Did Asteroid Bombardment Make Life Possible?

From a physical point of view, Venus and Earth are the two most similar planets in the solar system, almost identical in mass and close in orbit. Yet Earth has abundant life that luxuriates in mostly temperate conditions, while the other is hot enough to melt lead. The extra sunlight Venus receives is not enough, on its own, to explain such different outcomes.

Professor Mark Jellinek claims that the Earth’s more advanced plate tectonics and magnetic field were crucial to our planet taking the path to life. "The events that define the early formation and bulk composition of Earth govern, in part, the subsequent tectonic, magnetic and climatic histories of our planet, all of which have to work together to create the Earth in which we live," says Jellinek. "It's these events that potentially differentiate Earth from other planets."

Others have raised similar ideas before. However, Jellinek goes further, attributing our tectonic activity to the relative shortage of crustal uranium, thorium and potassium, which produce heat through radioactive decay. Plate tectonics is thought to require a large temperature gradient between crust and core. 

Jellinek attributes this deficiency to “impact erosion,” the removal of crust when large asteroids hit the Earth and threw material into space. A particularly large such event is thought to have created the Moon. In place of the crust we lost, we ended up with a crust containing a lot of material from chondrite meteorites.

Jellinek concedes this explanation contradicts what he calls the “canonical model of silicate Earth evolution,” but says this canonical model is contradicted by the fact that Earth has a lower ratio of Neodymium-142 to Nd-142 than meteorites. This ratio indicates the Earth once had a crust rich in Nd-142. Jellinek proposes this early crust also had more radioactive materials before being lost to space early on, while surviving on Venus. 

Jellinek thinks Venus's greater crustal radioactivity created huge swings in volcanism over billions of years, making life impossible.

"We played out this impact erosion story forward in time and we were able to show that the effect of the conditions governing the initial composition of a planet can have profound consequences for its evolution,” says Jellinek.

Disturbingly for the quest for life beyond the solar system, Jellinek thinks Venus's story is likely to be more common than Earth's. “Earth could have easily ended up like present day Venus. A key difference that can tip the balance, however, may be differing extents of impact erosion. It's a very special set of circumstances that make Earth,” he says.