Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment.

We describe how environmental context can help determine whether oxygen (O) detected in extrasolar planetary observations is more likely to have a biological source. Here we provide an in-depth, interdisciplinary example of O biosignature identification and observation, which serves as the prototype for the development of a general framework for biosignature assessment. Photosynthetically generated O is a potentially strong biosignature, and at high abundance, it was originally thought to be an unambiguous indicator for life.

The Habitability of Proxima Centauri b: Environmental States and Observational Discriminants.

Proxima Centauri b provides an unprecedented opportunity to understand the evolution and nature of terrestrial planets orbiting M dwarfs. Although Proxima Cen b orbits within its star's habitable zone, multiple plausible evolutionary paths could have generated different environments that may or may not be habitable. Here, we use 1-D coupled climate-photochemical models to generate self-consistent atmospheres for several evolutionary scenarios, including high-O, high-CO, and more Earth-like atmospheres, with both oxic and anoxic compositions.

Organic Haze as a Biosignature in Anoxic Earth-like Atmospheres.

Early Earth may have hosted a biologically mediated global organic haze during the Archean eon (3.8-2.5 billion years ago).

The Pale Orange Dot: The Spectrum and Habitability of Hazy Archean Earth.

Recognizing whether a planet can support life is a primary goal of future exoplanet spectral characterization missions, but past research on habitability assessment has largely ignored the vastly different conditions that have existed in our planet's long habitable history. This study presents simulations of a habitable yet dramatically different phase of Earth's history, when the atmosphere contained a Titan-like, organic-rich haze. Prior work has claimed a haze-rich Archean Earth (3.

Carbon Monoxide and the Potential for Prebiotic Chemistry on Habitable Planets around Main Sequence M Stars.

Lifeless planets with CO atmospheres produce CO by CO photolysis. On planets around M dwarfs, CO is a long-lived atmospheric compound, as long as UV emission due to the star's chromospheric activity lasts, and the sink of CO and O in seawater is small compared to its atmospheric production. Atmospheres containing reduced compounds, like CO, may undergo further energetic and chemical processing to give rise to organic compounds of potential importance for the origin of life.

IDENTIFYING PLANETARY BIOSIGNATURE IMPOSTORS: SPECTRAL FEATURES OF CO AND O RESULTING FROM ABIOTIC O/O PRODUCTION.

O and O have been long considered the most robust individual biosignature gases in a planetary atmosphere, yet multiple mechanisms that may produce them in the absence of life have been described. However, these abiotic planetary mechanisms modify the environment in potentially identifiable ways. Here we briefly discuss two of the most detectable spectral discriminants for abiotic O/O: CO and O.

C and N fractionation of CH/N mixtures during photochemical aerosol formation: Relevance to Titan.

The ratios of the stable isotopes that comprise each chemical species in Titan's atmosphere provide critical information towards understanding the processes taking place within its modern and ancient atmosphere. Several stable isotope pairs, including C/C and N/N, have been measured or probed spectroscopically by Cassini-borne instruments, space telescopes, or through ground-based observations. Current attempts to model the observed isotope ratios incorporate fractionation resulting from atmospheric diffusion, hydrodynamic escape, and primary photochemical processes.

Using biogenic sulfur gases as remotely detectable biosignatures on anoxic planets.

We used one-dimensional photochemical and radiative transfer models to study the potential of organic sulfur compounds (CS(2), OCS, CH(3)SH, CH(3)SCH(3), and CH(3)S(2)CH(3)) to act as remotely detectable biosignatures in anoxic exoplanetary atmospheres. Concentrations of organic sulfur gases were predicted for various biogenic sulfur fluxes into anoxic atmospheres and were found to increase with decreasing UV fluxes. Dimethyl sulfide (CH(3)SCH(3), or DMS) and dimethyl disulfide (CH(3)S(2)CH(3), or DMDS) concentrations could increase to remotely detectable levels, but only in cases of extremely low UV fluxes, which may occur in the habitable zone of an inactive M dwarf.

A revised, hazy methane greenhouse for the Archean Earth.

Geological and biological evidence suggests that Earth was warm during most of its early history, despite the fainter young Sun. Upper bounds on the atmospheric CO2 concentration in the Late Archean/Paleoproterozoic (2.8-2.