A new study proposes that deep layers of molten rock, known as basal magma oceans (BMOs), within super-Earth exoplanets could generate strong magnetic fields. These fields are considered vital for protecting planetary atmospheres and fostering conditions conducive to life, offering an alternative mechanism to the iron-core dynamos observed in planets like Earth.
Magnetic Fields and Planetary Habitability
Planetary magnetic fields play a critical role in preserving atmospheres and shielding planets from harmful cosmic radiation and high-energy particles. On Earth, the magnetic field is generated by the movement of liquid iron in its outer core. However, larger rocky exoplanets, termed super-Earths, might possess solid or entirely liquid cores that may not produce magnetic fields through a similar process.
Super-Earths are a class of exoplanets larger than Earth but smaller than ice giants, believed to be primarily rocky. They are frequently detected in the galaxy, with many orbiting within their stars' habitable zones, where liquid water could potentially exist.
Basal Magma Oceans as Dynamo Sources
Researchers at the University of Rochester, led by Associate Professor Miki Nakajima, investigated an alternative source for magnetic field generation: a basal magma ocean (BMO). A BMO is a deep layer of molten rock located at the bottom of a planet's mantle. The study, published in Nature Astronomy, suggests that if these molten rock layers become sufficiently electrically conductive under extreme pressure, they could sustain a magnetic field.
According to Nakajima, super-Earths have the potential to generate dynamos in their core and/or magma, which could enhance their habitability. While Earth likely had a BMO briefly after its formation, super-Earths, owing to their greater mass and higher internal pressures, could potentially retain these molten regions for billions of years.
Experimental Simulation and Findings
To simulate the extreme internal pressures characteristic of super-Earths, Nakajima's team conducted laser shock experiments at the University of Rochester's Laboratory for Laser Energetics. These experiments were complemented by quantum mechanical simulations and planetary evolution models, focusing on molten rock, specifically the mineral (Mg,Fe)O, commonly found in planetary mantles.
The research revealed that under super-Earth-like pressures, the deep-mantle molten rock becomes electrically conductive enough to support a magnetic dynamo. This implies that on super-Earths ranging from three to six times Earth's size, BMO-driven dynamos, powered by the movement of molten rock, could generate magnetic fields. The study suggests these fields could be comparable to or even stronger and more enduring than Earth's core-generated field.
Implications for Exoplanet Habitability
This mechanism expands the range of conditions under which a planet might be considered habitable, particularly for those with cores that may be inactive or unsuitable for generating a magnetic field. Such BMO-driven magnetic fields could provide long-term protection against solar wind and cosmic radiation, influencing a planet's ability to retain its atmosphere over cosmic timescales.
Super-Earths with active basal magma oceans could therefore represent significant candidates in the search for extraterrestrial life. Researchers anticipate that future magnetic field observations of exoplanets could help verify this hypothesis and further elucidate the role of deep molten rock in shaping exoplanet habitability.