Filed-Analog Study: Efficiency of microbial fossilization in sulfate-rich playas

Abstract:

The samples with the highest priority for sample return will be samples that contain potential life signatures. Sulfate-rich salts have been identified as important component of Mars sedimentary deposits, illustrating the importance of near-surface hydrological processes during the planet’s history. The presence of Noachian/early Hesperian sulfate-rich deposits have been identified by the MER’s Opportunity at Meridiani Planum and by MRO mission in sedimentary sequences within Gale crater, the Mars Science Laboratory landing site.

White Sands National Monument, which we are using as a terrestrial analog in our study, holds active playas that may contain different sizes of water bodies during the year. Alkali Flat contains also preserved deposits of Pleistocene Lake Otero. The presence of a range of modern playas and equivalent Pleistocene deposits allows for an excellent comparison of these different extreme transitional habitats and potential of preservation of organics in an evaporitic playa setting.

We have performed shallow drilling (1m) of the Lake Lucero deposits and of Lake Otero evaporitic sequence. The XRD analysis revealed that samples are mainly composed of gypsum and minor mineral phases as brushite, halite and quartz. SEM/EDS revealed the presence of amorphous phases such as halite, glauberite, magnesium chlorite salt, and diatom shells and biofilm. Biofilm is found in near surface samples, which may be attributed to microbial adaptation to desert environment and considered as one of the characteristics of modern and not fossil microbial communities. The compositional difference of salt precipitates associated with potentially geologically old biofilm and the modern counterpart are still ongoing and our findings will be presented at the conference. Further, comparison of environmental physicochemical conditions and molecular biology will be used to determine the characteristics of modern microbial habitats/deposits and to attempt to distinguish them from the fossil ones. Bioturbation and re-precipitation is a potential issue for any fossil finding, and especially for evaporitic settings. White Sands playas represent an excellent model system to evaluate preservation potential of evaporitic playa deposits and their potential value in a context of a sample return mission.