Mars once had a strong magnetic field, but now only traces remain

Scientists have long puzzled over why Mars, once protected by a global magnetic field like Earth, now shows only a magnetic imprint mostly concentrated in the planet’s southern hemisphere.

A new study from the University of Texas Institute for Geophysics (UTIG) offers a compelling explanation: perhaps Mars’ ancient magnetic field was one-sided from the start.

The experts present a model showing that a hemispheric, or one-sided, magnetic field could have emerged if Mars’ core had been entirely liquid during the time its magnetic field was active.

A mystery buried in Martian rock

Today, Mars does not have a global magnetic field. However, scientists believe that billions of years ago, the planet did possess one. It was likely generated by a planetary dynamo – a self-sustaining process powered by the movement of molten metal within Mars’s core.

While Earth’s magnetic field wraps around the entire planet, Mars left behind a lopsided magnetic signature. Most of the crustal magnetism – what’s essentially a fossil record of the planet’s magnetic past – is preserved in the southern hemisphere.

For decades, that imbalance has puzzled scientists. Chi Yan, a research associate at UTIG and lead author of the study, believes the explanation may be buried deep within Mars’ core.

“The logic here is that with no solid inner core, it’s much easier to produce hemispheric (one-sided) magnetic fields,” Yan explained. “That could have implications for Mars’ ancient dynamo and possibly how long it was able to sustain an atmosphere.”

Liquid core changes the game

Most prior models of Mars’ ancient magnetic field were based on assumptions drawn from Earth’s internal structure: a solid inner core surrounded by molten metal. But in recent years, that picture of Mars has started to shift.

In particular, NASA’s InSight mission, which studied Martian seismic activity, revealed that Mars’ core contains more light elements – such as sulfur and oxygen – than previously thought. These lighter elements lower the melting point of the core, making it more likely to be entirely molten.

“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,” said study co-author Sabine Stanley, a professor at Johns Hopkins University.

With a liquid core possibility in mind, the team ran a new set of simulations. Instead of a solid inner core, they modeled a young Mars with a fully liquid core.

The team then added a slight temperature difference between the planet’s northern and southern hemispheres. In the model, the northern mantle was made just a bit hotter than the southern mantle.

Heat flow shaped the magnetic field

The results were striking. As the simulation ran, heat from the molten core escaped preferentially through the cooler southern hemisphere.

That focused outflow of heat proved sufficient to power a magnetic dynamo – but only in the south. This simulated southern-hemisphere-only magnetic field matches what we observe today in Martian rock.

“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,” Stanley said.

These findings suggest that Mars didn’t necessarily have a planet-wide magnetic field that was later erased from its northern hemisphere – as some researchers have proposed. Instead, the field may have always been asymmetrical.

Rethinking the hemispheric mystery

The difference between Mars’ northern and southern hemispheres has long intrigued scientists. The north features relatively low, smooth terrain, while the south is higher, more rugged, and more strongly magnetized.

Some earlier theories suggested that massive asteroid impacts may have obliterated magnetic traces from the northern hemisphere. But the new study offers a different idea: what if the northern rocks were never magnetized in the first place?

“Mars is naturally interesting to look at because it’s like Earth in some ways and it’s the closest planet that we can imagine actually setting up shop on,” said UTIG planetary researcher Doug Hemingway, who was not involved in the study.

“But then, it’s got this dramatic hemispheric dichotomy where the topography, the terrain and the magnetic field of the northern hemisphere and southern hemisphere are dramatically different. Anything that gives a clue at what could account for some of that asymmetry is valuable.”

Ancient Mars and planetary habitability

Understanding how Mars lost its magnetic field is more than just a question of planetary history – it has implications for the planet’s climate and potential for life. A global magnetic field helps shield a planet’s atmosphere from solar wind, the stream of charged particles constantly blowing from the Sun.

When Mars lost its magnetic protection, the atmosphere thinned, water evaporated, and surface conditions became too harsh for life as we know it. The new research suggests that Mars’ magnetic dynamo may have been patchy from the beginning, raising the possibility that atmospheric loss began earlier than previously thought.

By revealing how a small thermal asymmetry in a planet’s mantle could produce a lopsided magnetic field, the study may also help scientists understand other planetary bodies with unusual magnetic features. This includes the planet Mercury and even some of the moons in our solar system.

As planetary scientists continue to digest InSight’s seismic data and refine their models of Mars’ interior, more findings may be on the way.

For now, this study marks a significant step toward explaining one of the Red Planet’s most persistent mysteries – and suggests that Mars’ inner life may have been every bit as strange as its landscape.

The study is published in the journal Geophysical Research Letters.

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