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'Aeronomie du CNRS, France, Ann-Carine Vandaele,
Institut d'Aeronomie 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 Patzold, Universitat zu Koln, 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'Aeronomie 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 Patzold, VeRa co-Investigator
Universitat zu Koln, Germany
Email: martin.paetzold @ geo.uni-koeln.de

Bernd Hausler, VeRa Principal Investigator
Institut fur Raumfahrttechnik, Universitat der Bundeswehr Munchen, Germany
Email: Bernd.Haeusler @ unibw-muenchen.de

Hakan 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_Ex...A373R8F_1.html ]