Flying over the cloudy world - science updates from Venus Express
European Space Agency
12 July 2006
On 20 April 2006, after its first 9-day, elongated orbit around Venus,
ESA’s Venus Express started to get closer to the planet, until it
reached its final 24-hour long orbit on 7 May. During this time, and up
to today, the spacecraft has been working relentlessly: the new data
coming in are already providing first glimpses on planetary features
never seen before.
If taking the first ever clear images of the double-eye vortex at
Venus’
south pole - imaged by Venus Express during its very first orbit - was
already a first in the history of planetary exploration and a very
pleasant surprise for the scientists, nobody could expect that the
vortex had a structure even more complicated than possibly foreseen.
Infrared images taken by the Ultraviolet/Visible/Near-Infrared
spectrometer (VIRTIS) on board the spacecraft not only provided the
first clear view of the vortex, but also gave a much closer insight
into
it when Venus Express flew over the south pole at the end of May this
year.
VIRTIS is an instrument that can operate at different wavelengths. Each
infrared wavelength provides a view of the Venusian atmosphere at a
different altitude, like a ‘cross-section’. “When we looked at this
gigantic vortex at different depths, we realised how much its shape is
varying over altitude,” said Pierre Drossart, VIRTIS co-Principal
Investigator, from the Observatoire de Paris, France. “It is like if we
were looking at different structures, rather than a single one. And the
new data we have just started gathering and analysing reveal even
stronger differences”.
The reason why the morphology of the vortex varies so extensively along
a ‘vertical’ line is still unexplained. “This is why we are organizing
a
campaign to observe the south polar vortex, fully dedicated to solve
this unexpected puzzle,” said Giuseppe Piccioni, VIRTIS co-Principal
Investigator. “First we want to understand how the structure is
organized - actually, with VIRTIS we are building a true 3D view of the
vortex. Then we hope to be able to better understand what are the
driving forces that shape it”.
Tracking clouds and winds
While Venus Express was flying over the planet, many other details from
the thick atmosphere have also started to emerge. Both the Venus
Monitoring Camera (VMC) and the VIRTIS instruments started to monitor
the cloud system and to track its complex dynamics, while the
SpicaV/SOIR spectrometers started retrieving information on the
atmospheric chemistry and temperature.
Ultraviolet images from the VMC camera show the complex morphology of
the cloud deck, characterised by very thin, low-contrast
stripe-features, possibly due to the presence of strong winds that
produce elongated structures. Set of periodic ‘wave’ patterns in the
clouds, possibly due to the local variation of temperature and
pressure,
or to a kind of tidal forces in action at Venus, can also be seen.
One of the most important confirmations from the first set of data
being
analysed by the scientists is the detection of the so called ‘UV
absorbers’- ultraviolet markings on the cloud top, also visible as
darker features in the VMC mosaic image. They are so called because
they
absorb almost half of the solar energy received by the planet. The
mysterious substance that causes this absorption still represents a
true
puzzle for the scientists.
“Understanding what is the origin of these ultraviolet markings and
what
makes their absorbing power so high is one of the major objectives of
Venus Express,” said Wojciech J. Markiewicz, VMC Principal
Investigator,
from the Max Planck Institute for Solar System Research in Lindau,
Germany. “We now have confirmation that we can actually see them, so we
can start working to understand what their source is. Because of their
amazing absorbing power, they are very important to understand the
overall radiative and thermal balance of the planet, and also the
atmospheric dynamics”.
Tracking cloud motion and starting to characterise the wind speed is an
exercise that the Venus Express scientists have already started. A
spectacular night view of the mid to low atmospheric layers over low
latitudes (between 20? and 90 ? south) by VIRTIS, show clouds being
clearly pushed by winds.
“We can now make a first qualitative assessment of the wind fields and
circulation, which is comfortably matching with previous measurement
from the Galileo mission over the north pole,” continued Giuseppe
Piccioni. “We are now collecting more data from different atmospheric
depths, to be able to provide the first precise numbers, possibly in
the
near future”.
“We are also collecting the first information on the minor chemical
components of the atmosphere, such as carbon monoxide,” added Pierre
Drossart. “With VIRTIS we can see in the atmosphere of the southern
hemisphere deeper than any other previous mission, and we started
gathering data on the yet unknown chemistry of the lower atmospheric
layers, to build a global picture. Studying the variation of minor
chemical compounds over different latitudes and depths is also a very
useful tracer for the atmospheric global motion.”
Surprise at the atmospheric ‘top’
When looking at the higher atmospheric layers with Venus Express, the
scientists were taken once more by surprise. It is in fact know that
the
Venusian cloud deck is about 20 kilometres thick and it extends up to
about 65 kilometres altitude over the planet. The first ‘stellar
occultation’ measurements ever done at the Venus thanks to the SpicaV
spectrometer, revealed that on the night side the cloud deck actually
extends up to 90 kilometres altitude in the form of a fully opaque
haze,
and then continues as a more transparent haze up to 105 kilometres.
