Have we finally solved mystery of magnetic moon rocks?
i ate a rock from the moon
Have we finally solved mystery of magnetic moon rocks?
Simulations show how effects of asteroid impact could amplify the early Moon's weak magnetic field.
Jennifer Ouellette
–
May 23, 2025 2:36 pm
|
5
NASA Lunar sample 60015 on display at Space Center Houston Lunar Samples Vault, at NASA's Johnson Space Center
Credit:
OptoMechEngineer/CC BY-SA 4.0
NASA Lunar sample 60015 on display at Space Center Houston Lunar Samples Vault, at NASA's Johnson Space Center
Credit:
OptoMechEngineer/CC BY-SA 4.0
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NASA's Apollo missions brought back moon rock samples for scientists to study. We've learned a great deal over the ensuing decades, but one enduring mystery remains. Many of those lunar samples show signs of exposure to strong magnetic fields comparable to Earth's, yet the Moon doesn't have such a field today. So, how did the moon rocks get their magnetism?
There have been many attempts to explain this anomaly. The latest comes from MIT scientists, who argue in a new paper published in the journal Science Advances that a large asteroid impact briefly boosted the Moon's early weak magnetic field—and that this spike is what is recorded in some lunar samples.
Evidence gleaned from orbiting spacecraft observations, as well as results announced earlier this year from China's Chang'e 5 and Chang'e 6 missions, is largely consistent with the existence of at least a weak magnetic field on the early Moon. But where did this field come from? These usually form in planetary bodies as a result of a dynamo, in which molten metals in the core start to convect thanks to slowly dissipating heat. The problem is that the early Moon's small core had a mantle that wasn't much cooler than its core, so there would not have been significant convection to produce a sufficiently strong dynamo.
There have been proposed hypotheses as to how the Moon could have developed a core dynamo. For instance, a 2022 analysis suggested that in the first billion years, when the Moon was covered in molten rock, giant rocks formed as the magma cooled and solidified. Denser minerals sank to the core while lighter ones formed a crust.
Over time, the authors argued, a titanium layer crystallized just beneath the surface, and because it was denser than lighter minerals just beneath, that layer eventually broke into small blobs and sank through the mantle. The temperature difference between the cooler sinking rocks and the hotter core generated convection, creating intermittently strong magnetic fields—thus explaining why some rocks have that magnetic signature and others don't.
Or perhaps there is no need for the presence of a dynamo-driven magnetic field at all. For instance, the authors of a 2021 study thought earlier analyses of lunar samples may have been altered during the process. They re-examined samples from the 1972 Apollo 16 mission using CO2 lasers to heat them, thus avoiding any alteration of the magnetic carriers. They concluded that any magnetic signatures in those samples could be explained by the impact of meteorites or comets hitting the Moon.
Bracing for impact
In 2020, two of the current paper's authors, MIT's Benjamin Weiss and Rona Oran, ran simulations to test whether a giant impact could generate a plasma that, in turn, would amplify the Moon's existing weak solar-generated magnetic field sufficiently to account for the levels of magnetism measured in the moon rocks. Those results seemed to rule out the possibility. This time around, they have come up with a new hypothesis that essentially combines elements of the dynamo and the plasma-generating impact hypotheses—taking into account an impact's resulting shockwave for good measure.
Amplification of the lunar dynamo field by an Imbrium-sized impact at the magnetic equator.
Credit:
Isaac S. Narrett et al., 2025
They tested their hypothesis by running impact simulations, focusing on the level of impact that created the Moon's Imbrium basin, as well as plasma cloud simulations. Their starting assumption was that the early Moon had a dynamo that generated a weak magnetic field 50 times weaker than Earth's. The results confirmed that a large asteroid impact, for example, could have kicked up a plasma cloud, part of which spread outward into space. The remaining plasma streamed around to the other side of the Moon, amplifying the existing weak magnetic field for around 40 minutes.
A key factor is the shock wave created by the initial impact, similar to seismic waves, which would have rattled surrounding rocks enough to reorient their subatomic spins in line with the newly amplified magnetic field. Weiss has likened the effect to tossing a deck of 52 playing cards into the air within a magnetic field. If each card had its own compass needle, its magnetism would be in a new orientation once each card hit the ground.
It's a complicated scenario that admittedly calls for a degree of serendipity. But we might not have to wait too long for confirmation one way or the other. The answer could lie in analyzing fresh lunar samples and looking for telltale signatures not just of high magnetism but also shock.Scientists are looking to NASA's planned Artemis crewed missions for this, since sample returns are among the objectives. Much will depend on NASA's future funding, which is currently facing substantial cuts, although thus far, Artemis II and III remain on track.
Science Advances, 2025. DOI: 10.1126/sciadv.adr7401 .
Jennifer Ouellette
Senior Writer
Jennifer Ouellette
Senior Writer
Jennifer is a senior writer at Ars Technica with a particular focus on where science meets culture, covering everything from physics and related interdisciplinary topics to her favorite films and TV series. Jennifer lives in Baltimore with her spouse, physicist Sean M. Carroll, and their two cats, Ariel and Caliban.
5 Comments
#have #finally #solved #mystery #magnetic
Have we finally solved mystery of magnetic moon rocks?
i ate a rock from the moon
Have we finally solved mystery of magnetic moon rocks?
