Astrobiology & the Coevolution of Earth and Life: The Kepler mission has discovered over 2000 confirmed exoplanets, which are estimated to be common in the Milky Way galaxy. Efforts are underway to characterize the habitability of these planets, in particular whether they contain liquid water and have conditions conducive to supporting life. Another critical next step is identifying characteristic biosignatures that could indicate the presence of life on these planets. While for most of these exoplanets biosignatures will have to be visible via remote sensing, those in our own solar system, such as Mars, are being physically explored, making possible a much broader range of potential biosignatures. Extreme environments on Earth are ideal for typing and exploring such biosignatures. A wide array of microorganisms are adapted to diverse pH, redox, and salinity gradients thought representative of conditions on Mars. It follows that exploring biosignatures in these terrestrial ecosystems may provide fingerprints of life (past or present) on other habitable worlds. There are ten broad categories of biosignatures, including stable isotope patterns, organic molecules, minerals, and micro- and macroscopic structures and textures. Organic molecules, such as lipid biomarkers, are considered among the best biosignatures for exoplanetary research. When combined with stable isotopes, mineralogy, and sedimentary structures these biomarkers become particularly powerful bodies of evidence for past life, and such an approach has proven fruitful in a number of extreme environments on Earth, including methane cold seeps, euxinic systems, and hypersaline systems. There are few links connecting environments on Earth to similar environments on Mars – so-called “analog” environments, many of which are considered extreme environments on Earth. We must investigate analogous Earth systems, both modern and ancient, for biosignatures in order to understand the potential meaning and reliability of biosignatures regarding life and environmental conditions on exoplanetary systems. Several lacustrine deposits have been identified in Gale Crater on Mars by the Curiosity Rover, with some evidence for variations in redox conditions and potentially acidic environments. Further evidence of evaporitic lakes on Mars has been observed with the Mars Exploration Rover (MER) via the presence of evaporite minerals such as anhydrite, basanite, and gypsum. Other recent studies have identified organic molecules in sediments from Gale Crater, including organic sulfur compounds, as well as significant variations in the sulfur isotopic composition of Martian minerals, and fatty acids have been identified in various Martian analogues. In order to enrich our understanding of such lacustrine and evaporitic sites on Mars, further investigation in relevant analog sites is required. The potential for such biosignature studies to inform our understanding of early Earth environments, the coevolution of Earth and life, and the potential for life on other planets is significant.
