Characterization of biosignatures within geologic samples analyzed using a suite of in situ techniques
I investigated the biosignature detection capabilities of several in situ techniques to evaluate their potential to detect the presence of extant or extinct life on other planetary surfaces. These instruments included: a laser desorption time-offlight mass spectrometer (LD-TOF-MS), an acousto-optic tunable filter (AOTF) infrared (IR) point spectrometer, a laser-induced breakdown spectrometer (LIBS), X-ray diffraction (XRD)/X-ray fluorescence (XRF), and scanning electron microscopy (SEM)/energy dispersive X-Ray spectroscopy (EDS). I measured the IR reflectance spectra of several speleothems in caves in situ to detect the presence of biomineralization. Microorganisms (such as those that may exist on other solar system bodies) mediate redox reactions to obtain energy for growth and reproduction, producing minerals such as carbonates, metal oxides, and sulfates as waste products. Microbes occasionally become entombed in their mineral excrement, essentially acting as a nucleation site for further crystal growth. This process produces minerals with a crystal lattice distinct from geologic precipitation, detectable with IR reflectance spectroscopy. Using a suite of samples collected from three subterranean environments, along with statistical analyses including principal component analysis, I measured subsurface biosignatures associated with these biomineralization effects, including the presence of trace elements, morphological characteristics, organic molecules, and amorphous crystal structures. I also explored the optimization of a two-step LD-TOF-MS (L2MS) for the detection of organic molecules and other biosignatures. I focused my efforts on characterizing the L2MS desorption IR laser wavelength dependence on organic detection sensitivity in an effort to optimize the detection of high mass (≥100 Da) organic peaks. I analyzed samples with an IR reflectance spectrometer and an L2MS with a tunable desorption IR laser whose wavelength range (2.7 - 3.45 microns) overlaps that of our IR spectrometer (1.6 - 3.6 microns), and discovered a IR resonance enhancement effect. A correlation between the maximum IR absorption of organic functional group and mineral vibrational transitions - inferred from the IR spectrum - and the optimal IR laser configuration for organic detection using L2MS indicates that IR spectroscopy may be used to inform the optimal L2MS IR laser wavelength for organic detection. This work suggests that a suite of instruments, particularly LD-TOF-MS and AOTF IR spectroscopy, has strong biosignature detection potential on a future robotic platform for investigations of other planetary surfaces or subsurfaces.
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