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.

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.