REFLECTANCE SPECTROSCOPY OF ORGANIC-BEARING ROCKS: APPLICATIONS TO CARBONACEOUS CHONDRITES AND PLANETARY SURFACES

Reflectance spectroscopy of bulk rocks can provide information on organic and inorganic (mineralogical) composition. The non-destructive, rapid, and remote nature of reflectance measurements is beneficial for analysis of valuable extraterrestrial samples and for large-scale mapping of planetary and asteroid surfaces. In this work we examine clay-rich sedimentary rocks and kerogen isolated from those rocks at visible to mid-infrared wavelengths (0.35 – 25 mm) as a precursor to the study of C chondrites. The strength of aliphatic organic (C-H) absorptions between 3.3 – 3.6 mm are compared to total organic carbon (TOC) of the bulk rock and H/C ratios of the organic matter. A quantifiable but non-unique relationship between sample properties (TOC, H/C) and band depth in spectra is observed. We also explore a Hapke-based radiative transfer model to predict modal mineralogy and organic content using mineral and organic spectral endmembers, and results are compared with abundances measured by X-Ray diffraction and elemental analysis. Preliminary results show that relative kerogen abundance can be accurately predicted with the spectral model. The third method employed in this study is a partial least squares regression (PLSR) to determine TOC and H/C using spectra of bulk rock powders. This approach has been demonstrated to accurately predict soil organic content and mineral and organic contents in drill cores and, likewise, works well for the limited range of samples in this study. Finally, we will present the results of similar spectral models as applied to a subset of C chondrite spectra acquired from the Reflectance Experimental Laboratory (RELAB) facility, corresponding to samples for which TOC and H/C have been independently measured in previous work. This study provides an important and necessary starting point for quantitative interpretation of organic signatures that may be observed in spectra of C-type asteroids by the upcoming Hayabusa2 and OSIRIS-REx encounters.