Tenuous Ozone Layer Discovered in Venus' Atmosphere (Venus Express)

http://sci.esa.int/science-e/www/object/index.cfm?fobjectid9412

Tenuous ozone layer discovered in Venus' atmosphere
European Space Agency
06 Oct 2011

Using observations of Venus performed with an instrument on ESA's Venus
Express scientists have detected, for the first time, a tenuous layer of
ozone in this planet's atmosphere. Located at an altitude of about 100
km, the layer is up to a thousand times less dense than the one found,
at a lower altitude, in the Earth's stratosphere, but both are dominated
by very similar chemical reactions. The discovery poses new challenges
to the characterisation of planetary atmospheres, especially in the
quest for biomarkers on extrasolar planets.

Ozone (O_3) is one of the most notable molecules populating the
atmosphere of the Earth. Present there only in moderate quantities with
respect to other, more abundant molecules - nitrogen, oxygen and carbon
dioxide, among others - ozone plays an essential role in preserving life
on Earth. Mostly concentrated in a dense layer in the stratosphere, at
altitudes between 15 and 50 km, atmospheric ozone absorbs the most
energetic ultraviolet photons from the Sun, those which are harmful to
living organisms. By absorbing this radiation, ozone is also responsible
for the temperature inversion in the stratosphere.

The Earth's layer of ozone is a result of the large abundance of
molecular oxygen (O_2) in our planet's atmosphere. When irradiated by
solar ultraviolet rays, oxygen molecules are broken into atoms, which
may, in turn, react with other oxygen molecules and create ozone. These
reactions also take place in the atmosphere of other planets, as long as
they harbour sufficient amounts of molecular oxygen. In fact, traces of
ozone were first detected forty years ago in the tenuous atmosphere of
Mars. Until now no evidence of this molecule had been found in the
atmosphere of Venus, in spite of theoretical predictions.

At last, observations performed with one of the instruments on ESA's
Venus Express spacecraft have now clearly identified a thin layer of
ozone in the upper atmosphere of Venus. The results are reported in a
paper published in the journal Icarus and are being presented at the
European Planetology Network and Division for Planetary Sciences of the
American Astronomical Society (EPSC-DPS) joint meeting, held this week
in Nantes, France.

"We were surveying the atmosphere of Venus and focussing on other
molecules when we stumbled on a very interesting feature in one of the
spectra," comments Franck Montmessin from the Laboratoire Atmosphߨres,
Milieux, Observations Spatiales (LATMOS) in Guyancourt, France.
Montmessin led the team who carried out this study. "The spectral
signature of ozone, a distinctive absorption band at UV wavelengths, was
rather pronounced and we could clearly discern it in our plots with the
naked eye," he adds.

The data have been gathered with the Spectroscopy for Investigation of
Characteristics of the Atmosphere of Venus (SPICAV) instrument on board
Venus Express using the stellar occultation method. This technique
allows astronomers to indirectly probe a planet's atmosphere by studying
its influence on the light from distant stars.

"As it orbits Venus, the spacecraft's line of sight to a given star
crosses the planet's atmosphere, meaning that starlight is prone to
being absorbed by the atmosphere," explains co-author Jean-Loup Bertaux
from LATMOS and Universitß© de Versailles-Saint-Quentin, France. Bertaux
is also the Principal investigator of SPICAV. "In the process, chemical
species present in the atmosphere leave their characteristic signatures
in the star's spectrum, enabling us to pinpoint them," he adds.

After they identified this first ozone feature in the spectrum from an
orbit of Venus Express, the team of astronomers ran a dedicated search
over data from nearly 300 orbits. The analysis revealed robust evidence
for this molecule in about ten per cent of the orbits. According to the
new data, ozone is located at varying altitudes in the Venusian
atmosphere, between 90 and 120 km, and is always confined to a rather
thin layer, measuring 5 to 10 km across. "The varying altitude and
narrow radial extent of the detected patches are particularly striking,
hinting that an as-yet-undetermined mechanism must be keeping the ozone
right there," notes Hakan Svedhem, ESA Venus Express Project Scientist.

