In a new study published in the journal Earth and Planetary Science Letters, a team of researchers from the Scripps Institution of Oceanography at the University of California San Diego provides new estimates for the thermodynamics of magnetic field generation within the liquid portion of the early Earth’s mantle and show how long that field was available. Their research coincides with two other studies that expand on the team’s concept and apply it in new ways.
The internal sctructure of Earth. Image credit: NASA / JPL / Université Paris Diderot / Institut de Physique du Globe de Paris.
“Currently we have no grand unifying theory for how Earth has evolved thermally. We don’t have this conceptual framework for understanding the planet’s evolution. This is one viable hypothesis,” said Dr. Dave Stegman, co-author of the study.
It has been a bedrock tenet of geophysics that Earth’s liquid outer core has always been the source of the dynamo that generates its magnetic field.
Magnetic fields form on Earth and other planets that have liquid, metallic cores, rotate rapidly, and experience conditions that make the convection of heat possible.
In 2007, researchers in France proposed a radical departure from the long-held assumption that the Earth’s mantle has remained entirely solid since the very beginnings of the planet.
They argued that during the first half of the planet’s 4.5-billion-year history, the bottom third of Earth’s mantle would have had to have been molten, which they call the ‘basal magma ocean.’
Six years later, Dr. Stegman and his colleague, Dr. Leah Ziegler, expanded upon that idea, publishing the first work showing how this once-liquid portion of the lower mantle, rather than the core, could have exceeded the thresholds needed to create Earth’s magnetic field during that time.
The Earth’s mantle is made of silicate material that is normally a very poor electrical conductor.
Therefore, even if the lowermost mantle were liquid for billions of years, rapid fluid motions inside it wouldn’t produce large electrical currents needed for magnetic field generation, similar to how Earth’s dynamo currently works in the core.
The researchers asserted the liquid silicate might actually be more electrically conductive than what was generally believed.
“Ziegler and Stegman first proposed the idea of a silicate dynamo for the early Earth,” said Dr. Lars Stixrude, a geophysicist at the University of California, Los Angeles and University College London and lead author of a paper published in the journal Nature Communications.
“The idea was met with skepticism because their early results showed that a silicate dynamo was only possible if the electrical conductivity of silicate liquid was remarkably high, much higher than had been measured in silicate liquids at low pressure and temperature.”
Dr. Stixrude and colleagues used quantum-mechanical computations to predict the conductivity of silicate liquid at basal magma ocean conditions for the first time.
“We found very large values of the electrical conductivity, large enough to sustain a silicate dynamo,” he said.
In another study, published in the journal Geophysical Research Letters, Dr. Joseph O’Rourke from the School of Earth and Space Exploration at Arizona State University applied Dr. Stegman team’s concept to consider whether it’s possible that Venus might have at one point generated a magnetic field within a molten mantle.
These new studies are signs that the premise is starting to take hold, but is still far from being widely accepted.
“No one is going to believe it until they do it themselves and now two other highly esteemed scientists have done it themselves,” Dr. Stegman said.
“The pioneering studies of Dave Stegman and his collaborators directly inspired my work on Venus,” Dr. O’Rourke said.
“Their recent paper helps answer a question that vexed scientists for many years: How has Earth’s magnetic field survived for billions of years?”
If Dr. Stegman’s premise is correct, it would mean the mantle could have provided the young planet’s first magnetic shield against cosmic radiation. It could also underpin studies of how tectonics evolved on the planet later in history.
“If the magnetic field was generated in the molten lower mantle above the core, then Earth had protection from the very beginning and that might have made life on Earth possible sooner,” Dr. Stegman said.
“Ultimately, our papers are complementary because they demonstrate that basal magma oceans are important to the evolution of terrestrial planets. Earth’s basal magma ocean has solidified but was key to the longevity of our magnetic field,” Dr. O’Rourke said.
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Nicolas A. Blanc et al. 2020. Thermal and magnetic evolution of a crystallizing basal magma ocean in Earth’s mantle. Earth and Planetary Science Letters 534: 116085; doi: 10.1016/j.epsl.2020.116085
L. Stixrude et al. 2020. A silicate dynamo in the early Earth. Nat Commun 11, 935; doi: 10.1038/s41467-020-14773-4
J.G. O’Rourke. 2020. Venus: A Thick Basal Magma Ocean May Exist Today. Geophysical Research Letters 47 (4); doi: 10.1029/2019GL086126
