Crusty jigsaw

Imagine a planet composed of liquids and gases. Close to its surface, the radiation of energy into the deep chill of space results in the crystallization of material. Varying compositions result in different solids forming, the vagaries of phase changes and atomic lattice arrangement produce a huge array of forms. Gravity keeps all this crystalline stuff tightly packed, a thin sheath around the planetary sphere, cracking here and there, floating and bobbling on top of the liquid interior. This is, of course, the nature of the Earth, and the presumed nature of any substantial rocky world with radiogenic internal heating, or enough latent heat of formation combined with youth.

It's quite sobering to be reminded that our picture of how all this crusty stuff operates, how the great plates of rock shift and slide around, is still very, very new. Fifty years ago and the idea of plate tectonics was only just taking proper shape. Fast forward and we now talk about the inevitability of plate tectonics on super-Earth's - rocky planets several times the mass of ours. We care about this because it seems that active plate tectonics and volcanism play a critical role in the long-term regulation of the Earth's climate - forcing surface temperatures into the regime where liquid water can exist.

It's intriguing therefore to see that our understanding of the physics behind plate tectonics is still a matter of intense debate and study. A couple of new results bubbled up during the summer. One is the claim of a new understanding of how the Earth's crust shifts and wiggles - based on essentially the same physics that explains how objects move through viscous fluids. In this picture then the way the planetary crust moves is very much a function of that crust itself, a bit like how your bobsled run is determined to a great extent by the mass and slipperiness of your ride, not just by the ice underneath. This runs against many previous models, where deep interior processes in the liquid part of the planet effectively determine what you see up top. Another work, employing state-of-the-art computer simulation has made recent claims to tie the deep ebb and flow of the Earth's interior to the frosty bump and drift of the outer plates. Rather nicely, this grand simulacrum also suggests that precisely how all the surface cracks and gaps, the faults and fault zones, fit together plays a critical role in determining how the overall plate tectonics of the planet operate.

Just like a fiendish jigsaw, you can't see the big picture until you know how all the small pieces go together. Perhaps not surprisingly, our crusty surface is a hugely non-linear system, hard to predict ab initio. This should raise some concerns for making claims about the nature of plate tectonics on distant exoplanets, but it also indicates a possible opportunity. It may well be that there are distinct types of crustal activity that can occur on rocky planets, from the kind of plate motions that we see on the modern Earth to more fractured styles, or more global styles. At some level these will all link into climate, history, and chemistry. Detecting the presence of atmospheric gases associated with geochemical processes - such as sulfur dioxide - could actually help tell us about the arrangement of continents (or not) on these distant worlds.