Science Gazette

How did Mars’ seas vanish?

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Experiments simulating Mars’ core might explain why the planet’s magnetic field has vanished.

Due to a protective magnetic field comparable to Earth’s, it has long been assumed that Mars formerly possessed seas. The magnetic field, on the other hand, vanished, and new study may now be able to explain why. The behavior of the molten metal estimated to be present in the core of Mars billions of years ago likely gave birth to a transient magnetic field that was destined to go away, according to researchers.

Mars has captivated people’s imagination for years, whether it’s owing to science fiction or the fact that it can be seen with your own eyes from Earth. It’s one of our nearest neighbors, and it’s been researched with a variety of scientific equipment onboard the several unmanned space missions that have and are continuing to study it. Despite this, there are a number of major outstanding issues concerning Mars, the solutions to which might throw light on our own distant history and future, given that Earth, Mars, and all of our surrounding planets were all formed from the same cosmic material.

Some of the most pressing concerns regarding Mars have already been addressed. Many observable features on Mars, for example, are indications that it formerly had seas and a protective magnetic field. Professor Kei Hirose of the University of Tokyo’s Department of Earth and Planetary Science had one issue in mind: There had to be a magnetic field surrounding Mars, but why was it there at all, and why was it there for such a short time? In the Hirose lab, a team lead by Ph.D. student Shunpei Yokoo developed a unique technique to test anything so far away in both time and space in order to address this issue.

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“The magnetic field of the Earth is driven by enormous convection currents of molten metals in its core. Other planets’ magnetic fields are believed to function similarly “Hirose said. “Though the interior composition of Mars is unknown, evidence from meteorites shows it is made out of molten iron with a sulphur enrichment. Furthermore, data from NASA’s InSight mission on Mars’ surface indicates that the planet’s core is bigger and less thick than previously assumed. These factors point to the existence of lighter elements like hydrogen. With this information, we develop iron alloys that we believe will form the core and test them.”

Diamonds, lasers, and an unexpected surprise were all part of the experiment. Yokoo created a sample of material comprising iron, sulphur, and hydrogen (Fe-S-H), which he and his colleagues believe was originally used to create the core of Mars. They squeezed the sample between two diamonds and heated it with an infrared laser while compressing it. This was done to approximate the core’s anticipated temperature and pressure. The researchers was able to observe what was going on during melting under pressure using X-ray and electron beams, as well as map how the sample’s composition changed throughout that period.

“We were taken aback when we saw a certain tendency that may explain a lot. Under these pressures, the previously homogenous Fe-S-H split into two different liquids with a degree of complexity never seen before “Hirose said. “One of the iron liquids was high in sulphur, while the other was high in hydrogen, and this is crucial to understanding how the magnetic field surrounding Mars was born and subsequently died.”

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Because hydrogen-rich, sulphur-poor liquid iron is less dense, it would rise above the denser sulphur-rich, hydrogen-poor liquid iron, creating convection currents. These currents would have produced a magnetic field capable of keeping hydrogen in an atmosphere surrounding Mars, allowing water to exist as a liquid, similar to those on Earth. It was not, however, to last. Unlike the Earth’s internal convection currents, which are exceedingly long-lasting, there would have been no more currents to produce a magnetic field after the two liquids had entirely separated. When this occurred, solar wind blew hydrogen from the atmosphere into space, causing water vapor to break down and ultimately the evaporation of the Martian oceans. And all of this would have happened about 4 billion years ago.

“With our findings in mind, we anticipate that subsequent seismic studies of Mars will confirm that the core is actually in separate strata as we predicted,” Hirose added. “If that’s the case, it’ll help us finish the tale of how the rocky planets, including Earth, came to be and explain their makeup. You may imagine that the Earth’s magnetic field would vanish at some point, but don’t worry, that won’t happen for at least a billion years.”

The Japan Society for the Promotion of Science (JSPS) KAKENHI program financed this research (Grant No. 16H06285 and 21H04506).

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