This true-colour view taken by NASA's Pathfinder rover in August 1997 shows clouds in the Martian eastern sky (30 degrees above the horizon), as imaged before sunrise. Observations of the Martian atmosphere by the SPICAM spectrometer on board ESA's Mars Express spacecraft, have revealed for the first time that carbon dioxide clouds form and exist at very high atmospheric layers, between 80 and 100 kilometres above the Martian surface. This makes them the highest clouds ever observed above any planetary surface. These clouds may be of the same type observed by Pathfinder. Credits: NASA Pathfinder
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.
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, Verrières-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.
http://www.planetary.brown.edu/m42/m42_33.pdfMARS ATMOSPHERE AND CLIMALOGY WITH MARS-EXPRESS: MAIN RESULTS OF
EXPERIMENTS WITH RUSSIAN PARTICIPATION.This paper highlights main
results on the Martian atmosphere and climate obtained
by three instruments, PFS, SPICAM and
OMEGA, and with Russian participation. Temperature
field, concentration of minor constitutes like
water vapor, ozone, methane and CO, H2O and CO2
ice clouds, opacity, aerosol content and vertical distribution,
were observed during almost one Martian
year. These data are critical in understanding the
dynamics, photochemical processes and history of
Martian atmosphere, challenging new theoretical
studies with general circulation models.Vertical distribution of aerosol: All spectrometers
are capable of studying the distribution
and composition of Martian aerosols. Observations
of water ice clouds, insight into their microphysics
and the discovery of fine fraction of submicron particles
extending high up in atmosphere are among
the important results of Mars-Express. In particular,
vertical profiles of aerosols were observed by
are obtained by SPICAM IR channel in range 1274
micron. Lower north polar profile contrasts with
high extended south middle latitude extinctions.
We believe that the specific features of the vertical
distribution of high-altitude aerosols, including
evident depression near the poles and subtle inversions
in midlatitudes, have dynamical origin. This
conclusion is based on the numerical experiments
with the GFDL's Mars GCM including interactive
transport and coupled with ab initio microphysical
description of water ice clouds. In the equinox season,
Hadley cell circulation lifts dust and cloud particles
in the equatorial latitudes up to 35-50 km and
then advects them out of the equator in the two
symmetric branches. In midlatitudes (approximately
at 45-60^o) the advective poleward flow fades.
Larger particles settle out, while smaller ones are
accumulated in the convergent areas of the circulation
pattern. It is this location where SPICAM solar
occultation data suggest inversion of the vertical
dust profile. Upper polar latitudes are characterized
by very weak circulation with dominating downdraft
vertical air motion. Therefore those particles trapped
in the polar atmosphere are transported downwards
until they either precipitate at the surface or advected
back to low latitudes within the low-altitude closing
flow of the Hadley cell. These causes substantially
lower, relative to midlatitudes, vertical extension of
the aerosol layer.Due to high spectral resolution, a fine structure
of temperature inversion in the polar region, related
to descending branch of Hadley cell (Ls=342°) was
observed as low as at 10-20 km. The winter temperature
inversion is caused by downdraft of the air
mass advected from the summer hemisphere by the
main Hadley cell branch, and subsequent adiabatic
heating. As the heating occurs at relatively high altitude
(30-60 km), lower atmosphere controlled by
radiative transfer processes appears colder, which
renders as sharp temperature inversion.Seasonal distribution of water vapour measured
by SPICAM IR (orbits 8-1640), and
areographical distribution of water in Northern
spring by PFS (LS=330°-60°). Apparent zonal
structure revealing a strong equatorial maximum at
10°-45°E and a weaker maximum at 200°-240° suggests
contribution from stationary planetary waves to
the global water cycle on Mars.A singlet oxygen O2 (1Δg) dayglow at 1.27 μm
was predicted just after the discovery of ozone on
Mars by Mariner 9 . On Mars the situation is
similar to Earth, where a strong airglow arises from
O2 (1Δg) produced by ozone photolysis. For the first
time this emission was observed from the ground at
high resolution by Noxon et al.. The mapping of
this emission was reported by Krasnopolsky and
Bjoracker . Krasnopolsky  argues that the O2
emission provides even better insight to photochemistry
than ozone, since it is more sensitive to the
variations of the water vapour saturation level (10-
35 km) than total ozone, which remains nearly constant.
MAPPING CLOUDS MICROPHYSICS WITH OMEGA/MEX
An express journey to
• Presence of methane in the atmosphere
(concurrent with ground observations), which
together with the formaldehyde (oxidation
product of methane) also found would indicate
that Mars either bears volcanic activity or
biological processes today.
• Clear correlation between water vapour in the
boundary layer and methane concentrations
observed from orbit, further illustrating the
volcanism vs. life debate.
• The correlation between water vapour,
methane and possible underground acquifers
(Mars Odyssey) points to a common
underground source for water and methane.ASPERA
• Characterisation of the planetary wind
composition (atomic & molecular O+ )
away from the solar wind.
• Solar wind scavenging of the atmosphere
down to 270 km altitude, representing a
major mechanism in neutral atmospheric
degassing and past climate change.
• Planetary heavy ions accelerated up to
very high energies.
• For the first time, ”radiation” of fast atoms
is observed at Mars.
• CO2 also escapes from Mars. H2O ?MaRS
• Build-up of the ionosphere shortly
• First successful bi-static radar
experiment by pointing of the high-gain
antenna towards Mars to infer surface
roughness and other soil properties in
regions of geological interest.
• Stable two layer structure of dayside
ionosphere; sporadic third layer due to
meteor interaction with ionosphere.
• Very cold atmosphere over the first few
kilometres (-143°C to -130°C).
• CO2 snow fall at high Southern latitudes.