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Since it began operating on 9 January, the SPICAM spectrometer has successfully acquired its first readings from Mars’ atmosphere. It has detected ozone, water vapour and carbon dioxide (CO2)—three gases familiar to us here on Earth.

Ozone and water vapour measured simultaneously for the first time ever

"This is the first time we have measured ozone and water vapour in Mars’ atmosphere simultaneously from a Mars orbiter," says Jean-Loup Bertaux, Director of Research at CNRS’ Service d’Aéronomie and Principal Investigator for the SPICAM instrument. The U.S. orbiter Mariner 9 discovered ozone on Mars in 1969, but subsequent Mars missions did not carry instruments able to measure it. The first measurements by SPICAM on Mars Express, covering latitudes from 0 to 50° North along the orbit track, show there is more water vapour where there is less ozone. This new discovery confirms what scientists suspected: that ozone and water vapour are anti-correlated, since atmospheric chemistry tells us that OH radicals produced by photodissociation of H2O are powerful catalysts in the process of ozone depletion, as are the chlorine compounds produced by freons responsible for the polar ozone hole on Earth.

The new element in SPICAM’s findings is the first clear evidence of this coupling between ozone and water vapour on Earth or Mars. This catalytic depletion process only plays a minor role in the Earth’s stratosphere (20 kilometres above the surface), where most of the ozone layer that filters out solar UV radiation is located. Water vapour is confined below the stratosphere, since it freezes before it can reach the ozone layer in the very cold lower stratosphere. However, climatologists are now concerned about a new looming threat to Earth: expected global warming will raise temperatures in this region of the atmosphere, releasing huge amounts of water vapour that will gradually deplete it the ozone layer. SPICAM will be able to study this depletion mechanism close up on Mars. Scientists could then extrapolate the results to Earth’s stratosphere and to the surface of Mars. By plugging ozone and water vapour data into photo-chemical models, we will be able to predict the amount of solar UV radiation reaching the surface (there is approximately 200 times less ozone on Mars than on Earth) and the number of oxidizing agents there—two factors not conducive to the survival of any life that might have existed on Mars.

Star occultation from Mars

SPICAM has also completed the first vertical sounding of the density of Mars’ atmosphere, using an innovative star occultation technique. The Mars Express orbiter was oriented to centre SPICAM’s field of view in the assumed direction of Zeta Puppis. This very hot star is a rich source of UV radiation within SPICAM’s observation spectrum, strongly absorbed by CO2. As Mars Express orbits the planet, SPICAM’s line of sight to the star intercepts the atmosphere at different altitudes. By comparing the star’s spectrum outside Mars’ atmosphere with that seen through it, we can measure how transparent the atmosphere is to UV radiation and, therefore, determine CO2 levels with respect to altitude.

Studying the atmosphere… and laying the groundwork for future missions

After acquiring these sounding data, Mars Express was re-oriented to transmit them to Earth. Using this technique, we will be able to measure the vertical profile of carbon dioxide density and temperature in Mars’ "air" throughout the Martian year and at all latitudes up to an altitude of 150 kilometres. This information will allow us to model Mars’ upper atmosphere—important for future Mars missions, since the region between 100 and 150 kilometres above the surface is where a satellite slows before going into orbit about the planet, and where its orbit is lowered by aerobraking. This orbit injection method saves on mass, since it reduces the amount of thruster fuel needed.

A satellite arriving from Earth could also be captured directly into Mars orbit from an altitude of about 80 kilometres, using atmospheric braking on its first pass to go into orbit without thrusters.

The star occultation sounding technique used by SPICAM was inherited from the GOMOS instrument on ESA’s Envisat satellite. GOMOS has been closely monitoring terrestrial ozone since March 2002, and has so far performed more than 200,000 star occultations.

To measure water vapour in the near infrared, SPICAM is carrying a new AOTF (Acousto Optic Tunable Filter) spectrometer developed in Russia. This very compact, lightweight spectrometer (0.8 kg) is being used for the first time on a planetary mission. Two other conventional infrared spectrometers on Mars Express—OMEGA and PFS—are also measuring water vapour to compare results with SPICAM and validate this new and very promising AOTF approach.

SPICAM is a dual UV and near-infrared spectrometer weighing just 4.5 kilograms. It was designed by three research laboratories: the Service d’Aéronomie attached to CNRS, at Verrières-le-Buisson; the Belgian space aeronomy institute in Brussels (contact: Paul Simon); and the IKI space research institute in Moscow (contact: Oleg Korablev). SPICAM was built on the heritage of a similar instrument lost when the Russian Mars 96 orbiter was destroyed on launch in November 1996. However, its ancestor was much heavier, since the first version of SPICAM tipped the scales at 45 kilograms. The newly developed instrument on Mars Express is a superb feat of engineering, funded by CNES.


Jean-Loup Bertaux
SPICAM Principal Investigator, Mars Express