A new laboratory emissivity and reflectance spectral library for the interpretation of Mars thermal infrared spectral data

Abstract

New spectral orbital thermal infrared data of Mars are being acquired by the thermal infrared channel TIRVIM (in honor of Vassily Ivanovich Moroz) of the Atmospheric Chemistry Suite (ACS) of spectrometers on board of ExoMars2016 mission. TIRVIM encompasses the spectral range of 1.7–17 μm. A major challenge brought by the analysis of these data of planetary bodies with atmospheres is the ability to extract from the data the relevant information about the surface. Thus, laboratory work plays an essential role, providing end-member and mixture spectral data of planetary analogs to fit the orbital data by means of deconvolution techniques. At the Planetary Spectroscopy Laboratory (PSL) of the German Aerospace Center (DLR), we are performing new laboratory experiments on Martian analogs in order to provide a new and updated library of spectra optimized for the interpretation of TIRVIM data. Emissivity measurements, recorded at increasing temperatures, are coupled with reflectance measurements on fresh and thermally processed samples acquired between 1.7 and 17 μm. Building on measurements previously collected on Martian analogues, we have paid particular attention to the study of the spectral behaviour of mixtures of carbonates and phyllosilicates. The main goal of this analysis is to study the variation of the main carbonate spectral features in mixtures with a phyllosilicate component, an important factor for understanding the story of carbonates detections on planetary surfaces and to provide insights for new detections. The results obtained in this work show that the presence of a phyllosilicate component affect the appearance of the carbonate spectral features in the spectral range studied, with a stronger effect in the range between 1.7 and 5 μm. Effects of mineral type and particle size are also investigated and shown to strongly affect the spectral behaviour of laboratory samples. Finally, deconvolution techniques of laboratory emissivity spectra are studied in preparation for the interpretation of atmospherically corrected TIRVIM spectral data, showing that modelled mixtures spectra represent an acceptable reproduction of laboratory spectra of mixtures.