Mars had a long-lasting magnetic field, extending the chances of life

CHICAGO-Once upon a time, according to scientists, Mars was far from the cold and inhospitable desert of today. Rivers carved out canyons, lakes filled craters, and a magnetic field may have repelled space radiation, preventing it from eating away at atmospheric moisture. As the Martian interior cooled, according to leading theories, its magnetic field died out, leaving the atmosphere defenseless and ending that hot, humid period when the planet could have supported life. . But researchers can’t agree on when it happened.

Now fragments of a famous Martian meteorite, studied with a new type of quantum microscope, have provided evidence that the planet’s field persisted until 3.9 billion years ago, hundreds of millions years older than many thought. Clues in the meteorite, a Martian rock that ended up on Earth after an impact blasted it from its home planet, could expand Mars’ window of habitability and reconcile conflicting early timelines of the history of the planet. Discussed last week at a meeting of the American Geophysical Union (AGU), the findings also support the idea that, as on Earth, Mars’ field has occasionally reversed – behavior that could shed light on the molten dynamo in the outer core that once powered it. .

“They’re able to paint a pretty good picture of what could have happened,” says Jennifer Buz, a paleomagnetist at Northern Arizona University who was not involved in the study. “The work they did just wasn’t possible with the technology before.”

When certain types of iron-bearing minerals crystallize in molten rock, their internal fields align with the planet’s field like tiny compasses, preserving a stamp of its orientation. Later impact events can heat parts of a rock, covering it with fields from later times and creating a magnetic palimpsest.

Orbiters around Mars have mapped these residual magnetic signatures in rocks on the surface of Mars. But some of the planet’s largest and oldest scars – the Hellas, Argyre and Isidis asteroid impact basins – don’t appear to contain any magnetized rocks at all. Most researchers believe this is because the magnetic dynamo had collapsed when these craters formed, around 4.1 billion years ago. Strangely, however, the orbiters detected magnetic signatures in lava a few hundred million years younger from other parts of Mars, suggesting that the field had somehow survived longer than the basins suggested. .

“It’s hard to say you really understand what happened in the past on another planet if you have these two fundamentally opposite timelines,” says Sarah Steele, a graduate student in Earth and Planetary Sciences at the Harvard University.

Steele wondered if Allan Hills 84001, a Martian meteorite recovered from Antarctica in 1984, might have something to say on the matter. Debunked claims from the 1990s that the meteorite contained fossilized bacteria made the 2-kilogram rock notorious, but researchers are still studying it today because at 4.1 billion years old it is the only pristine sample known for recording this critical era in the history of Mars.

meteorite ALH84001
Hints of an ancient magnetic field have been found in thin slices of the Martian meteorite Allan Hills 84001.Nasa

Steele and Harvard planetary scientist Roger Fu imaged three paper-thin slices of a 0.6-gram Allan Hills sample with Fu’s state-of-the-art quantum diamond microscope. One of the few in the world, it is based on the sensitivity of the diamond’s atomic impurities to minute variations in magnetic fields; it can map these changes to grains as small as a human hair. The improved resolution revealed something surprising: three distinct populations of iron sulfide minerals. Two were strongly magnetized in different directions, while one had no significant magnetic signature.

In a paper under review, Steele and Fu propose that these groupings reflect three known impact events recorded by the meteorite, which radioactive dating had placed around 4 billion, 3.9 billion, and 1.1 billion years ago. ‘years. Because the two older mineral populations are highly magnetized, Fu says, a global magnetic field must still have been present 3.9 billion years ago. The 3.9 billion year old field appears to be relatively strong: about 17 microtesla (about a third of the average strength of the Earth’s field).

At that strength, the field could have helped deflect harmful cosmic rays, shielding potential early life forms, says Ben Weiss, a planetary scientist at the Massachusetts Institute of Technology. It could also have shielded the atmosphere from the solar wind, a flow of particles that can accelerate the loss of water vapor and other constituents to space. “The longer the dynamo stays, the longer you can have a potentially habitable period on Mars,” says Weiss.

Rob Lillis, a planetary geophysicist at the University of California, Berkeley, is more cautious about this reasoning. He says a field could also accelerate atmospheric losses by funneling more solar wind towards the poles.

The minerals also hold a clue to the inner workings of the planet: the two magnetized populations register fields pointing in almost opposite directions, 138° apart. The researchers say it’s unlikely the rock simply rotated between the impacts. Instead, they propose that the Martian dynamo flipped its poles, as Earth does every few hundred million years. Computer simulations have shown that dynamos only reverse within a narrow range of convective conditions in a planet’s molten outer core, so Martian reversals could help limit the history and nature of its dynamo,” says Lillis.

A reverse dynamo could also help explain why many large ancient basins have no magnetic signal. In an AGU presentation, Steele used computer simulations to show that layers of alternating magnetic fields could essentially cancel out the basins’ net magnetic field, making them appear demagnetized. Reversals can “allow us to tie all the strings together once and for all,” Steele says.

As a bonus, magnetic reversals could provide a common time marker for rocks from different locations. “It’s exciting for me to hear that there is evidence for an inversion in a meteorite,” said Weiss, who offered to use inversions to date rocks on Mars in a separate AGU presentation. “Whether [Mars’s dynamo] reverses, this plan we have in mind here is suddenly much more achievable.

Fu says he is indebted to the Allan Hills meteorite, which sparked his love for science when he learned about the famous rock on television. “Early Mars is such a black box in many ways,” says Fu. “The fact that we’re taking a rock that’s been analyzed to death…and we can still glean new information from it, I think that’s really cool.”

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