Liquid water, it turns out, is no longer the only criteria considered important for the habitability of a world. Scientists at the University of California Santa Cruz (UCSC) have found that the amount of radioactive elements, particularly the longer-lived ones, incorporated into rocky planets may be a crucial factor in the potential for life.
Radioactive decay of heavy elements such as thorium and uranium in the core of Earth drives our plate tectonics and might even contribute to the generation of our magnetic field. That magnetic field helps protect us from dangerous solar and cosmic rays, which helps life survive on the surface. The new findings, published this week in Astrophysical Journal Letters, show that the supply of radioactive elements in the Earth’s core generates more than enough heat to drive the geodynamo that creates our magnetic field.
First author Francis Nimmo explained: What we realized was that different planets accumulate different amounts of these radioactive elements that ultimately power geological activity and the magnetic field. So we took a model of the Earth and dialed the amount of internal radiogenic heat production up and down to see what happens.
It turns out that if a planet has too much radiogenic heating, it cannot sustain a dynamo because the heavier elements end up in the mantle, and a hotter mantle becomes an insulator, preventing the core from losing enough heat quickly enough to generate the convection necessary for a magnetic field. You also end up turning the volcanism to full blast and cause more extinction events.
On the other hand, if the planet doesn’t have enough radiogenic heating, the planet has no volcanism and is geologically dead. Nimmo goes on to note: Just by changing this one variable, you sweep through these different scenarios, from geologically dead to Earth-like to extremely volcanic without a dynamo. Now that we see the important implications of varying the amount of radiogenic heating, the simplified model that we used should be checked by more detailed calculations.
So how does a geodynamo add to habitability? Scientist and exoplanet expert Natalie Batalha explains: It has long been speculated that internal heating drives plate tectonics, which creates carbon cycling and geological activity like volcanism, which produces an atmosphere. And the ability to retain an atmosphere is related to the magnetic field, which is also driven by internal heating.
Once again, we add more pieces to the puzzle of what drives life.
“Radiogenic Heating and Its Influence on Rocky Planet Dynamos and Habitability,” Francis Nimmo et al., 2020 Nov. 10, Astrophysical Journal Letters