A new study suggests that a massive asteroid impact may have temporarily strengthened the Moon’s weak magnetic field, leaving behind magnetized rocks that continue to perplex scientists. Image Source: Shutterstock
Where did the Moon’s magnetism go? For decades, scientists have been puzzled by this question, which has lingered since orbiters detected signs of high magnetic fields in lunar surface rocks. Today, the Moon itself has no inherent magnetism.
Now, scientists at the Massachusetts Institute of Technology (MIT) may have unraveled this mystery. They propose that an ancient, weak magnetic field combined with an impact that generated a large amount of plasma may have temporarily created a powerful magnetic field concentrated on the far side of the Moon.
In a study published in the journal Science Advances, researchers demonstrated through detailed simulations that an impact, such as one from a large asteroid, could produce a cloud of ionized particles that briefly enveloped the Moon. This plasma would flow around the Moon and concentrate on the opposite side from the initial impact location. There, the plasma would interact with the Moon’s weak magnetic field and temporarily amplify it. Any rocks in that area could record signs of the magnetic enhancement before the magnetic field rapidly dissipated.
This combination of events could explain the presence of highly magnetic rocks detected in the region near the Moon’s south pole. Coincidentally, one of the largest impact basins—the Imbrium basin—is located on the completely opposite side of the near side of the Moon. The researchers suspect that whatever caused that impact may have released a plasma cloud, initiating the scenario in their simulations.
Isaac Narrett, a graduate student in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS) and the lead author, said, “A large part of the Moon’s magnetism remains unexplained. However, most of the strong magnetic fields measured by orbiters can be accounted for by this process—especially on the far side of the Moon.”
Narrett’s co-authors include Rona Oran and Benjamin Weiss from MIT, Katarina Miljkovic from Curtin University, Yuxi Chen and Gábor Tóth from the University of Michigan, and Elias Mansbach from the University of Cambridge. Nuno Loureiro, a professor of nuclear science and engineering at MIT, also contributed insights and suggestions.
Beyond the Sun For decades, scientists have known that the Moon retains remnants of a strong magnetic field. Samples brought back by astronauts during NASA’s Apollo missions in the 1960s and 1970s, as well as global measurements from orbiters, have shown signs of remnant magnetism in surface rocks, particularly on the far side of the Moon.
The typical explanation for surface magnetism is a global magnetic field generated by an internal “dynamo” or a molten, swirling core. Today, Earth generates its magnetic field through a dynamo process, and it is believed that the Moon may have done so in the past, although its much smaller core would produce a much weaker magnetic field, which may not explain the observed highly magnetic rocks, especially on the far side.
Another hypothesis that scientists have occasionally tested involves massive impacts that produce plasma, which in turn amplifies any weak magnetic fields. In 2020, Oran and Weiss tested this hypothesis by simulating massive impacts on the Moon, combined with the magnetic field produced by the Sun (which is weak when it extends to the Earth and Moon).
In their simulations, they tested whether impacts on the Moon could amplify this solar magnetic field enough to account for the high magnetic measurements in surface rocks. The results showed otherwise, seemingly ruling out the possibility that plasma-induced impacts played a role in the Moon’s missing magnetism.
A Spike and a Jolt However, in their new research, the researchers took a different approach. They did not consider the solar magnetic field but instead assumed that the Moon once had a dynamo generating its own magnetic field, albeit a weak one. Given the size of its core, they estimated such a magnetic field to be about 1 microtesla, which is 50 times weaker than today’s Earth magnetic field.
Starting from this point, the researchers simulated a massive impact on the Moon’s surface, similar to the impact that created the Imbrium basin on the near side. Using impact simulations from Katarina Miljkovic, the team then modeled the plasma cloud generated by the impact as the surface material vaporized. They adapted a second code developed by their collaborators at the University of Michigan to simulate how the resulting plasma would flow and interact with the Moon’s weak magnetic field.
These simulations showed that as the plasma cloud rose from the impact, part of it would expand into space, while the rest would flow around the Moon and concentrate on the opposite side. There, the plasma would compress and briefly amplify the Moon’s weak magnetic field. Narrett noted that from the moment the magnetic field was amplified to when it decayed back to baseline, the entire process would be very quick—about 40 minutes.
Is that brief window enough for the surrounding rocks to record the momentary spike in magnetism? The researchers say yes, aided by another impact-related effect.
They found that an impact the size of the Imbrium basin would send a pressure wave across the Moon, similar to seismic shocks. These waves would converge on the opposite side, where the impact would “jolt” the surrounding rocks, briefly disturbing the electrons in the rocks—these subatomic particles naturally align their spin direction to any external magnetic field. The researchers suspect that as the plasma from the impact amplified the Moon’s magnetic field, the rocks were simultaneously jolted. When the electrons in the rocks re-stabilized, they would exhibit a new orientation consistent with the momentary high magnetic field.
Weiss said, “It’s like throwing a deck of 52 cards into the air in a magnetic field, with each card having a compass needle. When the cards fall back to the ground, they settle in a new direction. That is essentially the magnetization process.”
The researchers concluded that the combination of a dynamo, large impacts, and the shock waves from those impacts is sufficient to explain the highly magnetized surface rocks of the Moon—especially on the far side. One way to confirm this would be to directly sample the rocks for signs of impacts and high magnetism. This could be a possibility since these rocks are located on the far side, near the Moon’s south pole, where NASA’s Artemis program plans to explore (Background extension: The Artemis program is the U.S. manned space program to return to the Moon, aiming to land astronauts on the Moon’s south pole by 2025).
Oran said, “For decades, there has been a puzzle regarding the Moon’s magnetism—does it come from impacts or from a dynamo? Here, we say both are present. This is a testable hypothesis, which is great.”
The team’s simulations were conducted using MIT’s SuperCloud. This research was partially supported by NASA. (Background extension: The study was also supported by the National Science Foundation (NSF) under project number: PHY-2020249)
返回搜狐，查看更多