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The
light and dark of Venus
Thursday, February 21, 2008
This
is a picture of Venus’s atmosphere, taken by the Venus
Monitoring Camera (VMC) during Venus Express orbit number
443 on 8 July 2007. The view shows the southern hemisphere
of the planet.
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Credit:
ESA/ MPS/DLR/IDA
Venus Express has revealed
a planet of extraordinarily changeable and extremely large-scale
weather. Bright hazes appear in a matter of days, reaching from
the south pole to the low southern latitudes and disappearing
just as quickly. Such ‘global weather’, unlike
anything on Earth, has given scientists a new mystery to
solve.
The cloud-covered world of Venus is all but a
featureless, unchangeable globe at visible wavelengths of light.
Switch to the ultraviolet and it reveals a truly dynamic nature.
Transient dark and bright markings stripe the planet, indicating
regions where solar ultraviolet radiation is absorbed or
reflected, respectively.
Venus Express watches the
behavior of the planet’s atmosphere with its Venus
Monitoring Camera (VMC). It has seen some amazing things. In July
2007, VMC captured a series of images showing the development of
the bright southern haze. Within days, the high-altitude veil
continually brightened and dimmed, moving towards equatorial
latitudes and back towards the pole again.
Such global
weather suggests that fast dynamical, chemical and microphysical
processes are at work on the planet. During these episodes, the
brightness of the southern polar latitudes increased by about a
third and faded just as quickly, as sulphuric acid particles
coagulate.
“This bright haze layer
is made of sulphuric acid,” says Dmitri Titov, VMC
Co-Investigator and Venus Express Science Coordinator, Max Planck
Institute for Solar System Research, Germany. That composition
suggests the existence of a formation process to the VMC team.
At an altitude of about 70 km and below, Venus’s
carbon dioxide-rich atmosphere contains small amounts of water
vapor and gaseous sulfur dioxide. These are usually buried in the
cloud layer that blocks our view of the surface at visible
wavelengths.
However, if some atmospheric process lifts
these molecules high up above the cloud tops, they are exposed to
solar ultraviolet radiation. This breaks the molecules, making
them highly reactive. The fragments find each other and combine
quickly to form sulphuric acid particles, creating the haze.
“The process is a bit similar to what happens with
urban smog over cities,” says Titov. With over 600 orbits
completed, the VMC team now plan to look for repeating patterns
of behavior in the build-up and decrease of the haze layer.
What causes the water vapor and sulfur dioxide to well up
in the first place? The team does not know yet. Titov says that
it is probably an internal dynamical process in the planet’s
atmosphere. Also, the influence of solar activity on haze
formation has not been completely ruled out.
When the
team have worked out what causes the hazes and their vigorous
dynamics, there is still another problem waiting to be solved.
The dark markings on these images are one of the biggest
remaining mysteries of Venus’s atmosphere. They are caused
by some chemical species, absorbing solar ultraviolet radiation.
However, as yet, planetary scientists do not know the identity of
the chemical. Now that they can spot these dark patches quickly
with VMC, the team hopes to use another Venus Express instrument,
VIRTIS, to pinpoint the exact chemical composition of these
regions.
Source:
ESA
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