Dr. Mary Beth Wilhelm NASA Planetary Scientist and Astrobiologist

Dr. Mary Beth Wilhelm

Research Interests

PhD Dissertation Summary: 

The Preservation, Distribution, and Detectability of Lipid Biomarkers in the Atacama Desert and Implications for Mars

Desert environments on Earth are colonized by organisms adapted to desiccation. However, the limits of adaptation are not known. We hypothesized that extreme and prolonged dryness might impart too great a challenge for microbial survival. To test this, 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, 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 has been crossed in the driest hyperarid regions in the Atacama, and implies that it may also have been crossed on the surface of Mars, which is 100-1000 times drier. While dry Atacama soils in this region might not be habitable, dryness could lead to greater preservation of biomarkers generated during a wetter epoch. 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 and are poorly understood. We investigated the accumulation and preservation of lipid biomarkers in hyperarid soils in the Yungay region of the Atacama Desert where microbial activity might not exist.  Lipids from seven soil horizons in a 2.5 m vertical profile were extracted and analyzed using GC-MS and LC-MS. Diagnostic functionalized lipids and geolipids were detected and increased in abundance and diversity with depth. Deeper clay units contained fossil organic matter (radiocarbon dead) that has been protected from rainwater since the onset of hyperaridity. We showed 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). The degree of biomarker preservation allowed us to reconstruct major changes in ecology in the Yungay region that reflect a shift in hydrological regime from wet to dry since the early Quaternary. Our results suggest that hyperarid environments, which comprise 7.5% of the continental landmass, could represent a rich and relatively unexplored source of paleobiological information on Earth, and potentially Mars. Greater characterization of the organic material contained in ancient and/or modern 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 characterized 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. Firstly, Raman laser spectroscopy, slated to fly on both ESA and NASA Mars rovers, was not able to identify the most abundant biogenic lipid or hydrocarbon compounds contained in Yungay soils. Instead, only two non-specific carbon bands were detected in ~75% of sample points. These bands are commonly observed in non-biologic carbon-bearing samples. However, evolved gas analysis (EGA) techniques similar to what is being employed currently on the Curiosity Rover revealed organic ions in all samples analyzed. Organics contain in Yungay surface soils had the strongest indication of bioenicity. However, analysis of million-year-old buried halite revealed the presence of organic-bearing water inclusions. Our study suggests that sample acquisition and handling which includes preparation leas to increased quality of organic information.