Evidence suggests that Mars may have been teeming with life billions of years ago. Now cold, dry and stripped of what was once a potentially protective magnetic field, the Red Planet is a sort of forensic stage for scientists investigating whether Mars was indeed once habitable and, if so, when .
The “when” question in particular has been driving researchers at Harvard’s Paleomagnetic Laboratory in the Department of Earth and Planetary Sciences. A new paper in Nature Communications makes their most compelling case to date that Mars’ life-sustaining magnetic field could have survived until about 3.9 billion years ago, compared to previous estimates of 4.1 billion years—that’s hundreds of millions of years. later.
The study was led by Griffin Graduate School of Arts and Sciences student Sarah Steele, who used simulation and computer modeling to estimate the age of the Martian “dynamo,” or global magnetic field produced by convection in the planet’s iron core. , like on Earth. . Along with senior author Roger Fu, the John L. Loeb Associate Professor of Natural Sciences, the team has doubled down on a theory they first argued last year that the Martian dynamo, capable of deflecting harmful cosmic rays, was more longer than prevailing estimates. claim.
The researchers simulated the cooling and magnetization of large impact basins on Mars to protect against a subsequent dynamo shutdown.
Credit: Sarah Steele
Their thinking evolved from experiments simulating the cooling and magnetization cycles of large craters on the Red Planet’s surface. Known as weak magnetics, these well-studied impact basins have led researchers to hypothesize that they formed after the dynamo shut down.
This timeline was hypothesized using the basic principles of paleomagnetism, or the study of a planet’s prehistoric magnetic field. Scientists know that ferromagnetic minerals in rock match the surrounding magnetic fields when the rock is hot, but these small fields “shut off” after the rock has cooled. This effectively turns the minerals into fossilized magnetic fields that can be studied billions of years later.
Seeing basins on Mars with weak magnetic fields, scientists hypothesized that they first formed among hot rocks during a period in which there were no other strong magnetic fields—in other words, after the planet’s dynamo had gone away.
But the Harvard team says this early shutdown is not necessary to explain those largely demagnetized craters, according to Steele. Rather, they argue that the craters formed while the Martian dynamo was experiencing a polarity reversal—the north and south poles switching places—which, through computer simulation, could explain why these large impact basins have only weak magnetic signals today. Magnetic pole rotations also occur on Earth every few hundred thousand years.
“We’re basically showing that there may never have been a good reason to assume that the Martian dynamo would shut down early,” Steele said.
Their results build on previous work that first overturned existing timelines of Martian habitability. They used a famous Martian meteorite, Allan Hills 84001, and a powerful diamond quantum microscope in Fu’s lab to infer a more stable magnetic field up to 3.9 billion years ago by studying different magnetic populations in thin slices of the rock.
Steele says that poking holes in a long-held theory is a little nerve-wracking, but that they have been “broken down” by a community of planetary researchers who are open to new interpretations and possibilities.
“We’re trying to answer the big, important questions of how everything came to be the way it is, and even why the entire solar system is the way it is,” Steele said. “Planetary magnetic fields are our best probe for answering many of these questions, and one of the only ways we have to learn about the deep interiors and early histories of planets.”