http://www.esa.int/SPECIALS/Venus_Ex...5OV74TE_0.html

Complex meteorology at Venus
Venues Express
European Space Agency
13 October 2006

In its relentless probing of Venus's atmosphere, ESA's Venus Express
keeps revealing new details of the Venusian cloud system. Meteorology
at
Venus is a complex matter, scientists say.

New night-side infrared images gathered by the Ultraviolet, Visible and
Near-Infrared Mapping Spectrometer (VIRTIS) in July 2006, clearly show
new details of a complex cloud system.

The first (false colour) view - the composite of three infrared images
acquired by VIRTIS, was taken on 22 July when the spacecraft was flying
around the apocentre of its orbit (point of maximum distance from the
planet surface) at about 65 000 kilometres altitude. Venus was in the
night side.

Using its capability to observe at 1.7-micrometre wavelenght, VIRTIS
could probe at about 15-20 kilometres altitude, below the thick cloud
deck situated at about 60 kilometres from the surface. The thermal
radiation coming from the oven-hot surface of Venus is represented by
the intensity of the colours: the brighter the colour (towards white),
the more radiation comes from the surface, so the less cloudy the
region
in the line of sight between the view and the spacecraft is.

The edge of the images, taken at a time interval of about 30 minutes
from each other, do not precisely match. This is due to the fact that
clouds on Venus move very rapidly and constantly vary their shape.
Venus?s atmosphere is certainly the most dynamic among the
terrestrial
planets that have one, taking only four days to completely rotate
around
the planet.

"Clouds at Venus present repetitive patterns and recurrent features,
but
they are very variable in position both on short and long time scales,"
said Giuseppe Piccioni who, with Pierre Drossart, is the instrument
co-Principal Investigator. "This makes meteorology a very complex
matter
for this planet."

Since the thermal radiation from the surface of Venus is in practice
modulated by the presence of the clouds, taking the negative of the
image it is possible to see directly the clouds structure on the night
side, and so study its morphology and dynamics.

This was done for this other night-side image, acquired by VIRTIS on 29
July around apocentre from a distance of about 65 000 kilometres from
the surface.

The 2.3-micrometre wavelength used for this image brought the 'view'
again down below the cloud deck. The large cloud visible in the centre
of the image and extending toward the bottom-right part is about 2000
kilometres long and 500 kilometres wide.

This cloud presents the familiar and peculiar shape of clouds at Venus.
They are usually elongated due to the very high-speed winds ?
reaching
up the formidable velocity of 360 kilometres per hour and being caused
by the 'super-rotation' of the atmosphere.

The very cloudy region visible at the top-right of the image is
situated
beyond 60?? South latitude, and represents the transition to the
region
where the atmosphere is dominated by the effects of the powerful South
double polar vortex.

For more information

Hakan Svedhem, ESA Venus Express Project Scientist
Email: hakan.svedhem @ esa.int

Giuseppe Piccioni, VIRTIS co-Principal Investigator, IASF-INAF, Rome,
Italy
Email: giuseppe.piccioni @ iasf-roma.inaf.it

Pierre Drossart, VIRTIS co-Principal Investigator, Observatoire de
Paris, France
Email: pierre.drossart @ obspm.fr