Andrew Yee[_1_]
January 16th 08, 06:23 PM
ESA News
http://www.esa.int
16 January 2008
Ice clouds put Mars in the shade
Until now, Mars has generally been regarded as a desert world, where a
visiting astronaut would be surprised to see clouds scudding across the
orange sky. However, new results show that the arid planet possesses
high-level clouds that are sufficiently dense to cast a shadow on the
surface.
The results were obtained by the OMEGA Visible and Infrared Mineralogical
Mapping Spectrometer instrument on board ESA's Mars Express.
Mars is not entirely a haven for Sun worshippers. Clouds of water ice
particles do occur, for example on the flanks of the giant Martian
volcanoes. There have also been hints of much higher, wispy clouds made up
of carbon dioxide (CO2) ice crystals. This is not too surprising, since the
thin Martian atmosphere is mostly made of carbon dioxide, and temperatures
on the fourth planet from the Sun often plunge well below the 'freezing
point' of carbon dioxide.
Now, a team of French scientists has shown that such clouds of dry ice do,
indeed, exist. Furthermore, they are sometimes so large and dense that they
throw quite dark shadows on the dusty surface.
"This is the first time that carbon dioxide ice clouds on Mars have been
imaged and identified from above," said Franck Montmessin of the Service
d'Aeronomie, University of Versailles (UVSQ), lead author of the paper in
the Journal of Geophysical Research. "This is important because the images
tell us not only about their shape, but also their size and density.
"Previously, we had to rely on indirect information -- for example, from the
SPICAM instrument on board Mars Express -- to find out what the clouds are
made of. However, it is very difficult to separate the signals coming from
the clouds, the atmosphere and the surface."
Data from the SPICAM Ultraviolet and Infrared Atmospheric Spectrometer
indicated that any high altitude clouds are not very thick and made up of
much smaller particles, but the CO2 clouds detected by OMEGA are very
different. Not only are they surprisingly high -- more than 80 km above the
surface -- but they can be several hundred kilometres across. They are also
much thicker than expected. Instead of looking like the wispy ice clouds
seen on Earth, they resemble tall convectional clouds that grow as the
result of rising columns of warm air.
Even more surprising is the fact that the CO2 ice clouds are made of quite
large particles -- more than a micron (one thousandth of a millimetre)
across -- and they are sufficiently dense to noticeably dim the Sun.
Normally, particles of this size would not be expected to form in the upper
atmosphere or to stay aloft for very long before falling back towards the
surface.
"The clouds imaged by OMEGA can reduce the Sun's apparent brightness by up
to 40 per cent," said Montmessin. "This means that they cast quite a dense
shadow and this has a noticeable effect on the local ground temperature.
Temperatures in the shadow can be up to 10 C cooler than their surroundings,
and this in turn modifies the local weather, particularly the winds."
Since the CO2 clouds are mostly seen in equatorial regions, the OMEGA team
believes that the unexpected shape of the clouds and large size of their ice
crystals can be explained by the extreme variations in daily temperature
that occur near the equator.
"The cold temperatures at night and relatively high day-time temperatures
cause large diurnal waves in the atmosphere," explained Montmessin. "This
means there is a potential for large-scale convection, particularly as the
morning Sun warms the ground."
Bubbles of warm gas rise above the surface and when they reach high levels
they become cold enough for CO2 to condense. This process releases latent
heat, which causes the gas and the ice particles to rise even further.
What are the particles around which the CO2 ice condenses? On Earth, cloud
droplets form around tiny nuclei -- often particles of dust or salt. On
Mars, the answer remains uncertain. One possibility is that Martian dust is
carried to high altitudes. Another potential source of condensation nuclei
is particles left behind by micrometeorites entering the upper atmosphere.
Or the nuclei may simply be tiny crystals of water ice carried aloft on the
thermal updraughts.
"This discovery is important when we come to consider the past climate of
Mars," said Montmessin. "The planet seems to have been much warmer billions
of years ago, and one theory suggests that Mars was then blanketed with CO2
clouds. We can use our studies of present-day conditions to understand the
role that such high level clouds could have played in the global warming of
Mars."
Note for editors:
The results were published in the 13 November 2007 issue of the Journal of
Geophysical Research in a paper entitled "Hyperspectral imaging of
convective CO2 ice clouds in the equatorial mesosphere of Mars" by F.
Montmessin (Service d'Aeronomie, CNRS / IPSL / Universite Versailles
Saint-Quentin en Yvelines, France), B. Gondet, J.-P. Bibring, Y. Langevin
(Institut d'Astrophysique Spatiale, Orsay, France), P. Drossart, T. Fouchet
(Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique,
Observatoire de Paris, France), and F. Forget (Laboratoire de Meteorologie
Dynamique, CNRS / IPSL / Universite Pierre et Marie Curie, Paris, France).
