How the Death of Mars's Magnetic Field Turned a Living World Into a Desert

A Planet That Once Had Everything

Four and a half billion years ago, Mars boasted a thick atmosphere and abundant surface water — conditions that could have hosted life. It was, by all scientific reckoning, a world with a genuine shot at biology. What happened next is one of the most dramatic transformation stories in the solar system, and it carries a sobering lesson for life on Earth.

Mars once had a global magnetic field, generated deep inside the planet, strong enough to deflect the solar wind much as Earth's does today. That invisible shield was the linchpin holding everything together. Magnetic fields help a planet hold onto its oceans. If a planet ever loses its magnetic field, the atmosphere goes next — and, along with it, water.

What Killed the Dynamo

Researchers recreated conditions expected in the core of Mars billions of years ago and found that the behavior of the molten metal thought to be present likely gave rise to a brief magnetic field that was destined to fade away. The culprit was a chemical separation deep inside the planet. The liquid iron rich in hydrogen and poor in sulphur, being less dense, would have risen above the denser sulphur-rich, hydrogen-poor liquid iron, causing convection currents. These currents, similar to those on Earth, would have driven a magnetic field capable of maintaining hydrogen in an atmosphere around Mars, which in turn would have allowed water to exist as a liquid.

Unlike Earth's internal convection currents, which are extremely long lasting, once the two liquids had fully separated, there would have been no more currents to drive a magnetic field. When that happened, hydrogen in the atmosphere was blown out to space by solar wind, leading to the breakdown of water vapor and eventually the evaporation of the Martian oceans. This would all have taken place about 4 billion years ago.

A Slow Bleed, Not a Sudden Catastrophe

It's tempting to picture the death of Mars as a sudden, dramatic event — but the reality is more sobering. The idea of the atmosphere and oceans vanishing in a geologically brief period can leave the impression of a sudden catastrophe, and that is misleading. The loss played out over hundreds of millions to billions of years. It was a long, slow bleed, not a collapse.

NASA's MAVEN orbiter was built specifically to measure how Mars loses its atmosphere, and it found the planet is still bleeding air into space today, at a rate of roughly 100 grams every second. Crucially, MAVEN's data also imply that the loss rate would have been far higher long ago, perhaps hundreds of times higher, when the young Sun was more violent. The magnetic field was also not the only factor. Mars is a small world with weaker gravity, which by itself makes it easier for gas to escape. A great deal of the planet's water was not lost to space at all but locked away underground and bound into minerals in the crust, where signs of it are still being found.

What Mars Tells Us About Earth — and the Search for Life

Earth still runs an active core and keeps its magnetic shield, which is part of why it has held onto its air. Earth is the only one of the rocky planets in our solar system to have a strong magnetic field — a fact that looks less like luck and more like a prerequisite for complex life. The Mars story is essentially a controlled experiment showing what happens when that shield disappears.

If life had evolved in the watery regions of Mars in its early years, once the magnetic field disappeared it would not have survived, at least not on Mars's surface. That is why the next searches for whatever may have passed for life on Mars will need to drill down deeper. The discovery of a thriving biosphere full of microbial life at depths of a few hundred meters on Earth has given added hope that something like this may still be present on Mars. Mars may be a frozen desert on the surface, but its deeper secrets have not yet been fully told.