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Retrieving martian surface composition by means of the Surface Atmosphere Separation (SAS) method

Summary: Analysis of Martian visible and infra-red spectra from the OMEGA/MEX instrument to separate the atmospheric contribution from the surface signal and better infer the mineralogical composition.

An example of two OMEGA orbits covering the same region but with different atmospheric conditions: ORB1441_5 has been collected when a local dust storm was occurring, while during ORB3262_5 the atmosphere was clearer. – a) Comparison of the images at 1.2 μm. The thick black rectangle superimposed on orbit 1441_5 map indicates the footprint of orbit 3262_5. The coloured isolines give the 1.2 μm values (reflectance factor) of the dust cloud at values of 0.50, 0.40, 0.35 and 0.30 (from black to blue). – b) RGB maps for the two observations, where R = 0.73 μm, G = 0.58 μm and B = 0.51 μm.

Useful Skills: Programming, Planetology, Principal Component Analysis

OMEGA is the Visible and Infrared imaging spectrometer (Bibring et al, 2004) on board the ESA mission Mas Express (MEX) orbiting the Red Planet since 2004. OMEGA has been designed to better understand the martian surface composition in the 0.4-5.1 micron range and asses the mineralogical evolution. However, the planet atmosphere prevent the study of the surface.

Martian atmosphere is very tenuous (surface pressure around 6/7 mbar) and is composed mainly of CO2. Unfortunately, CO2 has deep absorptions at 1.5 and 2 micron where the most interesting minerals show diagnostic absorptions. Moreover, airborne dust (mainly composed of micron-sized) particles is always present in the atmosphere, masking the true surface composition. The amount of dust is controlled by global and local atmospheric circulation. Periodically, dust storms can completely obscure the surface features (Fig. 1, Oliva et al. 2018).

In order to remove the atmospheric contribution from both airborne dust and gases and obtain the spectral albedo of the surface (Reflectance Factor), the OMEGA spectra can be processed using the Surface Atmosphere Separation (SAS) method (Geminale et al., 2015) based on the principal component analysis (PCA) and Target Transformation (TT) technique. The PCA allows finding a set of eigenvectors whose linear combinations are able to reconstruct all the original measured spectra. The TT provides the means for physically interpreting the eigenvectors, by associating them to a set of trial vectors representing the possible physical contributions to the measured data.

This approach, although successful, is time consuming and so far it has been applied on a small portion of the OMEGA dataset. In this study we propose to optimize the method and exploit a larger number of OMEGA orbits.

References:

Bibring, J.-P. , Soufflot, A. , Berthé, M. , et al. ,2004. Omega: Observatoire pour La minéralogie, l’eau, Les Glaces Et l’activité. ESA SP-1240: Mars Express: The Sci- entific Payload. ESA Publications Division, Estec, Noordwijk, The Netherlands, pp. 37–49.

Geminale, A., Grassi, D., Altieri, F., Serventi et al., 2015. Removal of atmospheric features in near infrared spectra by means of principal component analysis and target transformation on Mars: I method. Icarus 253, 51–65. doi: 10.1016/j.icarus. 2015.02.012.

Oliva, F., Geminale A., D’Aversa E., Altieri F., et al., 2018. Properties of a Martian local dust storm in Atlantis Chaos from OMEGA/MEX data, Icarus, 300, doi 0.1016/j.icarus.2017.07.034.