Stellar occultation is a technique that allows to determine the
composition of a planet’s atmosphere by looking at the ‘sunset’ of a
pointed star through the atmosphere itself. “On Earth the atmosphere
becomes perfectly clear already above 20 kilometres altitude,” said
Jean-Loup Bertaux, SpicaV/SOIR Principal Investigator, from the Service
d’A?ronomie of CNRS, France.
“We were truly amazed to see how unexpectedly higher the haze at Venus
can get. Actually, on Earth as well as on Venus, at around 20
kilometres
it is sometimes possible to see droplets of sulphuric acid. On Earth
they come from volcanic eruptions. It makes us wonder if on Venus,
where
differently from Earth the droplets form very thick clouds, their
origin
is volcanic too.”
The haze phenomenon may be due to water condensation in ice crystals on
the night side, but it is too early to rule out other explanations.
“Now
we need to gather and study more data to understand this phenomenon in
the high atmosphere - an area that, before SpicaV, was still virtually
unexplored,” he concluded.
Bertaux also expressed his satisfaction for the atmospheric detection
of
‘heavy water’ - a molecule similar to water but with higher mass -
thanks to the SOIR spectrometer. “The detection of heavy water in the
atmosphere of a planet, and its percentage with respect to normal
water,
is very important to understand how much water was present on the
planet
in the past, and how much of it escaped,” added Bertaux.
"The amount of water vapour present today in the atmosphere of Venus
would be enough to cover the planet with a 3-centimetre deep liquid
layer. If we find out that heavy water - a trace of the original water
is massively present in the top atmospheric layers where it can more
easily escape, than the amount of water in the past may have well
corresponded to a layer up to a few hundred metres deep," Bertaux
concluded.
Studying the atmospheric escape process at Venus is actually one of the
major objective of another Venus Express instrument - ASPERA (Analyzer
of Space Plasma and Energetic Atoms). The instrument already detected
the massive escape of oxygen and tracked trajectories of other
planetary
ions such as singly-charged helium.
“This early detection confirms the strong interaction between the solar
environment and the atmosphere of Venus - a planet without a planetary
magnetic field to protect it from the incoming solar wind,” said
Stanislav Barabash, ASPERA Principal Investigator, from the Swedish
Institute of Space Physics in Kiruna, Sweden. “The study of this
interaction will provide important clues on the complex set of
mechanisms by which atmospheric gases get lost in space, and on the
influence that this may have had on Venus’ climate over geological time
scales”, he concluded.
The status of the spacecraft
On 4 July 2006 Venus Express passed an important exam. An ESA board
declared the conclusion of the spacecraft in-orbit commissioning phase
and declared that the spacecraft has met the requisites to officially
enter the operational phase of its scientific mission.
The Venus commissioning phase, started on 7 May when Venus Express
reached its final 24-hour orbit around the planet, and concluded on 4
June this year, is a series of operations aimed at validating the
performance of the spacecraft and its systems in the Venus environment,
of the scientific instruments, and of all ground systems and
operations.
The spacecraft and instruments are showing an overall good performance.
However, one of the instruments on board - the Planetary Fourier
Spectrometer (PFS) - showed a malfunctioning, that could not be fixed
yet in the series of attempts performed so far in space. The PFS
scanner
- the mirror needed by the instrument for pointing - is currently
blocked in a close position, preventing the instrument spectrometer
from
‘seeing’ its targets.
The commissioning review board endorsed a series of activities and
further in-orbit tests to be conducted in the next months, as well as a
series of independent investigations to examine the origin of the
problem. In the meantime, other instruments will cover some of the PFS
objectives.
PFS is designed to measure the chemical composition and temperature of
the atmosphere of Venus. It is also able to measure surface
temperature,
and so search for signs of volcanic activity.
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
Jean-Loup Bertaux, SpicaV Principal Investigator, Service d’A?ronomie
du
CNRS,Verri?res-le-Buisson, France
Email: Jean-Loup.Bertaux @ aerov.jussieu.fr
Wojciech Markiewicz, VMC Principal Investigator, Max Planck Institute
for Solar System Research, Lindau, Germany
Email: markiewicz @ linmpi.mpg.de
Stanislav Barabash, ASPERA Principal Investigator, Swedish Institute of
Space Physics, Kiruna, Sweden
Email: stas @ irf.se
Vittorio Formisano, PFS Principal Investigator, IFSI-CNR, Rome, Italy
Email: formisan @ ifsi.rm.cnr.it