Buon compleanno a Venus Express!

http://www.esa.int/esaSC/SEMCTD0CYTE_index_0.html

Happy birthday, Venus Express!
European Space Agency
9 November 2006

One year after its launch on 9 November 2005 and a few months into its
science phase, ESA’s Venus Express keeps working well and continues to
gather lots of data about the hot and noxious atmosphere of the planet.
Newly released images show additional details of the thick cloud deck
that surrounds Venus.

It was 11 April 2006 when, after a delicate manoeuvre, Venus Express
entered into orbit around Venus, and started a series of gradually
smaller loops around the planet to reach its 24-hour science orbit
(spanning between 66 000 over the South pole and 250 kilometres over
the
North pole) on 7 May 2006.

Atmospheric investigations by Venus Express

“From that time onwards this unique spacecraft, equipped with the most
advanced instruments ever used for atmospheric investigations at Venus,
has started gathering views and information on the thick atmosphere,
its
cloud system and its dynamics - during experiment tests in the
beginning, and on a nominal basis after 4 June 2006,” said Hakan
Svedhem, Venus Express Project Scientist.

From the very first images of the approaching planet and from the South
Pole views obtained on 12 April - the first of this area taken in the
infrared in the history of Venus’ exploration - scientists immediately
obtained novel glimpses about an extraordinarily complex weather
system.

This is ruled by huge and still largely unexplained forces at work in
the atmosphere, causing hurricane-force winds and generating the
amazing
double-eyed vortices over both poles.

It was indeed this European spacecraft to entirely reveal for the first
time the double vortex over the planet’s South pole, with its 3D
structure so much differentiated over different altitudes and similarly
shaped (but mirrored) to that present over the North pole.

Newly released images, obtained in July and September 2006 by the
Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on board
Venus Express, show new details of the cloud structure. This thick
layer
of clouds, located around 60 kilometres altitude over the surface,
blocks part of the thermal radiation coming from below.

However, VIRTIS is designed to catch the only radiation that can pass
through the lower atmopshere and the cloud deck, making use of the so
called ‘infrared windows’. Through these ‘windows’, thermal (infrared)
radiation at specific wavelengths can cross the lower atmosphere and
escape towards space. By doing so, it carries information about the
lower atmospheric layers and the surface that VIRTIS can collect from
orbit.

The first, false-colour night-side image (see top of the article), was
taken at a 1.7-micrometre wavelength on 23 September 2006, when the
spacecraft was flying at about 60 000 kilometres over the planet around
the point of furthest distance from the surface (apocentre). It shows
an
area close to the South pole (out of the field of view beyond the top
left of the image).

“The bright big spot on the left of the image corresponds to an area
where the cloud deck is thinner,” said Giuseppe Piccioni, VIRTIS
co-Principal Investigator. “Such regions, similar to large holes, allow
the thermal radiation from below the clouds layer to increase
significantly with respect to the surrounding areas, and make it
possible to probe very deep in the atmosphere with a limited
attenuation
from the clouds.”

The atmospheric layers below the cloud deck, and the fact that they are
blocking almost all thermal radiation coming from the surface, are the
main responsible for the tremendous green-house effect at work on the
planet - the most powerful found in the Solar System. It maintains the
surface temperature as high as 450??C!

The second, night-side image of the southern hemisphere was taken by
VIRTIS on 29 July 2006 from a distance of about 64 000 kilometres over
the surface (around the orbit apocentre) at a wavelength of 1.7
micrometres. The South pole is visible on the top left of the image.

This image provides a remarkable example of a wave structure, running
from the bottom to the top-right, each ‘wave’ extending about 150
kilometres. This peculiar cloud feature is often seen at a latitude of
about 55?? South.

“Despite the fact that the cloud structure is very variable and dynamic
at Venus, recurrent patterns and structures tend to appear in the same
locations,” said Pierre Drossart, VIRTIS co-Principal Investigator.
“They are mostly visible in the infrared, sometimes very easily, some
other times less. When they are very faint and hidden, we need to play
with the intensity of the images.”

The picture also shows a part of the polar vortex (top left). Regions
of
thinner clouds are present also in this image. They are visible as
bright spots (top left corner), as they allow more thermal radiation to
escape towards deep space from the hotter regions below.

The region between the black stripe around the pole and the wave
structure contains the so called ‘cold collar’, a region in which the
temperature of the clouds is lower than that of the surrounding area.
Similar cold-collar structures are present at both poles. The cold
collar over the South pole cannot be clearly seen in this image (1.7
micrometre wavelenght), but it was was imaged by VIRTIS at 5
micrometres
in April 2006.

In the meantime, VIRTIS and the other instruments on board Venus
Express
keep observing the mysterious atmosphere of Venus. Well done so far
and…happy anniversary, Venus Express!

For more information

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

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

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