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Rare high-altitude clouds found on Mars
European Space Agency
28 August 2006

Planetary scientists have discovered the highest clouds above any
planetary surface. They found them above Mars using the SPICAM
instrument on board ESA's Mars Express spacecraft. The results are a
new
piece in the puzzle of how the Martian atmosphere works.

Until now, scientists had been aware only of the clouds that hug the
Martian surface and lower reaches of the atmosphere. Thanks to data
from
the SPICAM Ultraviolet and Infrared Atmospheric Spectrometer onboard
Mars Express, a fleeting layer of clouds have been discovered at an
altitude between 80 and 100 kilometres. The clouds are most likely
composed of carbon dioxide.

SPICAM made the discovery by observing distant stars just before they
disappeared behind Mars. By looking at the effects on the starlight as
it travelled through the Martian atmosphere, SPICAM built up a picture
of the molecules at different altitudes. Each sweep through the
atmosphere is called a profile.

The first hints of the new cloud layer came when certain profiles
showed
that the star dimmed noticeably when it was behind the 90?100
kilometre
high atmospheric layer. Although this happened in only one percent of
the profiles, by the time the team had collected 600 profiles, they
were
confident that the effect was real.

"If you wanted to see these clouds from the surface of Mars, you would
probably have to wait until after sunset" says Franck Montmessin, a
SPICAM scientist with Service d'Aeronomie du CNRS,
Verri??res-le-Buisson,
France, and lead author of the results. This is because the clouds are
very faint and can only be seen reflecting sunlight against the
darkness
of the night sky. In that respect, they look similar to the mesospheric
clouds, also known as noctilucent clouds, on Earth. These occur at 80
kilometres altitude above our planet, where the density of the
atmosphere is similar to that of Mars? at 35 kilometres. The newly
discovered Martian clouds therefore occur in a much more rarefied
atmospheric location.

At 90-100 kilometres above the Martian surface, the temperature is just
-193?? Celsius. This means that the clouds are unlikely to be made
of
water. "We observe the clouds in super-cold conditions where the main
atmospheric component CO2 (carbon dioxide), cools below its
condensation
point. From that we infer that they are made of carbon dioxide," says
Montmessin.

But how do these clouds form? SPICAM has revealed the answer by finding
a previously unknown population of minuscule dust grains above 60
kilometres in the Martian atmosphere. The grains are just one hundred
nanometres across (a nanometre is one thousand-millionth of a metre).

They are likely to be the 'nucleation centres' around which crystals of
carbon dioxide form to make clouds. They are either microscopic
chippings from the rocks on the surface on Mars that have been blown to
extreme altitudes by the winds, or they are the debris from meteors
that
have burnt up in the Martian atmosphere.

The new high-altitude cloud layer has implications for landing on Mars
as it suggests the upper layers of Mars' atmosphere can be denser than
previously thought. This will be an important piece of information for
future missions, when using friction in the outer atmosphere to slow
down spacecraft (in a technique called 'aerobraking'), either for
landing or going into orbit around the planet.



Note to editors

These results are published online in the Icarus scientific magazine
(vol. 183, issue 2, August 2006), in the article titled: "Subvisible
CO2
ice clouds detected in the mesosphere of Mars", by F.Montmessin,
J.L.Bertaux (Service d'Aeronomie du CNRS, Verri??res-le-Buisson,
France
), et al.

Since the start of the Mars Express scientific operations at the
beginning of 2004, SPICAM has been probing the Martian atmosphere from
top to bottom. Another major achievement is the first global map of
Martian ozone. Latter this year, a special issue of the Journal of
Geophysical Research is being devoted to SPICAM?s results from Mars.



For more information

Frank Montmessin, Service d'Aeronomie du CNRS,
Verri??res-le-Buisson, France
Email: franck.montmessin @ aero.jussieu.fr

Jean-Loup Bertaux, SPICAM Principal Investigator
Service d'Aeronomie du CNRS, Verri??res-le-Buisson, France
Email : jean-loup.bertaux @ aerov.jussieu.fr

Agustin Chicarro, ESA Mars Express Project Scientist
Email : agustin.chicarro @ esa.int