Analysis of OMEGA/Mars express data hyperspectral data using a Multiple-Endmember Linear Spectral Unmixing Model (MELSUM): Methodology and first results

Research areas:
The mineralogical composition of the Martian surface is investigated by
a Multiple-Endmember Linear Spectral Unmixing Model (MELSUM) of the
Observatoire pour la Mineralogie, l'Eau, les Glaces et l'Activite
(OMEGA) imaging spectrometer onboard Mars Express. OMEGA has fully
covered the surface of the red planet at medium to low resolution (2-4
km per pixel). Several areas have been imaged at a resolution up to 300m
per pixel. One difficulty in the data processing is to extract the
mineralogical composition, since rocks are mixtures of several
components. MELSUM is an algorithm that selects the best linear
combination of spectra among the families of minerals available in a
reference library. The best fit of the observed spectrum on each pixel
is calculated by the same unmixing equation used in the classical
Spectral Mixture Analysis (SMA). This study shows the importance of the
choice of the input library, which contains in our case 24 laboratory
spectra (endmembers) of minerals that cover the diversity of the mineral
families that may be found on the Martian surface. The analysis is
restricted to the 1.0-2.5 mu m wavelength range. Grain size variations
and atmospheric scattering by aerosols induce changes in overall albedo
level and continuum slopes. Synthetic flat and pure slope spectra have
therefore been included in the input mineral spectral endmembers library
in order to take these effects into account. The selection process for
the endmembers is a systematic exploration of whole set of combinations
of four components plus the straight line spectra. When negative
coefficients occur, the results are discarded. This strategy is
successfully tested on the terrestrial Cuprite site (Nevada, USA), for
which extensive ground observations exist. It is then applied to
different areas on Mars including Syrtis Major, Aram Chaos and Olympia
Undae near the North Polar Cap. MELSUM on Syrtis Major reveals a region
dominated by mafic minerals, with the oldest crustal regions composed of
a mixing between low-calcium pyroxenes (LCPs) (orthopyroxenes (OPx)) and
high-calcium pyroxenes (HCPs) (clinopyroxenes (CPx)). The Syrtis
volcanic edifice appears depleted in LCP (OPx) and enriched in HCP
(CPx), which is consistent with materials produced with a lower partial
fusion degree at an age younger to the surrounding crust. Strong olivine
signatures are found between the two calderas Nili Patera and Meroe
Patera and in Nili Fossae. A strong signature of iron oxides is found
within Aram Chaos, with a spatial distribution also consistent with
thermal emission spectrometer (TES). Gypsum is unambiguously detected in
the northern polar region, in agreement with the study of Langevin et
a]. {[}2005. Sulfates in the north polar region of Mars detected by
OMEGA/Mars Express. Science 307(5715), 1584-1586]. Our results show that
the linear spectral unmixing provides good first order results in a
variety of mineralogical contexts, and can therefore confidently be used
on a wider scale to analyze the complete archive of OMEGA data. (c) 2008
Elsevier Ltd. All rights reserved.