Biosignatures in ancient rocks: a summary of discussions at a field workshop on biosignatures in ancient rocks

Life on Mars: Clues, Evidence or Proof?

The search for life on Mars is one of the main objectives of space missions. At “Pahrump Hills Field Site” (Gale Crater, Mojave target), inside the mudstones of the Murray lacustrine sequence, Curiosity rover found organic materials and lozenge shaped laths considered by NASA as pseudomorphic crystals. Besides it detected mineral assemblages suggesting both oxidizing (hematite) and reducing (magnetite) environments, as well as acidic (diagenetic and/or authigenic jarosite) and neutral (apatite) conditions, that might suggest bacterially mediated reactions. Our morphological and morphometrical investigations show that such diagenetic microstructures are unlikely to be lozenge shapes and, in addition to several converging features, they suggest the presence of remnants of complex algal-like biota, similar to terrestrial procaryotes and/or eukaryotes; possible microorganisms that, on the base of absolute dating criteria used by other scholars, lived on Mars about 2.12 +/−0.36 Ga ago.

The Characterization of Biosignatures in Caves Using an Instrument Suite

The search for life and habitable environments on other Solar System bodies is a major motivator for planetary exploration. Due to the difficulty and significance of detecting extant or extinct extraterrestrial life in situ, several independent measurements from multiple instrument techniques will bolster the community's confidence in making any such claim. We demonstrate the detection of subsurface biosignatures using a suite of instrument techniques including IR reflectance spectroscopy, laser-induced breakdown spectroscopy, and scanning electron microscopy/energy dispersive X-ray spectroscopy. We focus our measurements on subterranean calcium carbonate field samples, whose biosignatures are analogous to those that might be expected on some high-interest astrobiology targets. In this work, we discuss the feasibility and advantages of using each of the aforementioned instrument techniques for the in situ search for biosignatures and present results on the autonomous characterization of biosignatures using multivariate statistical analysis techniques. Key Words: Biosignature suites-Caves-Mars-Life detection. Astrobiology 17, 1203-1218.

Electron paramagnetic resonance study of a photosynthetic microbial mat and comparison with Archean cherts

Organic radicals in artificially carbonized biomass dominated by oxygenic and non-oxygenic photosynthetic bacteria, Microcoleus chthonoplastes-like and Chloroflexus-like bacteria respectively, were studied by Electron Paramagnetic Resonance (EPR) spectroscopy. The two bacteria species were sampled in mats from a hypersaline lake. They underwent accelerated ageing by cumulative thermal treatments to induce progressive carbonization of the biological material, mimicking the natural maturation of carbonaceous material of Archean age. For thermal treatments at temperatures higher than 620 °C, a drastic increase in the EPR linewidth is observed in the carbonaceous matter from oxygenic photosynthetic bacteria and not anoxygenic photosynthetic bacteria. This selective EPR linewidth broadening reflects the presence of a catalytic element inducing formation of radical aggregates, without affecting the molecular structure or the microstructure of the organic matter, as shown by Raman spectroscopy and Transmission Electron Microscopy. For comparison, we carried out an EPR study of organic radicals in silicified carbonaceous rocks (cherts) from various localities, of different ages (0.42 to 3.5 Gyr) and having undergone various degrees of metamorphism, i.e. various degrees of natural carbonization. EPR linewidth dispersion for the most primitive samples was quite significant, pointing to a selective dipolar broadening similar to that observed for carbonized bacteria. This surprising result merits further evaluation in the light of its potential use as a marker of past bacterial metabolisms, in particular oxygenic photosynthesis, in Archean cherts.

Critical issues in the history, philosophy, and sociology of astrobiology

Fifty years after serious scientific research began in the field of exobiology, and forty years after serious historical research began on the subject of extraterrestrial life, this paper identifies and examines some of the most important issues in the history, philosophy, and sociology of what is today known as astrobiology. As in the philosophy of science in general, and in the philosophies of particular sciences, critical issues in the philosophy and sociology of astrobiology are both stimulated and illuminated by history. Among those issues are (1) epistemological issues such as the status of astrobiology as a science, the problematic nature of evidence and inference, and the limits of science; (2) metaphysical/scientific issues, including the question of defining the fundamental concepts of life, mind, intelligence, and culture in a universal context; the role of contingency and necessity in the origin of these fundamental phenomena; and whether or not the universe is in some sense fine-tuned for life and perhaps biocentric; (3) societal issues such as the theological, ethical, and worldview impacts of the discovery of microbial or intelligent life; and the question of whether the search for extraterrestrial life should be pursued at all, and with what precautions; and (4) issues related to the sociology of scientific knowledge, including the diverse attitudes and assumptions of different scientific communities and different cultures to the problem of life beyond Earth, the public "will to believe," and the formation of the discipline of astrobiology. All these overlapping issues are framed by the concept of cosmic evolution-the 13.7 billion year Master Narrative of the Universe-which may result in a physical, biological, or postbiological universe and determine the long-term destiny of humanity.