Life Detection in Gypsum-Rich Environments: Lessons for Space Exploration

Abstract:

Our search for signs of life on other planets is necessarily influenced by chemical conditions present on Earth. These chemical conditions not only shape the limits of Terran life, but they also impact the possible preservational pathways for how signs of this life are preserved into the rock record. For instance, on Earth, there are many known environmental conditions that can preserve organic biomarkers, but on Mars, the equivalent environments are chemically unsuitable for preserving organic carbon compounds, in large part because the surface of Mars is extremely oxidizing and ionizing. Gypsum, a hydrated calcium sulfate mineral common on Mars, offers protection from many of these Mars-specific barriers, but its ability to preserve biosignatures is largely unknown, as it is not a standard target of life-detection searches. Thus, in this study, we used techniques drawn from chemistry, biology, and geology to explore the record of life preserved within gypsum samples from the Permian Blaine Formation of Kansas.

Field observation and petrographic microscopy revealed that this gypsum is likely primary, conclusions supported by previous researchers. Gas chromatography/mass spectroscopy analyses demonstrated that samples from the Blaine Formation contain biomarkers, while the shale units found stratigraphically above and below did not. Similarly, fluorescence microscopy established that the Blaine Formation contains autofluorescing compounds within equant gypsum crystals, unlike either of the adjoining units. Together, these techniques suggest that this gypsum contains the remnants of an ancient Permian microbial mat: the morphology of the gypsum crystals is consistent with those precipitated in the presence of organic inhibitors, the biomarker distribution is identical to that found in modern microbial mats growing in gypsum-precipitating environments, and the distribution and color of the autofluorescing compounds speaks to aliphatic hydrocarbons trapped within the mineral lattice. This study shows that combining techniques from various fields and examining samples from environments that are not usual targets for searching for signs of life on Earth can help develop further strategies for off-planet life detection.