Notes for editors:

The results appear in the 29 November issue of the scientific journal
Nature, in the papers: 'A warm layer in Venus' cryosphere and
high-altitude measurements of HF, HCL, H2O and HDO, by J-L.Bertaux, A-C.
Vandaele et al., and 'The structure of Venus' middle atmosphere and middle
ionosphere', by M.Patzold et al.

For more information:

Jean-Loup Bertaux, SPICAV/SOIR Principal Investigator
Service d'Aéronomie du CNRS
Email : Jean-Loup.Bertaux @ aerov.jussieu.fr

Anne-Carine Vandaele, SPICAV/SOIR co-Investigator
Belgian Institute for Space Aeronomie, Belgium
Email : annc @ bira-iasb.oma.be

Martin Pätzold, VeRa co-Investigator
Universität zu Köln, Germany
Email: martin.paetzold @ geo.uni-koeln.de

Bernd Häusler, VeRa Principal Investigator
Institut für Raumfahrttechnik, Universität der Bundeswehr München, Germany
Email: Bernd.Haeusler @ unibw-muenchen.de

Håkan Svedhem, ESA's Venus Express Project Scientist
Email: Hakan.Svedhem @ esa.int

[NOTE: Images supporting this release are available at
http://www.esa.int/SPECIALS/Venus_Express/SEM5A373R8F_1.html ]


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