Convection and Seismology
Stagnant Lid Convection
Unlike Earth with active plate tectonics, other rocky planets in our solar system are in another tectonic regime. Mars and Venus currently have no active moving plates on their surface, so we describe them as being in a stagnant lid regime. Stagnant lid convection occurs when the surface is locked as a single plate, and material is not downwelling (or subducting) into the mantle below. The mantle still rises and cools at the surface, due to convection, but the upper plate is not dragged back into the mantle, so the same lithospheric recycling process does not occur.
Convection in an episodic regime
The convection regime on Venus is slightly different from Mars. While it is currently in a stagnant lid regime (where the surface is locked and does not subduct into the mantle), it is thought that Venus had a large resurfacing event in the past. During this resurfacing event, the boundary layer devel- oped an instability and the entire surface was subducted into the mantle, recycling the lithosphere and creating new surface all over the planet. This helps explains the young age of Venus’s surface, given that the planet that formed at the same time as Earth.
Seismic tomography has provided much of our understanding of the in- ternal structure of the Earth. The approach measures travel times of seismic waves from an earthquake source to a receiver at the surface. There are two types of waves we can measure, compressional waves (P-waves) and shear waves (S-waves). These waves travel through the Earth at different speeds depending on the temperature, density and composition of the material it is travelling through. The shear waves (S-waves) cannot travel through a liquid, so they cannot travel through the hot liquid outer core of the Earth. The speed of both types of waves is compared to a mean speed at different depths in the Earth. Lower than average speeds suggest hotter temperatures and weaker rocks, while higher than average speeds suggest cold, stronger parts of the mantle. If the mantle rock is hotter, its density will be lower and it will be more likely to be rising, while the colder rocks will be more dense and likely to be sinking (or subducting) into the mantle.