USING PETROGRAPHY AND SPECTROSCOPY TO DETECT LIFE IN EVAPORITES AND IRON OXIDE CONCRETIONS: SUGGESTIONS FOR THE SEARCH FOR MODERN AND ANCIENT LIFE ON MARS

Acid brines are likely to have existed on Mars for much of its history. Much can be learned from the study of ephemeral acid saline lakes on Earth, which bear notable similarities to some sedimentary deposits on Mars. Recent work has shown that diverse communities of extremophile microorganisms live in these low pH, high salinity environments. Biosignatures, such as microorganisms and organic compounds, and even biota such as insects, are trapped within halite and gypsum as they precipitate from lake waters. Preservation includes fossilization in solid halite and gypsum, as well as entrapment within primary fluid inclusions in those minerals. A number of extremophilic microorganisms may remain viable while entrapped. Furthermore, salt minerals can become coated with early diagenetic minerals such as iron oxides, resulting in fossilization within iron oxide concretions.

Our results from optical petrography, UV-vis petrography, and laser Raman spectroscopy clearly indicated the presence of biosignatures within halite and gypsum from acid saline lakes in Chile and in Australia. Although species-level identifications cannot be made this way, bacteria/archea, fungi, and algae can be distinguished by their morphology, size, and UV fluorescent response. Some genera, such as Dunaliellaalgae, can be recognized based on morphological characteristics alone. The presence of a broad, double peak in our Raman spectra is indicative of the presence of microorganisms. In addition, we detected organic compounds by an UV fluorescent response and can be identified accurately with laser Raman spectroscopy.

We propose that the use of these instruments and non-destructive methods on Mars rovers and landers, and on future returned samples, will prove to be a more effectual way to search for modern and ancient life on Mars. Important considerations include the focus on salt minerals and high-quality, high magnification optics. This approach can become an important preliminary step to target the best samples sample return and further analysis by using traditional or molecular biological methods.