Recent measurements from NASA’s InSight mission show that Mars’ core is less dense than planetary scientists previously believed. This indicates that Mars probably never developed a solid inner core in the earliest times in its history. In a new study published on the journal Geophysical Research Letters, researchers from the University of Texas and elsewhere aimed to understand the effects of this lack of a solid inner core.
Computer simulation of a one-sided magnetic field on early Mars. Image credit: Ankit Barik / Johns Hopkins University.
“Like Earth, Mars once had a strong magnetic field that shielded its thick atmosphere from the solar wind,” said lead author Dr. Chi Yan from the University of Texas and colleagues.
“But now only the magnetic imprint remains. What’s long baffled scientists, though, is why this imprint appears most strongly in the southern half of the Red Planet.”
The team’s new study could help explain the one-sided imprint. It presents evidence that the planet’s magnetic field covered only its southern half.
“The resulting lopsided magnetic field would match the imprint we see today,” Dr. Yan said.
“It would also make Mars’ magnetic field different from Earth’s, which covers the entire globe.”
“The one-sided magnetic field could arise if Mars’ inner core was liquid.”
“The logic here is that with no solid inner core, it’s much easier to produce hemispheric (one-sided) magnetic fields.”
“That could have implications for Mars’ ancient dynamo and possibly how long it was able to sustain an atmosphere.”
In the study, the researchers used a computer simulation to model this scenario.
Until now, most studies of early Mars had relied on magnetic field models that gave the Red Planet an Earth-like inner core that’s solid and surrounded by molten iron.
The scientists were inspired to try simulating a fully liquid core after InSight found that Mars’ core was made of lighter elements than expected.
“That means the core’s melting temperature is different from Earth’s and therefore quite possibly molten,” said Johns Hopkins University’s Professor Sabine Stanley.
“If Mars’ core is molten now, it almost certainly would have been molten 4 billion years ago when Mars’ magnetic field is known to have been active.”
To test the idea, the authors prepared simulations of early Mars with a liquid core and ran them a dozen times on supercomputers.
With each run they made the planet’s northern half of the mantle a little hotter than the south.
Eventually, the temperature difference between the hotter mantle in the north and the cooler mantle in the south led to the heat escaping from the core to be released only at the southern end of the planet.
Channeled in such a way, the escaping heat was sufficiently vigorous to drive a dynamo and generate a strong magnetic field focused in the southern hemisphere.
A planetary dynamo is a self-sustaining mechanism that generates a magnetic field, typically through movement in the molten metallic core.
“We had no idea if it was going to explain the magnetic field, so it’s exciting to see that we can create a (single) hemispheric magnetic field with an interior structure that matches what InSight told us Mars’ interior is like today,” Professor Stanley said.
The finding offers a compelling alternative theory to a common assumption that involves asteroid impacts obliterating evidence of a planet-wide magnetic field in northern hemisphere rocks.
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C. Yan et al. 2025. Mars’ Hemispheric Magnetic Field From a Full-Sphere Dynamo. Geophysical Research Letters 52 (3): e2024GL113926; doi: 10.1029/2024GL113926
