Visible and near-infrared spectroscopy of terrestrial analogues of Mars by spectro-gonio radiometer measurements.

Spectroscopic observation is one of the main tools employed to explore the geological features of extraterrestrial environments. In particular, space missions equipped with high resolution imaging spectrometers provide hyperspectral images of planetary surfaces under variable illumination and observation geometries, which represent an essential approach for the understanding of the geochemical context. For instance, combined in situ measurements, remote-sensing observations and laboratory investigations have provided extensive evidence for the presence of water-lain sedimentary rocks on Mars. Coupled with morphological evidence for ancient lakes and valleys, these findings open up questions on the climatic history of Mars, and stimulate the efforts addressed at deciphering whether Mars could have experienced warmer, wetter, and more habitable conditions in its past and consequently if some biological processes could have developed on it. Since the advent of present and future in situ missions to Mars to investigate its properties and to search for traces of life, the analysis of the acquired data requires comparative laboratory measurements. In particular, soil reflection of the sunlight can be interpreted by studying the spectra of artificial and natural terrestrial analogues. Here we present the preliminary results obtained with the Spectro-Gonio Radiometer (working in the 0.3-4.5 μm spectral range) at the Institut de Planétologie et Astrophysique de Grenoble, a facility which mimics the remote and in situ spectroscopic techniques used for the observation of the Martian surface and simulate the conditions of the planet itself (temperature, pressure and composition of the atmosphere). The analysed samples are analogues of Martian brines formed in a sulfur reach environment collected in the Danakil Depression, a particularly extreme environment of the Afar triangle at the junction of the Arabian, Nubian and Somalian lithospheric plates.