Signatures of Silicification in Lava Tubes: Implications for Biosignature Preservation and Detection on Earth and...

Terrestrial lava tubes serve as analogs for potentially habitable subsurface environments on Mars due to the bioenergetic potential of basalts, formation of secondary minerals, presence of actively seeping fluids, and isolation from the surface. On Earth, a wide variety of microbial communities that thrive in lava tubes often leave behind detectable traces of their presence and activity. To further an understanding of the contribution that secondary minerals, such as amorphous silica, can make to the preservation of organic material in lava tubes, we analyzed a small set of silica-rich basaltic lava tube samples from Mauna Loa, Hawai'i, and Lava Beds National Monument, CA. We applied techniques relevant to current Mars missions (Raman spectroscopy, powder X-ray diffractometry, gas chromatograph-mass spectrometry, scanning electron microscopy, and energy-dispersive X-ray spectrometry) and compared our datasets with those collected previously from similar analyses of siliceous sinter deposits from Yellowstone National Park. Despite variations in major elemental and organic compositions, distributions, and concentrations, we did not observe differences in the Raman spectroscopy and X-ray diffractometer data of the different silica-rich samples. Organic molecules were detected by Raman spectroscopy in all samples with varying degrees of specificity, regardless of the presence of morphological evidence of microbes or apparent degree of silicification. Pyrolysis and thermochemolysis gas chromatograph-mass spectrometry analysis also revealed the presence of numerous organic fragments that were likely derived from the remains of microbial communities in the lava tube samples. Where microbial material was observed, there were often overlapping spectra in some of the biomolecular and amorphous silica Raman signatures, which required careful interpretation that included scanning electron microscopy and gas chromatograph-mass spectrometry. Our data indicate that amorphous silica formation in lava tubes can obscure over time the distinctive morphological characteristics of microbial remains, though the chemical signatures of biomolecular compounds can still be detectable. Our findings have implications for the types of instrument packages that could be selected to assess the chemical makeup and textures of silica-rich lava tube deposits and suggest that more targeted studies are warranted to understand the possible implications for biosignature preservation and detection in such environments.