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25
Aug 1st 2008
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First detection of hydroxyle in
the atmosphere of Venus
G. Piccioni, P. Drossart, L. Zasova,
A. Migliorini, JC. Gérard, FP. Mills,
A. Shakun, A. Garcia Munoz, N.
Ignatiev, D. Grassi, V. Cottini, FW.
Taylor, S. Erard and the VIRTISVenus Express team
Astronomy and astrophysics, vol.
483, pp. 29-33, mars 2008
A Dynamic upper atmosphere
of Venus as revealed by
VIRTIS on Venus Express
P. Drossart, G. Piccioni, JC. Gerard,
M. A. López-Valverde, A. SánchezLavega, R. Hueso, F. W. Taylor et
al.
Nature, vol. 450, pp 641-645,
novembre 2007.
... and Venus lost its water
Of all the planets in the Solar System, Venus is closest to the Earth in terms
of diameter, density and mass. Its atmosphere consists of much the same
elements as the Earth’s, but in very different proportions. The principal
component is carbon dioxide (96.5 % compared to 0.039 % on Earth)
followed by dinitrogen (3.5 % compared to 78.11 % on Earth). Traces of
other molecules are also found. Atmospheric pressure is 90 bars at ground
level (instead of1 bar on Earth) and ground temperatures remain at about
460° C both night and day because of the considerable greenhouse effect.
Being so close to the Sun, Venus has suffered a constant rise in
temperature, causing the planet’s large reserves of water to vaporise into the
atmosphere. These enormous quantities of water vapour in the atmosphere
intensified the greenhouse effect and the ground temperature continued to
rise. The Sun’s ultraviolet radiation broke down the water molecules in the
upper atmosphere, with the hydrogen ‘escaping’ into Space while the freed
oxygen combined with the carbon molecules to produce CO2.
Measured in Earth days, Venus orbits the Sun in 224.7 days and rotates on
its axis once in 243 days. The dense layer of clouds that hides its ground
moves round the planet in 4.2 Earth days. This movement is known as
super-rotation. Unlike the other planets in the Solar System, with the
exception of Uranus, Venus rotates in the opposite direction to the direction
in which it orbits the Sun.
In the upper atmosphere of planets, ultraviolet radiation causes numerous
chemical reactions. These reactions sometimes lead to unstable forms of
matter in so-called ‘excited’ states. During de-excitation, they emit radiation
that can be analysed by a spectrometer. Using this principle, the authors of
the article discussed here were the first to detect emissions by the hydroxyl
radical (OH) in the atmosphere of Venus using the VIRTIS instrument
onboard Venus Express.
Notes
Radical:
A radical is a chemical species
with one or more unpaired or
free electrons on its outer shell.
In chemical formulae, a radical is
denoted by a superscript dot
placed immediately to the right
of the chemical symbol. The
presence of a free electron
renders a radical highly unstable.
The European Space Agency’s Venus Express probe was launched on 9
November 2005 from the Baikonur cosmodrome in Kazakhstan. After a
voyage lasting 153 days and covering 350 million kilometres, the probe
settled into orbit round Venus on 11 April 2006. It has a quasi-polar elliptical
orbit with a periapsis at 250 km and an apoapsis at 66,000 km. Its mission
should be completed in May 2009.
The probe is carrying seven instruments for observing the planet. VIRTIS
(Visible and Infrared Thermal Imaging Spectrometer), which is sensitive to
ultraviolet, as well as visible and infrared radiation (0.25 - 5 µm), analyses
the clouds and the radiation emitted or reflected by every layer of the
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Aug 1st 2008
Notes
Luminescence:
Any light emitted by an object is
due to the de-excitation of
electrons excited by any mean.
When this excitation is provoked
by heating, this emission is
called incandescence. When it is
provoked by any other physical
or chemical mean, it is called
luminescence.
Nadir:
‘Lowest point’. Used to describe
a downward-pointing line of sight
when this passes through the
centre of the body concerned.
