FTIR Hyperspectral Imaging: a Top-Down Sequence to Investigate the Heterogeneity of Meteorites from Millimetric to Micrometric Scale

Abstract:

FTIR spectroscopy is a powerful tool to apprehend the complex assembly of organics and minerals which constitute extraterrestrial matter. In the laboratory, we can analyze their heterogeneity from millimetric to micrometric scales and reveal information of processes occurring on their parent body. Hayabusa2 and OSIRIS-REx sample return missions, require non-destructive methods to maximize the information extracted from the collected samples while minimizing the sample modification. IR spectra of asteroids Ryugu and Bennu have been collected by onboard spectrometers and remote observations. The combination of these data with laboratory measurements will help to apprehend multi-scale processes.

We established a top-down protocol combining FTIR measurements in the laboratory and applied it to several carbonaceous chondrites, as analogs of Ryugu and Bennu. Hyperspectral FTIR data were collected using an Agilent Cary 670/620 micro-spectrometer combined with a 128x128 pixels FPA detector, installed at the SMIS beamline of the SOLEIL synchrotron (France). Using the Globar source, we collected spectra in the 850-3950 cm^-1 spectral range. They were obtained in reflection mode using a 15x objective (pixel size of 5.5 µm) and in transmission mode using a 25x objective (pixel size of 0.66 µm). The spatial resolution was diffraction-limited for the whole investigated spectral range.

Analyses in reflection, comparable to remote sensing observations, were performed first on mm-sized samples to characterize their surface. Then, smaller fragments (30-50 µm) were mounted on needles to perform FTIR micro-tomography and characterize the whole volume. We explored the 3D distributions and investigated the links between the organic and mineral components. We found peculiar areas of interest inside some grains (such as regions with a higher concentration of organic matter). Then we sliced one of the grains in thin sections using a FIB. We analyzed them with complementary methods to characterize the matter at the sub-µm scale (e.g., Transmission Electronic Microscopy, Raman spectroscopy, Optical Photothermal IR Spectroscopy). With this sequence, we will reinforce the bridge between the remote sensing observations of planetary surfaces, and laboratory measurements on retrieved samples.