The ozone layer on Venus is very tenuous - up to a thousand times less
dense than that on Earth. Nonetheless, it could be readily detected
using SPICAV because the stellar occultation method, by peering
tangentially through the layers of the atmosphere, probes a large path
of the atmosphere and is thus sensitive to chemical species even in low
concentrations. A drawback of the method, however, is that it relies on
the availability of stars in the line of sight, meaning that it yields
an uneven coverage of the planet and is limited to the night side. This
means that investigations of the spatial or temporal distribution of
ozone across the planet are not straightforward.

"Interestingly, we expected to detect ozone at the anti-solar point,
where molecular oxygen is highly concentrated, but instead we found none
there," comments Montmessin. As highlighted by previous studies based
on Venus Express observations, sunlight illuminating the day side of the
planet breaks apart the carbon dioxide molecules, liberating oxygen
atoms. These are then carried to the anti-solar point, on the night
side, by the sub-solar-to-anti-solar circulation, a strong wind caused
by the high temperature gradient between the two sides of Venus. Oxygen
atoms transported to the anti-solar point give rise to molecular oxygen,
and some production of ozone is also expected.

"The lack of ozone detected there is explained if the molecules are
destroyed by chlorine-based compounds, which are funnelled to the
anti-solar point by the same mechanism that draws the oxygen there,"
adds Montmessin. The chlorine-catalysed destruction of ozone at Venus'
anti-solar point may be caused by reactions very similar to those
responsible for the Antarctic 'ozone hole' on Earth, highlighting once
again the similarity of atmospheric processes on these two planets.

Besides enabling comparative studies among the atmospheres of
terrestrial planets, this discovery has intriguing implications for
astrobiology and the quest for extrasolar planets. Future hunts for
planetary systems beyond our own will characterise the atmosphere of
these planets in search of biomarkers—chemical compounds, such as water,
carbon dioxide and molecular oxygen, that are associated, on Earth, with
living organisms. Thus far, ozone has also been regarded as a potential
biomarker, since it traces the distribution of molecular oxygen.
However, the detection of ozone in the atmosphere of Venus, a planet
that clearly does not host life, raises questions about the unambiguity
of biomarkers and will help assessing the role of chemical indicators of
biological activity in future extrasolar planet searches.

Notes for editors

The study presented here is based on data collected with the
Spectroscopy for Investigation of Characteristics of the Atmosphere of
Venus (SPICAV) instrument on board ESA's Venus Express spacecraft.

SPICAV is a remote-sensing spectrometer covering three distinct spectral
regions (ultraviolet, near- and mid-infrared). It is used to probe the
atmosphere of Venus above the clouds, which are located at an altitude
of about 50–60 km.

Venus Express, Europe's first mission to Earth's twin world, is
investigating the nature of our closest planetary neighbour. Launched
from the Baikonur Cosmodrome in Kazakhstan on 9 November 2005 upon a
Soyuz-Fregat launcher, it was inserted into Venus orbit on 11 April
2006, and is currently the only spacecraft in orbit around the planet.

Related publications

F. Montmessin, et al., "A layer of ozone detected in the nightside
upper atmosphere of Venus", 2011, Icarus, Volume 216, Issue 1, November
2011, Pages 82-85. DOI:10.1016/j.icarus.2011.08.010

Contacts

Franck Montmessin
Laboratoire Atmosphߨres, Milieux, Observations Spatiales (LATMOS)
and CNRS/UVSQ/IPSL
Guyancourt, France
Email: franck.montmessinlatmos.ipsl.fr
Phone: +33-1-80 28 52 85

Jean-Loup Bertaux
SPICAV Principal Investigator
Laboratoire Atmosphߨres, Milieux, Observations Spatiales (LATMOS)
and Universitß© de Versailles-Saint-Quentin
France
Email: jean-loup.bertauxlatmos.ipsl.fr
Phone: +33-1-80 28 50 62; +33-6-80 73 08 70

Hakan Svedhem
ESA Venus Express Project Scientist
Research and Scientific Support Department
Science and Robotic Exploration Directorate
ESA, The Netherlands
Email: Hakan.Svedhemesa.int