For decades, planetary scientists have puzzled over a major Martian mystery: why are the strongest traces of Mars’ ancient magnetic field found almost exclusively in the planet’s southern hemisphere? A new study may finally have the answer—and it lies at the heart of the Red Planet.
Researchers led by Chi Yan of the University of Texas and co-authored by planetary scientist Sabine Stanley of Johns Hopkins University have proposed that Mars’ magnetic field may have been hemispheric in nature, covering only the southern half of the planet. Their findings, published in Geophysical Research Letters, stem from supercomputer simulations that model an early Mars with a fully molten core—a structural contrast to Earth’s partially solid, iron-rich core.
This breakthrough suggests Mars’ magnetic field wasn’t wiped away by asteroid impacts in the north, as previously believed, but may have been intrinsically one-sided due to the planet’s internal dynamics.
Why Mars’ Magnetism Matters
Mars, like Earth, once had a global magnetic field that protected its thick atmosphere from being stripped away by the solar wind. But today, only faint traces of magnetism remain, locked in Martian rocks. Intriguingly, these magnetic remnants are concentrated in the planet’s southern hemisphere.
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“This lopsided magnetic field has been a longstanding mystery,” said Chi Yan, a research associate at the University of Texas Institute for Geophysics. “Our study suggests that the field may have only existed in the southern hemisphere from the start, driven by unique internal conditions.”
Understanding this asymmetry not only helps scientists learn more about Mars’ early habitability and atmosphere retention, but also reveals how planetary magnetic fields—key for shielding life—are formed and sustained.
The Liquid Core Hypothesis
The study challenges the traditional view that Mars had a solid inner core similar to Earth’s, which typically helps generate a symmetrical magnetic field through the process known as a planetary dynamo. Instead, Yan and Stanley ran simulations using a fully liquid core, inspired by findings from NASA’s InSight mission.
InSight’s seismic data revealed that Mars’ core contains lighter elements than Earth’s, lowering its melting point and suggesting a molten state even today. “If Mars’ core is molten now,” said Stanley, “it almost certainly was molten 4 billion years ago when the magnetic field was active.”
Using these insights, researchers modeled an early Mars where heat from the molten core escaped more readily through the southern hemisphere, which had a cooler mantle. This uneven heat flow drove a dynamo that generated a magnetic field primarily in the south.
Supercomputers and Simulations
The research team used supercomputers at the Maryland Advanced Research Computing Center to simulate a liquid-core Mars a dozen times. Each simulation introduced slight variations in mantle temperature, with the northern hemisphere modeled as being slightly hotter than the southern half.
As a result, the heat from the core escaped predominantly through the south, initiating stronger convection and supporting a localized magnetic dynamo. This would naturally result in a magnetic field confined to the southern hemisphere, matching current observations of Martian crustal magnetism.
“It’s exciting to see that we can create a hemispheric magnetic field with an interior structure that matches what InSight told us Mars’ interior is like today,” said Stanley, a Bloomberg Distinguished Professor and vice provost at Johns Hopkins.
Rethinking Martian History
Traditionally, scientists believed that the northern hemisphere lost its magnetic imprint due to large asteroid impacts, which effectively reset the magnetic signal in surface rocks. But this new theory suggests a more inherent asymmetry.
Doug Hemingway, a planetary researcher at the University of Texas who was not involved in the study, finds the idea compelling. “Mars is naturally interesting to look at because it’s like Earth in some ways, and it’s the closest planet we can imagine actually setting up shop on,” Hemingway said. “But it’s also got this dramatic hemispheric dichotomy. Anything that gives a clue about that is valuable.”
The idea that Mars may never have had a northern magnetic field shifts our understanding of the planet’s evolution and atmosphere loss. Without a global magnetic shield, Mars’ atmosphere would have been more vulnerable to solar wind erosion, helping to explain its current thin and dry state.
A Model for Future Exploration
This research may influence how scientists model magnetic fields on other planets and exoplanets. It also adds a new layer to our understanding of planetary dynamos—mechanisms that generate and sustain magnetic fields in planetary interiors.
More broadly, the findings support continued investment in Mars missions and planetary science. “This work is a reminder that planetary evolution is more complex than we assume,” Stanley noted. “Every piece of the puzzle brings us closer to understanding how planets—including potentially habitable ones—form and change over time.”
Looking Ahead
The research team hopes their work will inform future missions to Mars and beyond. The fact that Mars’ magnetic field might have been intrinsically one-sided offers exciting opportunities for revisiting older assumptions and refining future planetary models.
Funded by NASA’s InSight program, this study reaffirms how space missions can transform theoretical models by providing key data about the internal structure of other worlds.
As humanity inches closer to potential crewed missions to Mars, understanding the Red Planet’s magnetic history isn’t just about solving ancient mysteries—it’s essential knowledge for our future among the stars.
