Near-Infrared Reflectance Spectroscopy of Organics on Earth and Planetary Surfaces: New Insights from Laboratory and Spacecraft Data


Organic matter is present on many bodies in the solar system. The parent bodies of carbonaceous chondrite meteorites, for example, are known to contain a diverse array of organic compounds that may be relevant to prebiotic chemistry. This thesis explores near-infrared reflectance spectroscopy, a remote, rapid, and non-destructive method, for quantifying organic matter in geologic materials such as carbonaceous chondrites. Laboratory studies of clay-organic mixtures, sedimentary rocks, and carbonaceous chondrites with known organic and inorganic chemistry are used to constrain the effects of organic composition and abundance on the reflectance spectrum. Vibrational absorptions due to aliphatic and aromatic C-H bonds at 3.2 – 3.6 µm are specifically targeted as these absorptions have been detected by spacecraft and ground-based infrared observations of comets, asteroids, moons, and the interstellar medium. When organic composition is controlled, the 3.4 µm absorption strength increases linearly with abundance (wt.%) of organics in the sample. Variations in organic composition, as measured by atomic hydrogen to carbon ratio (H/C), likewise influence the strength of the aliphatic and aromatic absorptions. Reflectance spectra of pure organics (e.g. kerogen from sedimentary rocks and insoluble organic matter from meteorites) are collected to better quantify compositional variations within the spectrum and to provide a spectral library for future compositional modeling. Together the pure organic and bulk rock spectra enable the determination of detection limits for organic matter with reflectance spectroscopy in terms of organic abundance (wt.%) and composition (H/C). Ceres provides an ideal application of this laboratory work. In 2017, aliphatic organics were detected on the surface of dwarf planet Ceres with the Visible and InfraRed (VIR) spectrometer. This detection is considered within the context of meteorite, insoluble organic matter, and kerogen laboratory spectra with the goal of constraining organic abundance and composition. Spectral modeling estimates between 45 – 65% insoluble organic matter on the surface of Ceres, placing firm constraints on the origin of these organics. Other applications of this work include upcoming missions to primitive asteroids, where organics may be detected. Data and returned samples from these missions can help to test spectral models for determining the composition of planetary surfaces.
Thesis (Ph. D.)--Brown University, 2018

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Kaplan, Hannah Hackett, "Near-Infrared Reflectance Spectroscopy of Organics on Earth and Planetary Surfaces: New Insights from Laboratory and Spacecraft Data" (2018). Earth, Environmental and Planetary Sciences Theses and Dissertations. Brown Digital Repository. Brown University Library.