Applying Lessons from Earth’s Mass Extinction Events to the Search for Biosignatures on Exoplanets

The study of exoplanets is partially motivated by the search for life elsewhere in the universe, but where there is life, there is also death. There have been at least five mass extinctions in Earth history, which generally follow a pattern of an already-stressed system subjected to a geologically short-term shock. The most famous example is the Cretaceous-Paleogene (K-Pg) extinction event 66 million years ago, when an asteroid impact began a chain of events that ended 66% of life on land and in the oceans. The most destructive known mass extinction event is the Permian-Triassic (P-T) 251.9 million years ago, which has been linked to the violent and sustained eruption of the Siberian Traps. The volcanic activity released massive amounts of carbon dioxide, causing ocean anoxia and acidification and the loss of up to 95% of marine species on Earth and up to 70% of terrestrial species.

The amount of biomass decaying, the residence times for the post-event gases in Earth’s atmosphere, and the types and relative amounts of gases could be clues to a planet undergoing an intense extinction event. Biotic “necrosignatures” have never been investigated in the context of remote detectability, but the idea exists in the literature. We can pretend that Earth is an exoplanet being observed to leverage the in situ data from Earth to extrapolate to exoplanet mass extinctions.

We will estimate detectable signatures as a function of biomass, atmospheric scale height, observing strategy, and other parameters that will arise during the investigation. First, we will produce spectral signatures of Earth’s atmosphere before, during, and after the K-Pg and P-T mass extinction events, using previously published work to estimate atmosphere compositions without regard to decaying biomass, as benchmarks for event detectability. We will add parameters to represent the decaying biomass in various quantities to simulate different extinction-level events. We will look for differences in the quantities and stratospheric residence times of the different gases that are produced under these conditions. We will specifically look for the gases that reach the thresholds of detectability by missions like JWST. If no gases reach these minima, we will estimate the observing requirements that could potentially detect these gases, including time. Finally, we will estimate how long these gases will be detectable before the atmosphere re-equilibrates and any signature relationships between gaseous species indicative of a mass extinction event.