OMEGA: IR Mineralogical Mapping Spectrometer
Mapping the surface composition of Mars
Mars viewed from the Hubble Space telescope. Oxides of iron in exposed soil and rock give the planet its red colour. Dark areas are either exposed rock or accumulations of dark sand.
We have little more than a general knowledge of the surface composition of Mars. The north is made largely of a silica-rich volcanic rock (felsic) and the south of basaltic rock relatively poor in silica (mafic); the soil and surface layer of exposed rocks contains oxides of iron, which give the planet its reddish colour. Going beyond this general description to identify particular minerals or elements at particular points on the surface, has so far proved beyond the scope of remote-sensing instruments. None has yet flown with sufficient spatial resolution for the task. That will change, however, when Mars Express carries the Infrared Mineralogical Mapping Spectrometer (OMEGA) into Martian orbit.
Using the fact that different materials absorb and radiate light at different wavelengths, OMEGA will build up a map of surface composition by analysing sunlight that has been absorbed and re-emitted by the surface. The instrument will also glean information on the composition from the wavelengths of infrared radiation given off as the surface cools. As radiation travelling from the surface to the instrument must pass through the atmosphere, OMEGA will also detect wavelengths absorbed by some atmospheric constituents, in particular dust and aerosols.
The optical path for the IR channel before integration into the instrument.
OMEGA weighs 29 kg and is about the size of a small TV set. It has two channels, one for visible (0.5-1.0 µm wavelength) and the other for IR (1.0-5.2 µm wavelength) light. Both channels include a telescope, a spectrometer and an optical device to focus light on a CCD (charge coupled device), in the case of the visible channel, and on two InSb arrays in the case of the IR channel. The visible channel will measure the wavelength of incoming radiation to within about 7 nm and the IR channel to within 13-20 nm.
2.2 µm absorption over Valles Marineris recorded by ISM on the Phobos mission, a first generation instrument in the Omega lineage. Absorption at 2.2 µm is characteristic of pyroxenes, which are silica-rich minerals. Higher abundances of pyroxenes are found within the canyons (blue shading).
As Mars Express proceeds along its orbit, OMEGA will gradually build up a map of the planet in squares, or pixels, whose sides range from 4 km to 300 m in length. During the mission's lifetime, a spectral map of the entire surface will be generated at 1-4 km resolution and selected sites, making up 2-5% of the Martian surface, will be mapped at 300 m resolution. The higher resolution is available only when the spacecraft is at the lowest point in its orbit, hence its restriction to a limited area of the Martian surface. "In each channel, we make a spectrometer and an imager and couple the two. On each resolved pixel, we will have the entire spectrum from the visible to the IR," says Jean-Pierre Bibring from the Institut d'Astrophysique Spatiale, Orsay, France and Principal Investigator for OMEGA.
Are there carbonates on Mars?
OMEGA's high spatial resolution is unprecedented. This and its spectral range give it the ability to pinpoint specific minerals on Mars more accurately than any instrument on a previous spacecraft. "We want to see not only that there are silicates, but to determine their class - whether they are in the form of feldspar, pyroxene, olivine, for example. We also want to know the iron content of the surface, the oxidation level of the iron, the hydration of the rocks and clay minerals and the abundance of non-silicate materials such as carbonates and nitrates. We'll be able to measure all these abundances to within a few per cent within a 100 m square," says Bibring.
Simulation of Omega reflectance spectra, but at a lower resolution than will be obtained by Omega.
Such knowledge of the composition will throw light on many of the outstanding puzzles about Mars. Our understanding of the tectonic history of the planet, for example, will deepen with more accurate knowledge of the whereabouts and composition of igneous rocks (rocks that have been molten in the interior of the planet). Accurate measurements of dust on the surface and in the atmosphere will illuminate present day wind patterns and patterns of dust transport. And climate history and present day weather patterns will be revealed in accurate measurements of the water and carbon dioxide content of the polar caps, which varies with the seasons. Some of the most interesting riddles OMEGA will help unravel, however, have to do with the history of water on the planet and the possibility that Mars was once hospitable to life.
"Two of the big questions we have about Mars are: where is the carbon dioxide? and what happened to the water? "says Bibring. "There's carbon dioxide in the atmosphere, but the pressure's very low. So either Mars has lost its carbon dioxide, or it is now in the rocks in the form of carbonates. If the carbon dioxide is in the rocks, there must have been liquid water in the past."
Carbonates form when carbon dioxide dissolves in water and then reacts with metals, such as iron, magnesium or calcium, to form minerals such as iron, calcium or magnesium carbonate. On Earth, calcium carbonate (CaCO3), the most common carbonate, is created when water containing dissolved carbon dioxide (CO2) reacts with calcium from the hard parts (bones and shells) of dead creatures. The calcium carbonate was deposited on lake and seabeds eventually to form sedimentary rocks. The discovery on Mars of sedimentary rocks consisting of carbonates will be taken as strong evidence that the planet once supported large, long-lived bodies of water, which could possibly have supported communities of simple living organisms.
One of OMEGA's tasks will therefore be to plot a map, which shows how the intensity of the 3.6 µm and 3.9 µm band (carbonate) varies over the surface. Another will be to plot a similar map of the 3 µm (water) band. "This will show us where the most hydrated and the driest areas are, which could tell us where the water once was," says Bibring.
Close work with other Mars Express instruments
The scientific teams building OMEGA, the camera (HRSC) and the Planetary Fourier Spectrometer (PFS) have co-investigators in common because the interests of the three instruments overlap. "If we find some interesting minerals, we'll want to see what they look like and so will ask what the HRSC has seen," says Bibring. Similarly, if the HRSC has identified some interesting features, OMEGA will be able to determine their composition.
The closest overlap, however, will be between OMEGA and PFS, which will also analyse spectra but in a different waveband. The different wavelengths combined with PFS's lower spatial resolution but higher spectral resolution make it more suitable for analysing atmospheric gases than surface composition. But just as OMEGA can provide some information on atmospheric composition, so PFS can provide some surface composition data.
"If OMEGA identifies some carbonates, we might ask PFS to look for strong carbonate features in its part of the spectrum. If we see a signal with both instruments, we'll be able to say unambiguously that we have carbonates," says Bibring. OMEGA's atmospheric observations will be most useful for following changes in global atmospheric pressure and providing the carbon monoxide to carbon dioxide (CO/CO2) and water to carbon dioxide (H2O/CO2) ratios.
Principal Investigator Dr. Jean-Pierre Bibring, Institut d'Astrophysique Spatiale, Orsay, France
For further information see related links.
||MARSIS: Subsurface Sounding Radar/Altimeter
||PFS: Planetary Fourier Spectrometer
Last Update: 15 February 2010