Of Volcanoes and Fermi

Solution 41 in Where is Everybody? is entitled “Earth’s system of plate tectonics is unique”. Why should volcanism and plate tectonics have anything to do with life? Well, I don’t plan to go into detail here – you can always read the book – but there are various ways in which plate tectonics might play a role in the emergence of life and the long-term viability of a planet. In Earth’s case, for example, volcanoes vomit lots of CO2:  this is a greenhouse gas, which could have kept the surface of our young planet warm enough to allow water to remain in the liquid phase. As Earth aged, volcanism and plate tectonics played a key role in the carbon cycle: plate tectonics captures CO2 and takes it into the planet’s interior while volcanism releases CO2 back into the atmosphere. This carbon cycle, so it’s believed, plays a role in stabilising Earth’s climate – and a stable climate in turn may have been necessary for the development of complex life. And Earth’s magnetic field, which is driven by a rotating liquid metallic core, protects the atmosphere from high-energy cosmic radiation.

I doubt that in a universe as large as ours there is only one planet, Earth, that possesses plate tectonics. Recent work, however, suggests that the phenomenon may not be common – at least not on “super Earths” (rocky planets with a mass between 2-10 times that of Earth).

A team led by Vlada Stamenkovic has studied how temperatures within a super Earth are likely to change over time. There are some uncertainties in the work, but in the scenarios that Stamenkovic’s team studied it turned out that super Earths cool slowly: plate tectonics is unlikely to occur and a slow planetary-core formation (or even no core formation at all) means that magnetic fields are likely to be absent.

I’ve heard lots of people say recently that rocky super Earths, orbiting in the habitable zone, might be suitable places for life to originate and prosper. I think it much more likely that those planets are lifeless.

For full details of the work, see: Stamenkovic, V., Noak, L., Breuer, D. and Spohn, T. (2012) The influence of pressure-dependent viscosity on the thermal evolution of super-Earths. Astrophysical Journal, 748: 41.