Recovery of Lipid Biomarkers in Hot Spring Digitate Silica Sinter as Analogs for Potential Biosignatures on Mars: Results from Laboratory and Flight-Like Experiments.

Digitate siliceous sinter deposits are common in geothermal environments. They form via evaporation and precipitation of cooling silica-rich fluids and passive microbial templating. Increasing interest in these "finger-like" microstromatolitic sinters is related to their morphological and mineralogical resemblance to opaline silica-rich rocks discovered by NASA's Spirit rover in the Columbia Hills, Gusev crater, Mars. However, these terrestrial deposits remain understudied, specifically in terms of biosignature content and long-term preservation potential. In this study, six digitate, opaline (opal-A) sinter deposits were collected from five Taupō Volcanic Zonegeothermal fields, and their lipid biosignatures were investigated as Mars analogs. Samples were collected in pools and discharge channels of varied temperatures, pH, and water chemistries, with spicular to nodular morphologies. Results revealed the presence of biomarkers from unsilicified and silicified communities populating the hot spring sinters, including lipids from terrigenous plants, algae, and bacteria. Although DNA sequencing suggests that the composition and diversity of microbial communities are correlated with temperature, pH, and water chemistry of the springs, these environmental parameters did not seem to affect lipid recovery. However, the morphology of the sinters did play a role in lipid yield, which was higher in the finest, needle-like spicules in comparison to the broad, knobby sinters. The capability of current Mars flight mission techniques such as pyrolysis-gas chromatography-mass spectrometry to detect lipid biomarkers was also evaluated from a subset of samples in a pilot study under flight conditions. The early preservation of lipids in the studied sinters and their detection using flight-like techniques suggest that martian siliceous deposits are strong candidates for the search for biosignatures on Mars.