Habitability and Life, With and Without Plate Tectonics

Plate tectonics is widely considered an essential factor in the present habitability of Earth. The weathering of the continental crust and the exchange of volatiles with the interior help maintain favorable climate conditions through the carbonate-silicate cycle and provide essential nutrients, particularly phosphorous, to the biosphere. The subaerial emergence of continents and their lateral motion during continental drift have also furthered the evolution of life. However, plate tectonics may have emerged relatively late in Earth's history, after life began and proto-continents (cratons) formed, with the planet's habitability independent of its tectonic mode. Estimates of when plate  tectonics emerged range from the Archean to the early Phanerozoic, while LUCA, the last universal common ancestor, has been dated to 4.2 Ga b.p. Pre-plate tectonic modes may have included stagnant lid and squishy lid (or sluggish mobile lid) tectonics, which may thus have supported the biosphere for significant periods of geological time. During this time, the lid may have increased in mobility and plateness to eventually reach modern values. These tectonic scenarios may also apply to Earth-like exoplanets, though they may be difficult to infer. This complicates the interpretation of possible biosignatures. We may attempt to infer tectonic modes from exoplanetary observables using indirect evidence, such as atmospheric composition, albedo variations, and magnetic fields, which are even more challenging to observe.