For more information:
Franck Montmessin, OMEGA and SPICAM co-investigator
Email: franck.montmessin @ aero.jussieu.fr
Agustin Chicarro, ESA Mars Express Project Scientist
Email: achicarr @ rssd.esa.int
[NOTE: Images supporting this release are available at
http://www.esa.int/esaCP/SEM1DV3MDAF_index_1.html ]
http://www.esa.int
16 January 2008
Ice clouds put Mars in the shade
Until now, Mars has generally been regarded as a desert world, where a
visiting astronaut would be surprised to see clouds scudding across the
orange sky. However, new results show that the arid planet possesses
high-level clouds that are sufficiently dense to cast a shadow on the
surface.
The results were obtained by the OMEGA Visible and Infrared Mineralogical
Mapping Spectrometer instrument on board ESA's Mars Express.
Mars is not entirely a haven for Sun worshippers. Clouds of water ice
particles do occur, for example on the flanks of the giant Martian
volcanoes. There have also been hints of much higher, wispy clouds made up
of carbon dioxide (CO2) ice crystals. This is not too surprising, since the
thin Martian atmosphere is mostly made of carbon dioxide, and temperatures
on the fourth planet from the Sun often plunge well below the 'freezing
point' of carbon dioxide.
Now, a team of French scientists has shown that such clouds of dry ice do,
indeed, exist. Furthermore, they are sometimes so large and dense that they
throw quite dark shadows on the dusty surface.
"This is the first time that carbon dioxide ice clouds on Mars have been
imaged and identified from above," said Franck Montmessin of the Service
d'Aeronomie, University of Versailles (UVSQ), lead author of the paper in
the Journal of Geophysical Research. "This is important because the images
tell us not only about their shape, but also their size and density.
"Previously, we had to rely on indirect information -- for example, from the
SPICAM instrument on board Mars Express -- to find out what the clouds are
made of. However, it is very difficult to separate the signals coming from
the clouds, the atmosphere and the surface."
Data from the SPICAM Ultraviolet and Infrared Atmospheric Spectrometer
indicated that any high altitude clouds are not very thick and made up of
much smaller particles, but the CO2 clouds detected by OMEGA are very
different. Not only are they surprisingly high -- more than 80 km above the
surface -- but they can be several hundred kilometres across. They are also
much thicker than expected. Instead of looking like the wispy ice clouds
seen on Earth, they resemble tall convectional clouds that grow as the
result of rising columns of warm air.
Even more surprising is the fact that the CO2 ice clouds are made of quite
large particles -- more than a micron (one thousandth of a millimetre)
across -- and they are sufficiently dense to noticeably dim the Sun.
Normally, particles of this size would not be expected to form in the upper
atmosphere or to stay aloft for very long before falling back towards the
surface.
"The clouds imaged by OMEGA can reduce the Sun's apparent brightness by up
to 40 per cent," said Montmessin. "This means that they cast quite a dense
shadow and this has a noticeable effect on the local ground temperature.
Temperatures in the shadow can be up to 10 C cooler than their surroundings,
and this in turn modifies the local weather, particularly the winds."
Since the CO2 clouds are mostly seen in equatorial regions, the OMEGA team
believes that the unexpected shape of the clouds and large size of their ice
crystals can be explained by the extreme variations in daily temperature
that occur near the equator.
"The cold temperatures at night and relatively high day-time temperatures
cause large diurnal waves in the atmosphere," explained Montmessin. "This
means there is a potential for large-scale convection, particularly as the
morning Sun warms the ground."
Bubbles of warm gas rise above the surface and when they reach high levels
they become cold enough for CO2 to condense. This process releases latent
heat, which causes the gas and the ice particles to rise even further.
What are the particles around which the CO2 ice condenses? On Earth, cloud
droplets form around tiny nuclei -- often particles of dust or salt. On
Mars, the answer remains uncertain. One possibility is that Martian dust is
carried to high altitudes. Another potential source of condensation nuclei
is particles left behind by micrometeorites entering the upper atmosphere.
Or the nuclei may simply be tiny crystals of water ice carried aloft on the
thermal updraughts.
"This discovery is important when we come to consider the past climate of
Mars," said Montmessin. "The planet seems to have been much warmer billions
of years ago, and one theory suggests that Mars was then blanketed with CO2
clouds. We can use our studies of present-day conditions to understand the
role that such high level clouds could have played in the global warming of
Mars."
Note for editors:
The results were published in the 13 November 2007 issue of the Journal of
Geophysical Research in a paper entitled "Hyperspectral imaging of
convective CO2 ice clouds in the equatorial mesosphere of Mars" by F.
Montmessin (Service d'Aeronomie, CNRS / IPSL / Universite Versailles
Saint-Quentin en Yvelines, France), B. Gondet, J.-P. Bibring, Y. Langevin
(Institut d'Astrophysique Spatiale, Orsay, France), P. Drossart, T. Fouchet
(Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique,
Observatoire de Paris, France), and F. Forget (Laboratoire de Meteorologie
Dynamique, CNRS / IPSL / Universite Pierre et Marie Curie, Paris, France).
For more information:
Franck Montmessin, OMEGA and SPICAM co-investigator
Email: franck.montmessin @ aero.jussieu.fr
Agustin Chicarro, ESA Mars Express Project Scientist
Email: achicarr @ rssd.esa.int
[NOTE: Images supporting this release are available at
http://www.esa.int/esaCP/SEM1DV3MDAF_index_1.html ]