Mars had a long-lasting magnetic field, expanding the possibilities of life | Sciences

CHICAGO—Once upon a time, scientists believe, Mars was far from the cold and inhospitable desert of today. Rivers carved canyons, lakes filled craters, and a magnetic field may have prevented space radiation from devouring atmospheric moisture. As the Martian interior cooled, leading theories hold, its magnetic field quenched, leaving the atmosphere defenseless and ending this warm and humid period, when the planet could have supported life. But researchers can’t agree on when that happened.

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

“They can paint a pretty good picture of what might have happened,” says Jennifer Buz, a paleomagnetist at Northern Arizona University who was not involved in the study. “The work that they did was simply not possible with previous technology.”

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

Orbiters around Mars have mapped these remnant magnetic signatures in rocks on the surface of Mars. But some of the largest and oldest scars on the planet, the Hellas, Argyre and Isidis asteroid impact basins, do not appear to contain magnetized rocks at all. Most researchers believe this is because the magnetic dynamo had died down when these craters formed, some 4.1 billion years ago. Interestingly though, orbiters have detected magnetic signatures in lavas a few hundred million years younger, from other parts of Mars, suggesting that the field somehow survived longer than the basins allowed.

“It’s hard to say that 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 Harvard University.

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

meteorite ALH84001
Clues to an ancient magnetic field were found in thin slices of the Allan Hills 84001 Martian meteorite.POT

Steele and Harvard planetary scientist Roger Fu imaged three paper-thin slices of a 0.6-gram Allan Hills sample using Fu’s state-of-the-art quantum diamond microscope. One of the few in the world, it is based on the sensitivity of atomic impurities in diamond to small changes in magnetic fields; you can map these changes onto 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 lacked a significant magnetic signature.

In a paper now being reviewed, Steele and Fu propose that these groupings reflect three known impact events recorded by the meteorite, which radioactive dating had placed at about 4 billion, 3.9 billion and 1.1 billion ago. of years. Because the two oldest mineral populations are highly magnetized, Fu says, a global magnetic field must still have been around 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 Earth’s field).

With that strength, the field could have helped deflect harmful cosmic rays, protecting possible 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 stream of particles that can accelerate the loss of water vapor and other components into 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 that line of reasoning. He says a field could also speed up atmospheric losses by funneling more solar wind toward the poles.

The minerals also hold a clue to the planet’s inner workings: The two magnetized populations register fields pointing in nearly opposite directions, 138° apart. The researchers say there is little chance that the rock would simply rotate between the impacts. Rather, they propose that the Martian dynamo must have reversed its poles, as Earth does every few hundred million years. Computer simulations have shown that dynamos only reverse within a narrow range of convection conditions in a planet’s molten outer core, so Martian reversals could help constrain the history and nature of your dynamo, he says. lillis.

A reversing dynamo could also help explain why many large, old basins lack a magnetic signal. In an AGU presentation, Steele used computer simulations to show that alternating magnetic field layers could essentially cancel out the net magnetic field of the basins, making them appear demagnetized. Inversions 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 of an inversion in a meteorite,” says Weiss, who proposed using inversions to date rocks on Mars in a separate AGU presentation. “Yes [Mars’s dynamo] is being reversed, that plan that we have in mind here is suddenly much more feasible ”.

Fu says he is indebted to the Allan Hills meteorite, which sparked his love of science as a child when he heard about the famous rock on TV. “Early Mars is a black box in many ways,” says Fu. “The fact that we’re taking a rock that’s been analyzed ad nauseam… and we can still get new information out of it, I think that’s really cool.”

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