Habitable Planets Might Need Plate Tectonics

The second really interesting talk in the session on comparative planetology (that I wrote about here, and Pamela wrote about here) was about how plate tectonics  effects the “habitability” of a planet.

Plate Tectonics or Not: Lithospheric Stress on Terrestrial Planets and Super-Earths (O’Neill C.J., Lenardic A., Jellinek A.M.)

This presenter started by pointing out that plate tectonics are sometimes neglected in planetary science because while it happens here, it doesn’t appear to happen elsewhere. As with many other things we see on Earth and not on other planets, this brings up the question: are plate tectonics necessary for a planet to support life?

About a year ago, Gliese 581c was proclaimed to the media as “habitable” (in fact, Astronomy Cast covered some of that story in this episode). In this case, habitable is a space- and orbit-based definition. Planets need to be within what is called the “habitable zone” such that they receive optimal amounts of heat from their parent star. If they’re too close in, they get baked (like Mercury). Too far out? Frozen. Technically, we’re looking for a surface temperature between about 0 and 50 degrees Centigrade.

Now, this presenter says that more goes into making a planet habitable than just its position in space relative to its parent star. The problem with the current definition is it doesn’t take into account atmospheric composition. We can see how that affects planets even in our own solar system. Mercury’s surface temperature is 179C, but Venus – which is further from the Sun, has a surface temperature of 283C because of the runaway greenhouse effect caused by its thick atmosphere.

So, if atmosphere is connected to plate tectonics, it’s important to understand them. Plate tectonics are a complex thermomechanical system.

There are “essential ingredients” to simulate plate tectonics. The viscosity is extremely temperature-dependant – obeying the Arrhenius equation, or something similar. There needs to be a yield stress above which the plate brakes – the yield stress envelope. The plates need to be able to fault. Other important factors to take into account: strain weakening, elasticity, more complex viscosities, phase changes and so on.

If the convective stress is less than the yield strength, it leads to stagnant lid mantle convection, as seen in the Moon and Mercury. Plate tectonics is a far more rare form of convection. Venus has episodic convection. It’s sort of half way between the stagnant lid type observed on the Moon and Mercury, and plate tectonics as seen on Earth, where the convective strength is greater than the yield strength (the driving stresses are greater than the plate resistance).

So, if we scale Earth to a larger size, what happens with plate tectonics?

The increased depth of the convecting mantle means there’s a higher Rayleigh number (Ra) and a higher convective velocity. The higher gravity of a super-earth feeds into the Ra, but also increases pressure on surface faults, leading to a stronger effective lithosphere. Faults on super-earths are much, much stronger than on Earth.

All of this implies that super-earths may be stagnant. Are they habitable? Plate tectonics recycle water and carbon dioxide at subduction zones. Without plate tectonics, there’s a potent greenhouse effect (like on Venus).

Scaled-up earths are not at the same point in tectonothermal evolution as the Erath. The next steps are to map composition, temperature, tectonics parameter ranges (and mineral phases, atmospheric composition, etc). Essentially, we need an HR diagram for planets.

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