Radioactive elements may be crucial for the habitability of rocky planets

The amount of long-lived radioactive elements incorporated into a rocky planet as it forms may be a crucial factor in determining its future habitability, according to a new study by an interdisciplinary team of scientists at UC Santa Cruz.

This is because internal warming due to radioactive decay of heavy elements thorium and uranium drives plate tectonics and may be necessary for the planet to generate a magnetic field. The Earth’s magnetic field protects the planet from solar winds and cosmic rays.

Convection in the molten metal core of the Earth creates an internal dynamo (the “geodynamo”) that generates the planet’s magnetic field. Earth’s supply of radioactive elements provides more than enough internal warming to generate a persistent geodynamo, according to Francis Nimmo, professor of earth and planetary sciences at UC Santa Cruz and first author of a paper on the new discoveries, published 10 November in Letters from astrophysics journals.

“What we understand is that different planets accumulate different amounts of these radioactive elements which ultimately fuel geological activity and the magnetic field,” explained Nimmo. “So we took a model of the Earth and increased and decreased the amount of internal radiogenic heat production to see what happens.”

What they found is that if the radiogenic warming is greater than that of the Earth, the planet cannot permanently support a dynamo, as the Earth did. This happens because most of the thorium and uranium end up in the mantle and too much heat in the mantle acts as an insulator, preventing the molten core from losing heat fast enough to generate the convective motions that produce the magnetic field.

With more radiogenic internal warming, the planet also has much greater volcanic activity, which could produce frequent mass extinction events. On the other hand, too little radioactive heat results in the absence of volcanism and a geologically “dead” planet.

“By simply changing this variable, you go through these different scenarios, from geologically dead to Earth-like to extremely volcanic without dynamos,” Nimmo said, adding that these results warrant more detailed studies.

“Now that we see the important implications of varying the amount of radiogenic heating, the simplified model we used should be checked with more detailed calculations,” he said.

Habitability

A planetary dynamo has been linked to habitability in several ways, according to Natalie Batalha, a professor of astronomy and astrophysics whose Astrobiology Initiative at UC Santa Cruz sparked the interdisciplinary collaboration that led to this article.

“It has long been hypothesized that internal warming results in plate tectonics, which creates the carbon cycle and geological activity such as volcanism, which produces an atmosphere,” Batalha explained. “And the ability to hold an atmosphere is related to the magnetic field, which is also driven by internal heating.”

Co-author Joel Primack, professor emeritus of physics, explained that stellar winds, which are fast-moving streams of material ejected from stars, can constantly erode a planet’s atmosphere if it has no magnetic field.

“The lack of a magnetic field is apparently part of the reason, along with its low gravity, why Mars has a very thin atmosphere,” he said. “It had a denser atmosphere and for a while it had surface water. Without the protection of a magnetic field, a lot more radiation passes through and the planet’s surface becomes even less habitable.”

Primack noted that the heavy elements crucial for radiogenic heating are created during neutron star mergers, which are extremely rare events. The creation of these so-called r-process elements during neutron star mergers was the focus of research by co-author Enrico Ramirez-Ruiz, professor of astronomy and astrophysics.

“We would expect considerable variability in the amount of these elements incorporated into stars and planets, because it depends on how close the matter that formed them was to where these rare events occurred in the galaxy,” Primack said.

Astronomers can use spectroscopy to measure the abundance of different elements in stars, and the compositions of planets are expected to be similar to those of the stars they orbit. The rare earth element europium, which is easily observed in stellar spectra, is created by the same process that makes the two longest-lived radioactive elements, thorium and uranium, so europium can be used as a tracer to study variability of those elements in the stars and planets of our galaxy.

Natural range

Astronomers have obtained measurements of europium for many stars in our galactic neighborhood. Nimmo was able to use these measurements to establish a natural range of inputs for his X-ray heating models. The composition of the sun is in the middle of that range. According to Primack, many stars have half as much europium as the sun’s magnesium, and many stars have up to twice as much as the sun.

The importance and variability of radiogenic warming opens up many new questions for astrobiologists, Batalha said.

“It’s a complex story, because both extremes have implications for habitability. You need enough X-ray heating to withstand plate tectonics, but not so much to shut down the magnetic dynamo,” he said. “Ultimately, we are looking for the most likely abodes of life. The abundance of uranium and thorium appear to be key factors, perhaps even another dimension to define a Goldilocks planet.”

By using europium measurements of their stars to identify planetary systems with varying amounts of radiogenic elements, astronomers can start looking for differences between planets in those systems, Nimmo said, especially once the James Webb Space Telescope has been deployed. “The James Webb Space Telescope will be a powerful tool for characterizing the atmospheres of exoplanets,” he said.

In addition to Nimmo, Primack, and Ramirez-Ruiz, co-authors of the paper include Sandra Faber, emeritus professor of astronomy and astrophysics, and postdoctoral scholar Mohammadtaher Safarzadeh.