Omega : science introduction omega primary goals

OMEGA : science introduction

OMEGA was selected in 1988, to fly onboard the Mars 96 Russian mission. Its design was made soon after, to achieve the goals conceived as prime at that time, with the available flight qualified technology.
At this time, almost no surface compositional data were available, except from those derived from the two Viking Lander APXS sets of measurements : our understanding of Mars state, history and evolution was essentially based on optical imaging.
The goal for a large community was thus to produce compositional data to be coupled with imaging data already acquired or to be acquired (inferring the geomorphological context).
Together with US colleagues (VIMS/Mars Observer) we considered that NIR spectral imaging was the right answer to achieve the above quoted goal. For planetary objects of temperature ~ 300K, NIR is the domain of reflectance spectroscopy (crossover to thermal emission: 3 to 4 µm).

OMEGA was decided at the time ISM / Phobos was launched, in 1988. ISM constituted the first ever designed and flown NIR spectral imager. For transfer of technology constraints, it was highly limited technically. However, it demonstrated the potential of NIR reflectance spectrometry to derive atmospheric and surface composition, and the benefit of coupling imaging and spectrometry to acquire the composition of each resolved pixel.
Consequently, OMEGA specifications were derived for:
imaging: space sampling (IFOV) and FOV
spectrometry: spectral sampling and spectral range
radiometry: SNR

imaging:
ISM demonstrated that surface diversity exists at all observed space scales (down to a few km). Thus, the goal for OMEGA was to get as high as possible a sampling. Trade off is thus between sampling and coverage. Given the lack of Mars compositional knowledge at that time, we wanted to have the capability to acquire the global coverage of the surface. With the envisioned downlink profile and mission duration, km-scale coverage was a realistic goal. For a given orbit, this drives the IFOV, thus the surface sampling. We chose an
IFOV of 1.2 mrad ( ~ 4.1 arcmin).
The associated surface sampling depends on the altitude of observation (along the elliptical orbit):
300 m from 250 km
4.8 km from 4000 km

spectrometry:
NIR reflectance spectrometry is the means to derive composition from specific quantum transition to excited states through (solar) photon absorption.
Identification is made by comparison with library spectra (potential bias and limitation).
1. Spectral domain
VIS + NIR corresponds to electronic states (marginally), and primarily to vibration modes (radical and molecules). Most non symmetrical species (permanent dipole) have diagnostic transitions in the NIR, which constitutes the reflectance domain (λmT = 2900 µm.K → reflectance/thermal crossover @ ~3 µm, thermal dominates above 5 µm). For OMEGA, we chose:
0.35 – 5.1 µm

2. Spectral sampling
2.1. Atmospheric (gaseous) transitions lead to very narrow (rotational) lines (<< 1 nm width): FS is the means to achieve such a sampling over a wide spectral range. However, it requires moving parts, and is difficult to couple to imagery (in particular too large spectral sampling over a large spectral domain precludes large spatial sampling, for data volume constraint). Grating spectroscopy is easier to implement in an imaging mode. However, given the achievable resolution (typically λ/Δλ ~100 to 500), sampling of a ~10 nm is achievable. We chose for OMEGA:
7nm from 0.35 to 1 µm
13 nm from 1 to 2.5 µm
20 nm from 2.5 to 5.1 µm
With such a reduced sampling, individual lines are summed, which reduces sensitivity, but enables (in a few cases) to unambiguously identify species: Martian CO2, CO, H2O and O2 can be identified along a nadir line, and even along limb sounding.

2. Spectral sampling
2.2. Surface (solid) transitions lead to rather broad features, summing up a diversity of environments at a microscopic scale: 10 to 20 nm is adequate. Other solid compounds (aerosols, even clouds) do exhibit diagnostic signatures.
The position of the maximum absorption is diagnostic of the bounding, while the shape gives potentially access to other parameters, such as : mean grain size, temperature etc…Most vibration transitions in this domain are not fundamental modes, but combination of harmonics: in a few cases, several transitions are present in the NIR domain.
Species potentially identified range from surface minerals to frosts, to atmospheric aerosols and clouds.