The two sides of Venus
atmosphere. It also measures the ground temperature enabling scientists to
investigate chemical interaction phenomena between the surface and the
atmosphere.
VIRTIS has already shown that the nocturnal emissions of atmospheric
luminescence from nitrogen monoxide (NO) and dioxygen (O2) [1], that have
long been noted in nadir-pointing observations of Venus, are not uniformly
distributed in either time or space. The carbon dioxide molecules are broken
down by sunlight on the daytime side of the planet, producing atomic oxygen.
This then recombines to form dioxygen molecules in an excited state. The
rapid transportation of atmospheric compounds due to the atmosphere’s
super rotation has made it possible for VIRTIS to detect their de-excitation
emission in the infrared spectrum on the night time side.
Limb:
‘Edge’. Used to describe a line
of sight that grazes or is
tangential to the edge of the disk
of the body concerned.
Fig 1: Comparison of light emissions obtained for different wavelengths in the infrared
spectrum with the synthetic spectrum of the hydroxyl radical at a temperature of 250 K (green
curve). The spectrum measured is shown by a black line. Maximum emissions occur during
de-excitation of the oxygen molecules for the bands centred on the 1.27 µm and 1.58 µm
wavelengths. The other peaks are characteristic of the hydroxyl radical at 1.44 µm and
2.80 µm. The coloured curves in the inset show the spectra for OH at different temperatures.
Peaks marked with an asterisk indicate thermal emissions from the underlying atmosphere.
The images show the spatial distribution of the intensity of the emission in the atmosphere of
Venus for a wavelength (arrow) of 1.58 µm (oxygen de-excitation emission) and 2.80 µm
(hydroxyl radical de-excitation emission). The yellow curve shows an altitude of 0 and the
green curve an altitude of 100 km. The use of different horizontal and vertical scales gives
these figures an elliptical appearance.
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25
Aug 1st 2008
Divorce, Venus-style
Contacts
Pierre Drossart
Laboratoire d'Etudes Spatiales
et d'Instrumentation en
Astrophysique (LESIA)
Meudon
[email protected]
More on the web
Venus Express web site
LESIA web site
INSU web site
ESA web site
th
Observations were made during the 317 orbit on 4 March 2007. Using a line
of sight pointed at the limb of the planet, the imaging spectrometer was able
to analyse the entire thickness of the atmosphere in a single acquisition. The
measurements were taken from an altitude varying between 10,800 and
13,700 km, providing a vertical resolution of less than 3.5 km. While the
probe was moving through this part of its orbit, VIRTIS acquired about a
hundred spectra over its entire range of operating wavelengths in
approximately half an hour. The area swept extends between 15 and
25°North. In order to improve the signal-to-noise ratio, spectra for identical
altitudes have been merged for the latitudes between 15 and 30° North and
for local times between 00:00 and 00:30 hours (Fig. 1). This enabled the first
detection of the hydroxyl radical in the atmosphere of Venus. It is mainly
found in a narrow layer of the atmosphere about 10 km thick, at an altitude of
96 ± 2 km.
Based on the models chosen for explaining the composition and evolution of
the atmosphere of Venus, two chemical reactions might explain the origin of
this radical:
.
H + O3 → OH + O2
Réaction (1) or Bates-Nicolet mechanism
O + HO2 → OH + O2
Réaction (2)
.
Although the observed data are compatible with both reactions, which
involve atomic hydrogen, ozone, atomic oxygen and the hydroperoxyl radical
(HO2), the models used to describe photochemical reactions in the
atmospheres of the telluric planets are based on the hypothesis that the
hydroxyl radicals are produced by the Bates-Nicolet mechanism, Reaction
(1). Based on the variable local distribution of the atmospheric luminescence
in the night sky at the limb and the nadir, it should be possible to plot the
distribution of these chemical species in the upper atmosphere of Venus.
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