The Preservation, Distribution, and Detectability of Lipid Biomarkers in the Atacama Desert and Implications for Mars
Space Telescope Science Institute (STScI) Bahcall Auditorium 3700 San Martin Drive Baltimore, MD 21218
Mary Beth Wilhelm (NASA Ames Research Center)
Desert environments on Earth are colonized by organisms adapted to desiccation. However, the limits of adaptation are not well understood. We surveyed biomolecular proxies for soil microorganism activity across a steep rainfall gradient from the driest region within the Atacama Desert in Chile that receives just a few millimeters of precipitation per decade to a few millimeters a year. Lipid biomarker proxies for membrane response to environmental stress, degree of amino acid racemization, and integrity of stress proteins suggest that organisms in the driest soils in the Atacama are not or very minimally metabolically active. This suggests that the dry threshold for soil habitability may have been crossed on the surface of Mars, which is 100-1000 times drier. While dryness imparts a great challenge to the "habitability" of Atacama soils, it also leads to a greater quality of preservation of biomarkers. Our understanding of long-term organic matter preservation comes mostly from studies in aquatic systems. In contrast, taphonomic processes in extremely dry environments are relatively understudied. We investigated the accumulation and preservation of lipid biomarkers in hyperarid soils in the Yungay region of the Atacama Desert via GC-MS and LC-MS. Buried clay units in this region contain fossil organic matter (radiocarbon dead) that has been protected from rainwater since the onset of hyperaridity. We show that these clay units contain lipids in an excellent state of structural preservation with functional groups and unsaturated bonds in carbon chains. This indicates that minimal degradation of lipids has occurred in these soils since the time of their deposition between >40,000 and 2 million years ago. The exceptional structural preservation of biomarkers is likely due to the long-term hyperaridity that has minimized microbial and enzymatic activity, a taphonomic process we term xeropreservation (i.e. preservation by drying). Finally, greater characterization of the organic material contained in martian sediments will be one of the the primary astrobiological goals in the next few decades. Life detection on other planets rests on the ability to interpret positive or negative results as well as contextualization with naturally-occurring terrestrial samples. We took advantage of the above-mentioned Atacama soil samples which contain both viable and fossil biomass, and are biomarker-poor and perchlorate-rich to assess the organic detection capability of current and future Mars mission flight-instrumentation including Raman laser spectroscopy and evolved gas analysis (EGA) techniques similar to what is being employed currently on the Curiosity Rover.
Light lunch (provided) starts at 12pm; talk starts at 12:30pm.
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