22.12.2022

Study of famed meteorite by quantum diamond microscope also reveals flips in martian field

Mars once held a magnetic field that may have shielded atmospheric moisture from space radiation.NASA/JPL/GSFC

CHICAGO—Once upon a time, scientists believe, Mars was far from today’s cold, inhospitable desert. Rivers carved canyons, lakes filled craters, and a magnetic field may have fended off space radiation, keeping it from eating away the atmospheric moisture. As the martian interior cooled, leading theories hold, its magnetic field died out, leaving the atmosphere undefended and ending this warm and wet period, when the planet might have hosted life. But researchers can’t agree on when that happened.

Now, fragments from a famous martian meteorite, studied with a new kind 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 had thought. The clues in the meteorite, a Mars rock that ended up on Earth after an impact blasted it from its home planet, could extend Mars’s window of habitability and reconcile 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, as on Earth, Mars’s field sometimes flipped around—behavior that could shed light on the molten dynamo in the outer core that once powered it.

“They are able to 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 they did was just not possible with the technology before.”

When certain kinds of iron-bearing minerals crystallize out of molten rock, their internal fields align with the planet’s field like tiny compasses, preserving a stamp of its orientation. Subsequent impact events can heat up parts of a rock, glazing it with fields from later times 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 planet’s largest, most ancient scars—the Hellas, Argyre, and Isidis asteroid impact basins—don’t appear to contain magnetized rocks at all. Most researchers think that’s because the magnetic dynamo had subsided by the time these craters formed, about 4.1 billion years ago. Strangely, though, orbiters have detected magnetic signatures in lavas a few hundred million years younger, from other parts of Mars, suggesting the field had somehow survived longer than the basins let on.

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

Steele wondered whether Allan Hills 84001, a martian meteorite retrieved from Antarctica in 1984, might have something to say on the question. Debunked 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 is the only known pristine sample to record this critical era of Mars’s history.

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 only a handful in the world, it relies on the sensitivity of atomic impurities in diamond to tiny changes in magnetic fields; it can map these changes across grains as small as a human hair. The enhanced resolution revealed something surprising: three distinct populations of iron-sulfide minerals. Two were strongly magnetized in different directions, whereas one lacked a significant magnetic signature.

In a paper now under review, 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 years ago. Because the two older mineral populations are highly magnetized, Fu says, a global magnetic field must still have been present at 3.9 billion years ago. The 3.9-billion-year-old field appears to be relatively strong: about 17 microtesla (about one-third the average strength of Earth’s field).

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

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 accelerate atmospheric losses by funneling more solar wind to the poles.

The minerals also hold a clue to the planet’s internal workings: The two magnetized populations record fields pointed in nearly opposite directions—138° apart. The researchers say there’s little chance the rock simply rotated between impacts. Rather, they propose the martian dynamo must have flipped its poles, as Earth’s does every few hundred million years. Computer simulations have shown dynamos only reverse within a narrow range of convection conditions in a planet’s molten outer core, so the martian reversals could help constrain the history and nature of its dynamo, Lillis says.

A reversing dynamo could also help explain why many large, ancient basins lack a magnetic signal. In an AGU presentation, Steele used computer simulations to show layers of alternating magnetic fields could essentially cancel out the net magnetic field of the basins—making them appear to be demagnetized. The reversals may “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 to me to hear that there’s evidence for a reversal in a meteorite,” says Weiss, who proposed using reversals to date Mars rocks in a separate AGU presentation. “If [Mars’s dynamo] is reversing, that plan we have in mind here is suddenly a lot more feasible.”

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

Quelle: AAAS