Thermal infrared spectroscopy as a function of temperature for Mercury rocky-forming minerals: applications to remote sensing

Kurzfassung

It is well known that the surface of Mercury shows temperature variations extremely large at specific locations (Strom, 1987). The temperature can range in 44 earth days between 70 and 725 K at different latitudes and such variations affect significantly the crystal structure and density of the minerals present on the surface of the planet. As a consequence, several kinds of remote information linked to mineral structures, including relative spectral signatures, are strictly dependent on the environmental conditions and vary according to the surface temperature,. By “looking at the planet” in remote sensing, Mercury may change its outer face through time and from place to place as a function of solar irradiation. This could drive to important misinterpretations and limit our capability of inferring compositions and rheological properties of materials from remote sensing acquisition. Thus, we need to investigate the behavior of planetary geological materials in situ, or under extreme temperature environments, and improve the geological interpretation and compositional inferences by applying the knowledge acquired to the analysis of available remote sensing data. Here we present a new in situ multi-methodological laboratory approach comprehensive of X-ray diffraction (single-crystal and powder materials) and spectroscopy. In particular, we used Thermal Infrared (TIR) spectroscopy, in the range 7 to 14 µm on olivines and clinopyroxenes in order to evaluate if temperature variation may causes the same density variation due to a cation substitution. In such wavelength range the spectra can be used very effectively to identify the fine-scale structural properties of minerals (Hamilton, 2010). We performed laboratory measurements at the Planetary Emissivity Laboratory (PEL) at the German Aerospace Center (Deutsches Zentrum fuer Luft- und Raumfahrt in Berlin), where is available a high-temperature and vacuum spectrometer, to acquire emissivity spectra of several single mineral phase with the aim to monitor how their specific spectral features changes as a function of temperature, reproducing the most likely environmental conditions of the hermean surface. In fact, although a significant number of minerals have been well-characterized by means of TIR spectroscopy (Christensen et al. 2002) in our knowledge no TIR data acquired at high-temperature have been published so far with the exception of those by Herbert and Maturilli (2009) on a labradorite plagioclase. The PEL has been build up over the last 5 years to study systematically the effect of temperature on TIR spectral signature and build up a new spectral library taking these effects into account. This is of special significance, since the Bepi Colombo mission, scheduled for launch in 2014, will carry the first thermal infrared imaging spectrometer (Mercury Radiometer and Thermal Infrared Spectrometer, MERTIS) to map the surface of Mercury from orbit, acting in the wavelength region from 7 to 14 µm. The instrument is under construction at Deutsches Zentrum fuer Luft- und Raumfahrt and at the same time the spectrometer of the Planetary Emissivity Laboratory is working for real measurements on minerals. The produced data allows to build up a new TIR mineral database that will be extremely useful in interpreting the future data acquisitions by remote sensing.