VIS-NIR DIFFUSE REFLECTANCE MICRO-SPECTROSCOPIC ANALYSIS OF IDPs

Introduction: Meteorites seem to come from a small number of primary parent bodies [1]. B-, C-, Cb-, Cg-, P- and D-types, representing not less than 66% of the mass of the main belt have no analogues clearly identified in the meteorite collections [2]. However, meteorites are not the only cosmomaterials found on Earth since no less than 30 000 tons of interplanetary dust particles (IDPs) enter the Earth’s atmosphere each year [3]. IDPs originate from different parent bodies throughout the solar system [4, 5, 6]. The link between IDPs and asteroids can be investigated thanks to Vis-NIR spectroscopy commonly used for the classification of asteroids (0.4 - 2.5 µm). The reflectance measurements in the visible range (0.4 - 0.8 μm) performed on IDPs in the 90s [7] and the simulated visible near infrared (Vis-NIR) spectra of IDPs with comparison of mid infrared (Mid-IR) spectra [2] have shown that IDPs may be good analogues to some asteroids and in particular to the classes not sampled by meteorites. But Vis-NIR reflectance measurements of IDPs is challenging and we must understand how the measurement on an isolated micro-metric particle can be affected by physical parameters of the sample such as size, composition, and roughness. We report here the requirements, the abilities as well as the limitations of the technique and the results obtained on 15 IDPs particles ranging 7-31 µm in size in the Vis-NIR range (0.45 - 1.0 µm). Experiments: Our setup, installed in a clean room, consists of a Vis-NIR spectrometer (Maya2000 Pro from Ocean Optics) coupled to a macroscope (Leica Z16 APO). A Vis-NIR optical fiber (100 or 50 μm in diameter) is used to collect the light diffused by the sample which is unilaterally illuminated by a halogen source through a 1000 μm diameter fiber (phase angle of ~ 45°). By changing the magnification and/or the diameter of the collection fiber it is possible to adapt the collection spot to the grain size down to 7 μm size. Results and discussion: To obtain a reliable reflectance spectrum of a micro-metric grain with this setup, we show that it is necessary to average spectra taken at different azimuth angles, by rotating the particle several times in the observation plane with respect to the incident light. Based on the study of spectral slopes we found that for particles with sizes below ~ 17 µm the spectral slope increases linearly with decreasing particle sizes. This behavior is due to a bias encountered in the reflectance measurement in this size range, inducing thus a loss of the chemical information. For particle sizes larger than ∼ 17 µm the spectral slopes seem randomly distributed between ∼ -0.3 and 0.4 µm−1, and the spectra must therefore carry chemical information of the particles. We found that the visible reflectance levels of the IDPs show a multimodal distribution. There is a lack of IDPs with reflectance level ~ 5 and ~ 8%. In addition, the majority of IDPs have rather low reflectance levels (< 10%). Some particles have reflectance levels that may be influenced by the presence of magnetite, which is sometimes found in extraterrestrial materials and could form upon atmospheric entry. Among the studied particles we identified an IDP (L2079C18) exhibiting a feature at 0.66 µm which is similar to the one observed by remote sensing at the surface of hydrated asteroids. This is the first detection of a hydration band in the reflectance spectrum of an IDP which could indicate a possible link between hydrated IDPs with hydrated asteroid surfaces. Acknowledgments: We are grateful to the CAPTEM NASA for providing the IDPs. This work is supported by the Programme National de Planétologie (PNP) of CNRS/INSU, co-funded by CNES. The authors also thank the ANR RAHIIA SSOM and the P2IO LabEx (ANR-10-LABX0038) in the framework Investissements d’Avenir (ANR11-IDEX-0003-01) for their supports. We thank O. Mivumbi and Y. Longval for their help and technical support for the development of the device. References: [1] R. Greenwood et al. (2020) Geochimica et Cosmochimica Acta 277, 377-406. [2] P. Vernazza et al. (2015) The Astrophysical Journal 806 :204. [3] Love and Brownlee. (1993) Science, 262, 550-553. [4] Dermott et al. (1994) Nature, 369, 719-723. [5] Liou et al. (1996) Icarus, 124, 429-440. [6] Brunetto et al. (2011) Icarus, 212, 896-910. [6] Bradley, J. P. (2003) Treatise on Geochemistry, 1, 689. [7] Bradley, et al. (1996) Meteoritics & Planetary Science, 31, 394-402.