Cool(ing) World

NASA is listening to Mars, and finally getting a picture of its interior:

Its crust is split into two or three layers of volcanic chocolate. The mantle below has a surprisingly sizable and rigid toffee-like filling. And the planet’s core is surprisingly light — less nougaty center, more syrupy heart.

I also love this simple explanation of plate tectonics:

A planet’s major volcanic and tectonic activity is essentially powered by the movement of heat from a planet’s inner sanctum to its outermost shell. 

It’s just cooling off! That’s how planets cools off. This swirling detritus around a sun coalesces and compacts into an “infernal engine” at the bottom of a gravity well. The heavy stuff sinks but not all the way, the light stuff floats but not all the way. And all that heat radiates back up and bakes the surface dry. Or, if there’s liquid there, the surface churns atop the rolling boil of the mantle.