OMEGA, the imaging spectrometer on board Mars-Express.

Stéphane Erard and Jean-Pierre Bibring. I.A.S., bât.121, 91405 Orsay campus, France.

Introduction. OMEGA is a second generation instrument developed after ISM/Phobos-2. OMEGA was originally developed for the Mars-96 mission by IAS and DESPA in France, IFSI in Italy and IKI in Moscow. OMEGA spare unit has been selected to fly on board the Mars Express ESA mission, planned to launch in 2003. The main differences with ISM performances include extended spectral range at both shorter and longer wavelengths (0.5-5.2 Ám), and increased angular and spectral resolutions. The instrument is made up of two co-aligned channels (visible and NIR), each using a dedicated telescope and foreoptics.

Visible channel. The visible channel works in pushbroom mode, using a Thomson CCD matrix 384 x 288 pixels (spatial x spectral). The total field of view is 8.8░. In baseline modes, pixels are binned either 3x3 or 3x2, providing a spatial sampling (IFOV) of 4.1 arcmin and a spectral sampling of 7 nm and 4 nm respectively: the beam is spread through an holographic grating over the spectral range 0.5-1.1 Ám, over 96 or 144 "spectels".

Infrared channel. The NIR channel works in whiskbroom mode, with an IFOV of 4.1 arcmin. The incoming radiation is divided in two subchannels, each using a grating spectrometer working in the first order. A crosstrack scanning mechanism is used to build swaths of 16, 32, 64 or 128 IFOV width, depending on the spacecraft altitude.

The two subchannels operate in the ranges 1.0-2.7 Ám and 2.6-5.2 Ám with a spectral resolution of 13 and 20 nm respectively. The detectors consist in linear arrays of 128 InSb detectors, cooled down to 70K, each by a dedicated cryocooler. The two spectrometers are cooled down to 190K by a passive radiator.

The measured signal to noise ratio is higher than 100 over the entire spectral range, and exceeds 300, at 5 ms integration time, with no summing, over half the spectral range.

Operating modes. For OMEGA, the major goals are to perform a global coverage of the surface, with a spatial resolution < 5 km, and to cover a fraction of he surface at high resolution (< 500 m). This is achieved in operating at different altitudes, and thus with different operation parameters. 8 sampling modes are available, both for the visible and the IR channels. These modes consist in combinations of integration times, swath widths in the NIR channels, and binning modes, adapted to different orbital configurations. Integration times vary from 2.5 to 20 ms (nominal 5 ms). The main two modes are adapted to global mapping from altitudes 1500 to 4000 km, and to high resolution snapshots from low altitude (pericenter).

From orbit pericenter (altitude < 300 km), the resolution reaches 400 m. The NIR channel acquires swaths 16 pixels large (~8 km) and 1500 km long. The 16 pixels swaths are scanned in 100 ms, corresponding to a drift of ~400 m, thus leading to contiguous swaths along the track.

For global mapping, typical sessions will have swaths of 128 pixels large (500 km) and 3000 km in length.

On-board signal processing. An on board compression unit, wavelet based, allows compression expected to reduce the downlink data by a factor 3 to 10, therefore permitting longer observation sessions in the allocated memory. Typical sessions of 200 Mbits last 400 s close to pericenter, up to 20 min from higher altitudes.

Scientific observations. The spectral range used gives access to spectral features of major (CO2) and minor (H2O, CO) atmospheric species, aerosols (suspended dust and ices), and surface minerals.

CO2 absorptions will provide on each resolved pixel the altimetric determination, in a way similar to Mariner 9 and ISM. Water vapor and CO absorptions will permit monitoring of minor species with location, altitude and time, at both diurnal and seasonal scales, linked to the atmospheric photochemistry processes.

The 1-2.5 Ám range is sensitive to aerosols scattering. Overlapping spatial observations will therefore permit to estimate aerosols size distribution, again at different locations and times.

Both H2O and CO2 ices have typical spectral features in the 1-5.2 Ám range that will make possible the study of ice clouds distribution.

Surface minerals have characteristic features at 1 and 2 Ám (pyroxenes, oxides) indicative of mineral abundance once the aerosols scattered contribution is removed from the signal. Salts (carbonates, sulfates, nitrates...) and alteration minerals (oxides, phyllosilicates...) have subdued features in particular at 2.3 Ám and between 3 and 5 Ám, that can be studied with the instrument signal to noise ratio.

This is an abstract from the workshop, "Spectroscopy of the Martian Surface: What Next?", held at the Lunar and Planetary Institute, Houston. More information is available here.

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