“The so-called ‘faint young Sun paradox’ has long been a topic of debate because its resolution bears important ramifications for the basic factors structuring climate regulation and the long-term habitability of Earth and Earth-like exoplanets.” So begins Chris Reinhard’s new paper in Nature.

Reinhard is a Principal Investigator at the Alternative Earths Team of NASA’s Astrobiology Institute, which has a goal of “unraveling the evolving redox state of Earth’s early atmosphere as a guide for exoplanet exploration” and eventual habitability.

The paradox at issue is that, three billion-ish years ago, our Sun was about 25-percent dimmer than it is today. Yet geological records suggest that the Earth was even warmer then than it is now. Most solutions to the paradox figure that there must have been high levels of greenhouse gasses in the atmosphere. Two big questions are related to that, though: which gasses, and what sort of processes put them there? Geological, chemical, and biological factors have all been suggested, with a different mix of gasses depending on the cause.

To tackle the paradox, Reinhardt and colleagues investigated how the lifeforms on the early Earth could have impacted the atmosphere. Their working hypothesis was that oxygen-producing photosynthesis had not yet evolved, although that is definitely not a given. The team used a computer model that assessed primitive forms of photosynthesis, any of which might have evolved prior to the photosynthetic processes we see today.

The model combined the different ecosystems that resulted from different putative atmospheric conditions. The results indicate that a mixture of two groups of organisms would generate enough methane to keep the early Earth toasty warm, even with a dimmer Sun. One group would metabolize hydrogen while the other scavenged electrons from iron. In fact, only that combination would work—neither type of organism alone would generate enough heat.

Oxygen was not prevalent in the Earth’s atmosphere until around two billion years ago. Currently, photosynthesis is the primary process putting oxygen into our atmosphere, but when this oxygen-producing photosynthesis evolved is not at all clear. The Great Oxygenation Event of two billion years ago could have possibly been due to the origin of photosynthesis; alternatively, it could be that photosynthesis evolved long beforehand but didn’t contribute much oxygen to the atmosphere. In that case, the Great Oxygenation Event would have been driven by tectonic activity.

Reinhardt and NASA care because, if the latter scenario is true, it can teach us about the early Earth’s oxygen cycle (things like the fact that iron is essential). Knowing how our atmosphere became so oxygenated might help us recognize potential biosignatures on other planets.

Nature Geoscience, 2017. DOI: 10.1038/s41561-017-0031-2 (About DOIs).