How Mars Lost Its Magnetic Field—and Then Its Oceans

Chemical transformations inside Mars’s core led to the loss of its magnetic field, which in turn caused the planet to lose its oceans. But how did this happen?

Mars’s surface today is barren and dry, with most of its remaining water trapped in polar icecaps or possibly hidden underground. However, a closer look reveals features resembling shorelines or canyons that hint at ancient, massive floods.

Billions of years ago, Mars likely had a thicker atmosphere and slightly warmer temperatures. Evidence from Martian deltas, similar to river deltas on Earth, suggests that oceans once partially covered the planet. Analysis of Martian meteorites, which reveal the planet’s past chemistry, also supports this idea. These studies suggest that around four billion years ago, Mars’s northern hemisphere was covered by a vast ocean.

Today, that ocean is long gone. A study led by the University of Tokyo and published in 2022 in Nature Communications offers an explanation: Mars lost its magnetic field billions of years ago. Without this protective shield, the atmosphere was gradually stripped away, and the oceans eventually vanished as water vapor escaped into space.

Magnetic Fields and Oceans

The Solar System is a dangerous place. The Sun, which gives us life, can also be destructive. It emits intense radiation that, without Earth’s magnetic field, would strip away our atmosphere and evaporate our oceans, leaving Earth as dry and barren as Mars.

Earth is the only rocky planet in our Solar System with a strong magnetic field, which likely explains the stark differences between Earth and Mars. But billions of years ago, Mars also had a strong magnetic field. So what went wrong?

How Mars Lost Its Magnetic Field and Oceans

To understand, a research team led by Shunpei Yokoo at the University of Tokyo recreated the conditions of Mars’s core in a laboratory. They combined iron, sulfur, and hydrogen—elements thought to be present in Mars’s core.

Evidence of sulfur comes from Mars meteorites, which lack elements that commonly pair with sulfur, indicating sulfur is more concentrated in the core. Hydrogen is thought to be abundant in Mars’s core because of the planet’s proximity to the “snow line,” where water ice was plentiful during the Solar System’s formation. Yokoo suggested that the Martian core might be composed of a mixture of iron, sulfur, and hydrogen, but this hypothesis needs further verification from marsquake observations, which NASA’s InSight mission could provide.

The team heated this iron, sulfur, and hydrogen mixture between two diamonds using a laser, mimicking the extreme conditions inside a rocky planet’s core. The result was two distinct liquids—one of iron and sulfur, and another of iron and hydrogen. The less dense hydrogen-rich liquid floated to the top, and as the liquids separated, convective currents emerged.

This process mirrors what likely happened in early Mars, where convective currents generated by the separating iron-sulfur-hydrogen mixture created a magnetic field. However, these currents were temporary. Once the liquids fully separated, the currents ceased, and the magnetic field disappeared, leading to the eventual loss of Mars’s atmosphere and oceans.

Similar Physics in the Earth’s Core

This separation of iron-sulfur and iron-hydrogen liquids also occurs in Earth’s core, but with a crucial difference: temperature.

“The temperature of Earth’s core (~6,740°F) is much higher than that of Mars’ core,” Yokoo explained. At these higher temperatures, the iron-sulfur and iron-hydrogen liquids mix, preventing full stratification. Instead, the Earth’s core shows separation only at the top, where temperatures are lower. According to Yokoo, it would take about a billion years for Earth’s core to fully stratify, meaning Earth has plenty of time left.

These findings also impact the search for habitable exoplanets. Currently, the presence of liquid water on a planet’s surface, situated in the habitable zone, is a primary criterion for potential life. However, these results suggest that a strong magnetic field might be just as crucial for a planet to retain its water, hinting that magnetic fields as robust as Earth’s may be relatively rare in the cosmos.