Earth's magnetic field is generated by movement in its liquid iron outer core - a process known as a dynamo - but larger rocky worlds like super-earths might have solid or fully liquid cores that cannot produce magnetic fields in the same way.
In a paper published in Nature Astronomy, University of Rochester researchers, including Miki Nakajima, an associate professor in the Department of Earth and Environmental Sciences, report an alternative source: a deep layer of molten rock called a basal magma ocean (BMO). The findings could reshape how scientists think about planetary interiors and has implications for the habitability of planets beyond our solar system.
"A strong magnetic field is very important for life on a planet," Nakajima says, "but most of the terrestrial planets in the solar system, such as Venus and Mars, do not have them because their cores don't have the right physical conditions to generate a magnetic field. However, super-earths can produce dynamos in their core and/or magma, which can increase their planetary habitability."
Super-earths are larger than Earth but smaller than ice giants such as Neptune. Scientists believe they are primarily rocky like Earth, with solid surfaces rather than layers of gas such as those surrounding Jupiter or Saturn. Super-earths are the most common class of exoplanets detected in our galaxy, but they are curiously absent from our own solar system. Despite their name, "super-earth" refers only to size and mass, not to whether these planets resemble Earth in other ways.
Because super-earths appear so frequently, they offer a crucial window into how planets form and evolve. Many super-earths orbit within their stars' habitable zones, where liquid water could exist. By studying their compositions, atmospheres, and magnetic fields, scientists are uncovering clues about the origins of planetary systems and signs of conditions that might allow life to thrive elsewhere.
Scientists believe that shortly after Earth formed, it likely had a BMO. This layer of partially or fully molten rock at the base of a planet's mantle can affect its magnetic field, heat transport, and chemical evolution. Because super-earths are larger than Earth and experience much higher internal pressures, they are more likely to have long-lasting BMOs - making BMOs a key factor in understanding the interiors, magnetic fields, and habitability of super-earths.
To recreate the extreme pressures inside super-earths, Nakajima and her colleagues conducted laser shock experiments at URochester's Laboratory for Laser Energetics, combined with quantum mechanical simulations and planetary evolution models. They focused on studying molten rock under conditions similar to those expected in a BMO.
The researchers discovered that under those crushing pressures, deep-mantle molten rock becomes electrically conductive - enough to sustain a powerful magnetic field for billions of years. This suggests that on super-earths more than three to six times the size of Earth, BMO dynamos - driven by the movement of molten rock - could generate stronger, longer-lasting magnetic fields than those produced by Earth's core, potentially creating habitable conditions for life across the galaxy.
"This work was exciting and challenging, given that my background is primarily computational and this was my first experimental work," Nakajima says. "I'm very grateful for the support from my collaborators from various research fields to conduct this interdisciplinary work. I cannot wait for future magnetic field observations of exoplanets to test our hypothesis."
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