TIR Emissivity Spectra of Thermally Processed Sulfates, Carbonates and Phyllosilicates as Analog Materials for Asteroid Surfaces

Kurzfassung

At the Planetary Emissivity Laboratory (PEL) of the German Aerospace Center (DLR) in Berlin we are building a database of spectral measurements of several meteorites and other analogs for asteroid surfaces. Bi-directional reflectance of samples in the 1 to 100 µm spectral range, are measured by using an evacuated (10-4 bar) Bruker Vertex 80V FTIR spectrometer and a Bruker A513 reflection unit, allowing phase angles between 26° and 170°. Emissivity in the 1 to 100 µm spectral range is measured with the same instrument coupled with an external emissivity chamber, for sample temperatures ranging from low (50° C) to very high (above 800° C). We present here new measurements on sulfates, carbonates, and phyllosilicates in various grain size ranges. The setup was configured to simulate the thermal history of surface minerals on the asteroid 2008 EV5 during its revolution around the Sun. This asteroid is the scientific target of the ESA Marco Polo-R mission. The samples in vacuum (< 0.8 mbar) are measured at surface temperature around 70° C, then the same samples are heated to 220° C, and maintained at this temperature for one hour. Slowly the sample temperature is reduced back again to 70° C and a second measurement is taken. Emissivity spectra before and after thermal processing of the samples are complemented with reflectance measurements on samples fresh and after thermal processing. This comparison show us that for some minerals no spectral/structural changes appear, while others show signs of dehydration and among them some species show structural changes. We conclude that a proper spectral library of emissivity spectra for asteroid analogue materials must include thermally processed samples, reproducing the thermal evolution for the asteroid that is target of the actual investigation.