The erosion of large primary atmospheres typically leaves behind substantial secondary atmospheres on temperate rocky planets.

Exoplanet exploration has revealed that many-perhaps most-terrestrial exoplanets formed with substantial H-rich envelopes, seemingly in contrast to solar system terrestrials, for which there is scant evidence of long-lived primary atmospheres. It is not known how a long-lived primary atmosphere might affect the subsequent habitability prospects of terrestrial exoplanets. Here, we present a new, self-consistent evolutionary model of the transition from primary to secondary atmospheres.

The case and context for atmospheric methane as an exoplanet biosignature.

Methane has been proposed as an exoplanet biosignature. Imminent observations with the James Webb Space Telescope may enable methane detections on potentially habitable exoplanets, so it is essential to assess in what planetary contexts methane is a compelling biosignature. Methane’s short photochemical lifetime in terrestrial planet atmospheres implies that abundant methane requires large replenishment fluxes.

Carbonate-silicate cycle predictions of Earth-like planetary climates and testing the habitable zone concept.

In the conventional habitable zone (HZ) concept, a CO-HO greenhouse maintains surface liquid water. Through the water-mediated carbonate-silicate weathering cycle, atmospheric CO partial pressure (pCO) responds to changes in surface temperature, stabilizing the climate over geologic timescales. We show that this weathering feedback ought to produce a log-linear relationship between pCO and incident flux on Earth-like planets in the HZ.

A Maximum Subsurface Biomass on Mars from Untapped Free Energy: CO and H as Potential Antibiosignatures.

Whether extant life exists in the martian subsurface is an open question. High concentrations of photochemically produced CO and H in the otherwise oxidizing martian atmosphere represent untapped sources of biologically useful free energy. These out-of-equilibrium species diffuse into the regolith, so subsurface microbes could use them as a source of energy and carbon.

Exoplanet Biosignatures: A Framework for Their Assessment.

Finding life on exoplanets from telescopic observations is an ultimate goal of exoplanet science. Life produces gases and other substances, such as pigments, which can have distinct spectral or photometric signatures. Whether or not life is found with future data must be expressed with probabilities, requiring a framework of biosignature assessment.

Constraining the climate and ocean pH of the early Earth with a geological carbon cycle model.

The early Earth's environment is controversial. Climatic estimates range from hot to glacial, and inferred marine pH spans strongly alkaline to acidic. Better understanding of early climate and ocean chemistry would improve our knowledge of the origin of life and its coevolution with the environment.

Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life.

Chemical disequilibrium in planetary atmospheres has been proposed as a generalized method for detecting life on exoplanets through remote spectroscopy. Among solar system planets with substantial atmospheres, the modern Earth has the largest thermodynamic chemical disequilibrium due to the presence of life. However, how this disequilibrium changed over time and, in particular, the biogenic disequilibria maintained in the anoxic Archean or less oxic Proterozoic eons are unknown.

Constraining climate sensitivity and continental versus seafloor weathering using an inverse geological carbon cycle model.

The relative influences of tectonics, continental weathering and seafloor weathering in controlling the geological carbon cycle are unknown. Here we develop a new carbon cycle model that explicitly captures the kinetics of seafloor weathering to investigate carbon fluxes and the evolution of atmospheric CO and ocean pH since 100 Myr ago. We compare model outputs to proxy data, and rigorously constrain model parameters using Bayesian inverse methods.

On Detecting Biospheres from Chemical Thermodynamic Disequilibrium in Planetary Atmospheres.

Atmospheric chemical disequilibrium has been proposed as a method for detecting extraterrestrial biospheres from exoplanet observations. Chemical disequilibrium is potentially a generalized biosignature since it makes no assumptions about particular biogenic gases or metabolisms. Here, we present the first rigorous calculations of the thermodynamic chemical disequilibrium in Solar System atmospheres, in which we quantify the available Gibbs energy: the Gibbs free energy of an observed atmosphere minus that of atmospheric gases reacted to equilibrium.

Transient Sulfate Aerosols as a Signature of Exoplanet Volcanism.

Geological activity is thought to be important for the origin of life and for maintaining planetary habitability. We show that transient sulfate aerosols could be a signature of exoplanet volcanism and therefore of a geologically active world. A detection of transient aerosols, if linked to volcanism, could thus aid in habitability evaluations of the exoplanet.