The magnetic field was destined to fade the moment it formed.
Four billion years ago, Mars held oceans beneath the shelter of a magnetic field — a fleeting grace that planetary science has long struggled to explain. Researchers at the University of Tokyo have now recreated the conditions of the ancient Martian core in miniature, discovering that its iron-rich interior briefly separated into two distinct liquids, generating the convective currents that powered the field before inevitably stilling into silence. The oceans that followed that silence did not drain away but vanished upward, stripped into space by the solar wind. In understanding why Mars lost its shield, we are also reminded, with quiet reassurance, that Earth's own remains steadfast for at least a billion years to come.
- Mars once harbored oceans, but its magnetic field — the invisible barrier that made liquid water possible — collapsed with a finality that left the planet barren for billions of years.
- The mystery of why that field vanished so quickly has haunted planetary scientists, because a world that could lose its shield so swiftly raises uncomfortable questions about planetary stability everywhere.
- University of Tokyo researchers crushed iron, sulphur, and hydrogen between diamonds under laser-heated, extreme pressure, watching in real time as the molten mixture split into two chemically distinct liquids — a phenomenon never before observed at such conditions.
- That liquid separation temporarily drove convection currents powerful enough to generate a magnetic field, but once the layers fully settled and stratified, the churning stopped, the field collapsed, and the solar wind claimed the atmosphere and oceans within a geologically brief window.
- NASA's InSight probe data on the Martian core may now confirm whether the layered structure the lab predicts actually exists, potentially closing the final chapter on Mars's watery past.
- Earth's magnetic field, far more robustly sustained, offers at least a billion years of continued protection — a finding that transforms this story of planetary loss into one of measured terrestrial reassurance.
Mars was not always the rust-colored desert we observe today. Billions of years ago, it held oceans — their memory preserved in ice caps, dried riverbeds, and geological scars etched across the surface. What made those oceans possible was a magnetic field, an invisible shield against the solar wind. Then the shield failed, the oceans vanished, and planetary scientists were left with a haunting question: why did it collapse so quickly?
Professor Kei Hirose and PhD student Shunpei Yokoo at the University of Tokyo set out to answer that question by reconstructing the Martian core in miniature. They compressed a mixture of iron, sulphur, and hydrogen between two diamonds and heated it with an infrared laser, recreating the pressure and temperature conditions that existed deep within Mars roughly four billion years ago. X-ray and electron beams allowed them to observe the material as it melted under these extremes.
What emerged was both unexpected and clarifying. The uniform mixture separated into two distinct liquids — one rich in iron and hydrogen, the other rich in iron and sulphur. The lighter hydrogen-rich liquid rose above the denser sulphur-rich layer, generating convection currents of the kind that power Earth's magnetic field today. For a time, those currents gave Mars its shield, holding hydrogen in the atmosphere and allowing water to persist on the surface.
But the separation was also the field's undoing. Once the two liquids fully stratified, convection ceased. Without it, the magnetic field collapsed. The solar wind stripped away the unprotected atmosphere, water vapor broke apart, and the oceans evaporated into space — all within a relatively brief window of planetary history.
The research carries implications beyond Mars. Data from NASA's InSight probe, which has already revealed the Martian core to be larger and less dense than expected, may confirm whether that layered structure actually exists. And for those concerned about Earth's own magnetic future, Hirose offered a grounding reassurance: our field is far more durable than Mars's ever was, and even under the most pessimistic scenarios, it would take at least a billion years to disappear.
Mars was not always the barren, rust-colored world we see today. Billions of years ago, it held oceans. Evidence is written across its surface—the ice caps, the dried riverbeds, the geological scars of a wetter past. What made those oceans possible was something we take for granted on Earth: a magnetic field, an invisible shield that protects a planet from the solar wind and allows water to persist. But Mars lost that shield. The oceans evaporated. The question that haunted planetary scientists was simple and profound: if Mars once had a magnetic field, why did it vanish so quickly?
Professor Kei Hirose at the University of Tokyo decided to answer that question by building a laboratory replica of Mars's ancient core. Working with PhD student Shunpei Yokoo, Hirose's team constructed a sample of what they believed the Martian interior once contained: iron, sulphur, and hydrogen. They placed this material between two diamonds and subjected it to crushing pressure while heating it with an infrared laser, recreating the temperature and density conditions that would have existed deep within Mars roughly four billion years ago. X-ray and electron beams allowed them to watch what happened as the sample melted under these extreme conditions.
What they observed was unexpected and elegant. The initially uniform mixture of iron, sulphur, and hydrogen separated into two distinct liquids—a phenomenon never before documented at such pressures. One liquid was rich in iron and hydrogen but poor in sulphur; the other was iron-rich in sulphur but hydrogen-poor. This separation was the key to understanding Mars's magnetic history. The lighter hydrogen-rich liquid would have risen above the denser sulphur-rich liquid, creating convection currents in the core—the same mechanism that generates Earth's magnetic field today. Those currents would have generated a magnetic field strong enough to hold hydrogen in the Martian atmosphere, which in turn allowed water to remain liquid on the surface.
But the field was always temporary. Once the two liquids had fully separated and stratified, the convection stopped. Without those churning currents, the magnetic field collapsed. Hydrogen, no longer protected by the magnetic shield, was stripped away by the solar wind. Water vapor broke down. The oceans that had once covered Mars simply evaporated into space. All of this, the researchers estimate, unfolded over a relatively brief window in planetary history, roughly four billion years ago.
The implications extend beyond Mars. Understanding how the rocky planets formed and why they evolved so differently speaks to Earth's own deep past and distant future. Hirose noted that further seismic studies of Mars—using data from NASA's InSight probe, which has already revealed the Martian core to be larger and less dense than previously thought—could confirm whether the core is indeed layered as the lab work suggests. If so, it would help complete the story of how planets like ours came to be. There is one reassurance embedded in this research: Earth's magnetic field, driven by its own internal convection, is far more robust and long-lasting than Mars's was. Even if Earth's field were to weaken, Hirose said, it would take at least a billion years for it to disappear entirely. For now, our shield remains intact.
Citas Notables
The initially homogeneous material separated into two distinct liquids with a level of complexity never before seen under these pressures, and this separation is key to explaining both the birth and death of Mars's magnetic field.— Professor Kei Hirose, University of Tokyo
Further seismic study of Mars will hopefully verify the core is indeed in distinct layers as predicted, which would help complete the story of how rocky planets formed.— Professor Kei Hirose
La Conversación del Hearth Otra perspectiva de la historia
So the lab experiment showed that Mars's core material split into two liquids. Why does that matter so much?
Because that separation is what created the convection currents—the churning motion that generates a magnetic field. It's the same thing that keeps Earth's field alive today.
But if it created a magnetic field, why didn't it last?
That's the crucial part. Once those two liquids fully separated and settled into layers, there was nothing left to churn. The convection stopped. No movement, no field.
And then what happened to the oceans?
Without the magnetic field to shield the atmosphere, the solar wind stripped away the hydrogen. Water broke down. The oceans just evaporated into space over time.
How long did the whole process take?
The magnetic field itself probably lasted millions of years, but the loss of the oceans played out over a longer span. It all happened roughly four billion years ago.
Does this tell us anything about Earth's future?
It's actually reassuring. Earth's convection currents are far more stable than Mars's were. Even if our field weakened, it would take at least a billion years to disappear. We're not in immediate danger.