MARSIS (Sub-Surface Sounding Radar/Altimeter)

Marsis will map the sub-surface structure to a depth of a few kilometres. The instrument's 40 m long antenna will send low frequency radio waves towards the planet, which will be reflected from any surface they encounter. For most, this will be the surface of Mars, but a significant fraction will travel through the crust to be reflected at sub-surface interfaces between layers of different material, including water or ice (see page 14 for more information on how Mars Express will help to solve the riddle of Mars's missing water).

"We should be able to measure the thickness of sand deposits in dune areas, or determine whether there are layers of sediment sitting on top of other material," says Giovanni Picardi, MARSIS Principal Investigator (PI) from Universita di Roma 'La Sapienza', Rome, Italy.

MARSIS will also study the ionosphere, as this electrically-charged region of the upper atmosphere will reflect some radio waves.

The HRSC (High Resolution Stereo Camera)

The HRSC will image the entire planet in full colour, 3D and with a resolution of about 10m. Selected areas will be imaged at 2m resolution. One of the camera's greatest strengths will be the unprecedented pointing accuracy achieved by combining images at the two different resolutions. Another will be the 3D imaging which will reveal the topography of Mars in full colour.

"As the 2 meter resolution image is nested in a 10 meter resolution swath, we will know precisely where we are looking. The 2 meter resolution channel will allow us to just pick out the Beagle 2 lander on the surface," says Gerhard Neukum, HRSC Principal Investigator (PI) from the Institut fur Weltraumsensorik und Planetenerkundung, Berlin, Germany.

OMEGA (Visible and Infrared Mineralogical Mapping Spectrometer)

OMEGA will build up a map of surface composition in 100 m squares. It will determine mineral composition from the visible and infrared light reflected from the planet's surface in the wavelength range 0.5-5.2 mm. As light reflected from the surface must pass through the atmosphere before entering the instrument, OMEGA will also measure aspects of atmospheric composition.

"We want to know the iron content of the surface, the water content of the rocks and clay minerals and the abundance of non-silicate materials such as carbonates and nitrates," says Jean-Pierre Bibring, OMEGA PI from the Institut d'Astrophysique Spatiale, Orsay, France.

SPICAM (Ultraviolet and Infrared Atmospheric Spectrometer)

SPICAM will determine the composition of the atmosphere from the wavelengths of light absorbed by the constituent gases. An ultraviolet (UV) sensor will measure ozone, which absorbs 250 nm light, and an infrared (IR) sensor will measure water vapour, which absorbs 1.38 micron light.

"Over the lifetime of the mission, we should be able to build up measurements of ozone and water vapour over the total surface of the planet for the different seasons," says Jean-Loup Bertaux, SPICAM PI from the Service d'Aeronomie du CNRS, Verrieres-le-Buisson, France.

PFS (Planetary Fourier Spectrometer)

PFS will determine the composition of the martian atmosphere from the wavelengths of sunlight (in the range 1.2-45 mm) absorbed by molecules in the atmosphere and from the infrared radiation they emit. In particular, it will measure the vertical pressure and temperature profile of carbon dioxide which makes up 95% of the martian atmosphere, and look for minor constituents including water, carbon monoxide, methane and formaldehyde.

"We hope to get many, many measurements so that by taking the average of thousands we'll be able to see minor species," says Vittorio Formisano, PFS PI from Istituto Fisica Spazio Interplanetario, Rome, Italy.

ASPERA (Energetic Neutral Atoms Analyser)

ASPERA will measure ions, electrons and energetic neutral atoms in the outer atmosphere to reveal the numbers of oxygen and hydrogen atoms (the constituents of water) interacting with the solar wind and the regions of such interaction. Constant bombardment by the stream of charged particles pouring out from the Sun, is thought to be responsible for the loss of Mars's atmosphere. The planet no longer has a global magnetic field to deflect the solar wind, which is consequently free to interact unhindered with atoms of atmospheric gas and sweep them out to space.

"We will be able to see this plasma escaping the planet and so estimate how much atmosphere has been lost over billions of years," says Rickard Lundin, ASPERA PI from the Swedish Institute of Space Physics in Kiruna, Sweden.

MaRS (Mars Radio Science Experiment)

MaRS will use the radio signals that convey data and instructions between the spacecraft and Earth to probe the planet's ionosphere, atmosphere, surface and even interior. Information on the interior will be gleaned from the planet's gravity field, which will be calculated from changes in the velocity of the spacecraft relative to Earth. Surface roughness will be deduced from the way in which the radio waves are reflected from the martian surface.

"Variations in the gravitational field of Mars will cause slight changes in the speed of the spacecraft relative to the ground station, which can be measured with an accuracy of less than one tenth the speed of a snail at full pace," says Martin Patzold, MaRS PI from Koln University, Germany.