The Effects of M-Dwarf Age-Dependent Ultraviolet Activity on Detecting and Interpreting Biosignatures

In the coming decades, next generation space- and ground-based observatories may be capable of characterizing the atmospheres of terrestrial exoplanets and initiating the search for biosignatures. Due to their size, abundance, and orbital geometries, M-dwarf stars will arguably be the best host candidates for characterizing resident terrestrial exoplanets. However, the ultraviolet (UV) flux from a M-dwarf star — which drives atmospheric photochemistry — is distinct in spectral shape from our Sun’s and influenced by stellar activity that weakens with age. Ultraviolet flux is known to drive planetary photochemical processes that impact atmospheric composition and, therefore, habitability and biosignatures. For example, methane has been shown to have a longer lifetime in terrestrial atmospheres around M-dwarfs due to weakened Near-UV (NUV) flux that produces destructive radicals. Similarly, ozone production is stronger in M-dwarf planetary atmospheres due to higher Far-UV (FUV) flux that destroys carbon dioxide and molecular oxygen. However, as an M-dwarf grows older and its rotation rate slows, its total UV flux decreases and the ratio of FUV to NUV flux changes. Therefore, characterization of this stellar-age-dependent UV flux is a necessary component of biosignature assessment and a useful target-selection metric for future habitability surveys. To explore the evolution Earth-like atmospheric compositions due to this time-variable UV flux from M-dwarf stars, we used a coupled photochemical-climate model to simulate two atmospheres — the Pre-Industrial Earth and Archean Earth — that were climatically and photochemically consistent with model spectra of M4V and M8V stars at ages between 650 Myr and 5 Gyr. We found that methane was more abundant, often by several orders of magnitude, in both atmospheres for all host-star ages. Most notably, for planets orbiting the youngest (650 Myr) stars, carbon dioxide photolysis in the Archean Earth atmosphere produced a build-up of ozone that mimicked previously suggested biosignatures that could potentially be detected in the UV with a future space-based IR/O/UV telescope. Our results also identified two stellar spectral regions, 118–150nm and 165–200nm, that are key to the chemistry of our atmospheres and are critical for the interpretation of potential biosignatures in terrestrial exoplanet atmospheres around M-dwarf stars.