ASTROBIOLOGY AND NASA’S PROPOSED MARS 2020 ROVER

Mars exploration is poised to search for evidence of past life using protocols that have been validated during investigations of Earth’s early biosphere. Orbital observations have documented diverse localities that sustained liquid water in the past and that also might have been habitable. But the existence of biosignatures (“signatures of life”) in ancient rocks requires past habitable environments, biota that produced biosignatures, and subsequent conditions that allowed the preservation of biosignatures (the conditions had high “biosignature preservation potential,” or BPP). Categories of potential biosignatures (PBS) consist of chemical, isotopic, mineralogical and morphological features that can be created by life and appear to be inconsistent with nonbiological processes. Each PBS category differs from the others regarding the processes most important for altering or destroying PBS. Thus assessing the BPP of any PBS necessitates interpretation of past environments and processes, which requires measurements of rock chemistry, mineralogy, oxidation state, rock texture, morphology and geologic context. Orbiters have mapped phyllosilicates, carbonates, evaporites and silica deposits; on Earth these preserved evidence of our early biosphere. The ability to correlate observations of multiple PBS categories in their geologic contexts at several spatial scales is essential. The confidence in interpreting the origin(s) of PBS increases with the number of PBS categories identified. Thus rover-based lateral and stratigraphic surveys of multiple geologic deposits are required to assess BPP and any PBS in deposits from multiple paleoenvironments. The rover must be able to detect organic matter in order to credibly search for PBS; fortunately several flight-compatible spectroscopic methods can detect organics. Thus the proposed Mars 2020 rover should perform the following observations: context imaging and mineralogy; fine-scale imaging, mineralogy and elemental abundances; and organic matter detection. This measurement suite could find and document the geologic contexts of mineral deposits having high BPP and also document any PBS. The most promising samples must then be returned to Earth-based state-of-the-art laboratories in order to authenticate any definitive martian biosignatures.