Natural solid bitumens as possible analogs for cometary and asteroid organics: 1. Reflectance spectroscopy of pure bitumens

Abstract

Natural solid oil bitumens (asphaltites, kerites, anthraxolites) appear to be useful spectral analogs for cometary and asteroid refractory organics. They show similarities to the surfaces of dark asteroids and cometary nuclei, as well as compositional and structural resemblances to organic components of carbonaceous chondrites. These dark solids are composed of a variety of organic compounds, most of which are aromatic and aliphatic hydrocarbons. Spectral reflectance measurements in the range of 0.5 - 16 µm have been performed on a suite of solid bitumen powders. The samples were also characterized by chemical analyses, solid state 13C-NMR, and X-ray difractometry. The observed spectral variations between bitumen samples correlate with their chemical composition and structure. In the near-infrared region spectral parameters have been found, which make it possible to derive carbon aromaticities (f_a) and the most important elemental ratios /C/(C+H+N+S+O) or H/C). Moreover, these parameters can be used to estimate some structural characteristics of the aromatic component (an average interlayer spacing d_002 and vertical chrystallite size L_c). The use of the same spectral parameters may help to reveal the presence of mineral admixtures when their content in an organic-rich assemblage is too low to show any distinct absorption bands of the minerals.