Simulations show how effects of asteroid impact could amplify the early Moon's weak magnetic field.
Jennifer Ouellette
–
May 23, 2025 2:36 pm
|
5
NASA Lunar sample 60015 on display at Space Center Houston Lunar Samples Vault, at NASA's Johnson Space Center
Credit:
OptoMechEngineer/CC BY-SA 4.0
NASA Lunar sample 60015 on display at Space Center Houston Lunar Samples Vault, at NASA's Johnson Space Center
Credit:
OptoMechEngineer/CC BY-SA 4.0
Story text
Size
Small
Standard
Large
Width
*
Standard
Wide
Links
Standard
Orange
* Subscribers only
Learn more
NASA's Apollo missions brought back moon rock samples for scientists to study. We've learned a great deal over the ensuing decades, but one enduring mystery remains. Many of those lunar samples show signs of exposure to strong magnetic fields comparable to Earth's, yet the Moon doesn't have such a field today. So, how did the moon rocks get their magnetism?
There have been many attempts to explain this anomaly. The latest comes from MIT scientists, who argue in a new paper published in the journal Science Advances that a large asteroid impact briefly boosted the Moon's early weak magnetic field—and that this spike is what is recorded in some lunar samples.
Evidence gleaned from orbiting spacecraft observations, as well as results announced earlier this year from China's Chang'e 5 and Chang'e 6 missions, is largely consistent with the existence of at least a weak magnetic field on the early Moon. But where did this field come from? These usually form in planetary bodies as a result of a dynamo, in which molten metals in the core start to convect thanks to slowly dissipating heat. The problem is that the early Moon's small core had a mantle that wasn't much cooler than its core, so there would not have been significant convection to produce a sufficiently strong dynamo.
There have been proposed hypotheses as to how the Moon could have developed a core dynamo. For instance, a 2022 analysis suggested that in the first billion years, when the Moon was covered in molten rock, giant rocks formed as the magma cooled and solidified. Denser minerals sank to the core while lighter ones formed a crust.
Over time, the authors argued, a titanium layer crystallized just beneath the surface, and because it was denser than lighter minerals just beneath, that layer eventually broke into small blobs and sank through the mantle. The temperature difference between the cooler sinking rocks and the hotter core generated convection, creating intermittently strong magnetic fields—thus explaining why some rocks have that magnetic signature and others don't.
Or perhaps there is no need for the presence of a dynamo-driven magnetic field at all. For instance, the authors of a 2021 study thought earlier analyses of lunar samples may have been altered during the process. They re-examined samples from the 1972 Apollo 16 mission using CO2 lasers to heat them, thus avoiding any alteration of the magnetic carriers. They concluded that any magnetic signatures in those samples could be explained by the impact of meteorites or comets hitting the Moon.
Bracing for impact
In 2020, two of the current paper's authors, MIT's Benjamin Weiss and Rona Oran, ran simulations to test whether a giant impact could generate a plasma that, in turn, would amplify the Moon's existing weak solar-generated magnetic field sufficiently to account for the levels of magnetism measured in the moon rocks. Those results seemed to rule out the possibility. This time around, they have come up with a new hypothesis that essentially combines elements of the dynamo and the plasma-generating impact hypotheses—taking into account an impact's resulting shockwave for good measure.
Amplification of the lunar dynamo field by an Imbrium-sized impact at the magnetic equator.
Credit:
Isaac S. Narrett et al., 2025
They tested their hypothesis by running impact simulations, focusing on the level of impact that created the Moon's Imbrium basin, as well as plasma cloud simulations. Their starting assumption was that the early Moon had a dynamo that generated a weak magnetic field 50 times weaker than Earth's. The results confirmed that a large asteroid impact, for example, could have kicked up a plasma cloud, part of which spread outward into space. The remaining plasma streamed around to the other side of the Moon, amplifying the existing weak magnetic field for around 40 minutes.
A key factor is the shock wave created by the initial impact, similar to seismic waves, which would have rattled surrounding rocks enough to reorient their subatomic spins in line with the newly amplified magnetic field. Weiss has likened the effect to tossing a deck of 52 playing cards into the air within a magnetic field. If each card had its own compass needle, its magnetism would be in a new orientation once each card hit the ground.
It's a complicated scenario that admittedly calls for a degree of serendipity. But we might not have to wait too long for confirmation one way or the other. The answer could lie in analyzing fresh lunar samples and looking for telltale signatures not just of high magnetism but also shock.Scientists are looking to NASA's planned Artemis crewed missions for this, since sample returns are among the objectives. Much will depend on NASA's future funding, which is currently facing substantial cuts, although thus far, Artemis II and III remain on track.
Science Advances, 2025. DOI: 10.1126/sciadv.adr7401 .
Jennifer Ouellette
Senior Writer
Jennifer Ouellette
Senior Writer
Jennifer is a senior writer at Ars Technica with a particular focus on where science meets culture, covering everything from physics and related interdisciplinary topics to her favorite films and TV series. Jennifer lives in Baltimore with her spouse, physicist Sean M. Carroll, and their two cats, Ariel and Caliban.
5 Comments
#have #finally #solved #mystery #magnetic
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