Tuesday, May 17, 2022

Study simulate Mars’ core may explain the loss of its magnetic field

How did Mars lose its oceans?

Mars was once thought to be wet to cover its surface with ocean water. This history of water on Mars raises the possibility that Mars was once home to life.

It is thought that after Mars lost its protective magnetic field, solar radiation and the solar wind stripped it of much of its air and water. A new study from the University of Tokyo that recreated conditions expected in the core of Mars billions of years ago may explain the reason behind the disappearance of Mar’s magnetic field.

Scientists 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.

Several mysteries of Mars have already been solved. Still, one question specifically boggled the mind of Professor Kei Hirose from the University of Tokyo‘s Department of Earth and Planetary Science: There must have been a magnetic field around Mars, so why was it there at all, and why was it there so briefly?

Constrained to address this question, a team led by Ph.D. student Shunpei Yokoo in the Hirose lab investigated a novel method for testing something so distant from us in reality.

Hirose said, Earth’s magnetic field is driven by inconceivably huge convection currents of molten metals in its core. Magnetic fields on other planets are thought to work the same way.”

“Though the internal composition of Mars is not yet known, evidence from meteorites suggests it is molten iron enriched with sulphur. Furthermore, seismic readings from NASA’s InSight probe on the surface tell us Mars’ core is larger and less dense than previously thought. These things imply the presence of additional lighter elements such as hydrogen. With this detail, we prepare iron alloys that we expect to constitute the core and subject them to experiments.”

In their experiments, scientists used diamonds, lasers, and a surprise. They made a sample of iron, sulfur, and hydrogen (Fe-S-H). According to scientists, the core of Mars was once made of this material.

Placing the sample between two diamonds and compressing it while heating could simulate and estimate the temperature and pressure at the core. Sample observations with X-ray and electron beams allowed the team to imagine what was happening during melting under pressure. They were also able to map how the sample’s composition changed during that time.

Hirose said, “We were very surprised to see a particular behavior that could explain a lot. The initially homogeneous Fe-S-H is separated into two different liquids with a level of complexity that has not been seen before under these kinds of pressures. One of the iron liquids was rich in sulphur, the other rich in hydrogen, and this is key to explaining the birth and eventual death of the magnetic field around Mars.”

The liquid iron- enriched in hydrogen and poor in sulphur- being less dense, would have risen above the denser sulfur-rich, hydrogen-poor liquid iron, causing convection currents. These currents would have prompted a magnetic field to maintain hydrogen in an atmosphere around Mars. This, in turn, has allowed water to exist as a liquid. However, it was not to last.

diamond anvil
Under pressure. The diamond anvil used in the experiments © 2022 Yokoo et al.

Once the two liquids had fully separated, there would have been no more currents to drive a magnetic field. The solar wind blew out the hydrogen in the atmosphere during this event. This causes the breakdown of water vapor and eventually the evaporation of the Martian oceans.

Scientists noted, “All this would have taken place about 4 billion years ago.”

Hirose said“With our results in mind, further seismic study of Mars will hopefully verify the core is indeed in distinct layers as we predict. If that is the case, it will help us complete the story of how the rocky planets, including Earth, formed and explain their composition. And you might be thinking that the Earth could one day lose its magnetic field as well, but don’t worry, that won’t happen for at least a billion years.”

Journal Reference:

  1. Shunpei Yokoo, Kei Hirose, Shoh Tagawa, Guillaume Morard, Yasuo Ohishi. Stratification in planetary cores by liquid immiscibility in Fe-S-H. Nature Communications, 2022; 13 (1) DOI: 10.1038/s41467-022-28274-z

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