The Orientale (right) and Crisium (left) basins are both associated with regions of enhanced surface magnetism that are antipodal to the basin’s location on the Moon. Credit: Orientale: NASA; Crisium: Robert Reeves
Twentieth-century explorations answered many questions about our satellite and its place in the solar system. But they also raised many new and challenging questions that still remain unanswered.
One of these enduring mysteries has been the lunar magnetic field. The Moon generates no magnetic field of its own today. Yet analysis of Apollo rock samples and measurements by orbiting spacecraft have revealed magnetism in individual rocks and even large swaths of the surface with high levels of magnetism.
A team of scientists led by Issac Narrett at MIT has put forth a new explanation for the mysterious magnetization: asteroid impacts generated Moon-spanning clouds of plasma that temporarily created high magnetic fields in the region globally opposite where they occurred. The research was published today in Science Advances.
A brief history of the Moon’s magnetic field
When the Moon was young and about 10 times closer to Earth than it is today, both the heat of its fiery creation and tidal squeezing by Earth’s gravity kept its interior molten. The early Moon also rotated faster, magnifying the effect of Earth’s tidal pull on the Moon. Prior to about 3.56 billion years ago, the dynamo effect from these factors combined created a magnetic field, albeit one much weaker than Earth’s present magnetic field (which measures about 50 microteslas, or 0.00005 tesla). As the Moon cooled and receded from Earth, its magnetic field slowly faded. But traces of its past still hide in pockets of residual magnetism on the surface.
Most Apollo samples show what is called natural remanent magnetization. Researchers think these samples cooled while in the presence of a magnetic field over millions of years. Other samples appear to have been magnetized by high shock pressures, such as the impact of an asteroid.
Over the past two decades, spacecraft observations have revealed regions of the lunar crust spanning tens of miles (10 to 100 kilometers) that show magnetic anomalies — higher local levels of magnetization than their surroundings. The size of these regions implies they were magnetized by a strong ancient magnetic field.
But the dynamo effect of the Moon’s small core (only about 90 miles [140 km] wide) could produce magnetic fields only one-tenth of that needed to magnetize these large surface regions. And perhaps most intriguing, some of the strongest crustal magnetic anomalies are antipodal, or on the exact opposite side of the Moon, from the Crisium, Imbrium, Orientale, and Serenitatis basins.
An impactful explanation
Narrett’s team has a new solution for the magnetizing large regions of the crust in a short period of time. Using the MIT SuperCloud shared computing resource, the researchers modeled the effect of cataclysmic asteroid impacts on the primordial Moon. They found that the globe-circling clouds of plasma created by large impacts could temporarily compress and concentrate the early lunar magnetic field.
As the plasma cloud expanded from the impact point and radiated around the Moon, it carried the magnetic field along with it. As the cloud converged from all directions onto the antipodal point, it progressively squeezed the magnetic field lines closer together, intensifying the strength of the magnetic field there. The plasma wave sustained the amplified magnetic field for more than 30 minutes before dissipating.
The work is based on a previous study by team members Rona Oran and Benjamin Weiss. They investigated whether such impact-generated plasma waves could concentrate and amplify not the Moon’s inherent magnetic field, but the solar magnetic field carried by the solar wind. But this solar-induced magnetic field — just tens of nanoteslas, at most — is too weak, even when enhanced, to magnetize lunar rocks.
The new simulations instead assume the Moon had a dynamo-generated magnetic field of one to two microteslas, some 50 times weaker than Earth’s but still 100 times stronger than the solar-generated field. Based on this, they show that the plasma-compressed magnetic field strength at an impact’s antipodal point could reach a maximum 180 microteslas above the surface at an altitude of 435 miles (700 km), an amplification by a factor of 120. Although the field strength at the surface was diminished by dissipation within the lunar crust, the resulting field strength at the surface was still about 43 microteslas.
The MIT study also showed how polar impacts preferentially led to regions of higher magnetic concentration, as the radiating plasma cloud followed the natural magnetic field lines extending from the polar regions. Plasma radiating from equatorial impacts encountered magnetic resistance and did not concentrate as densely at the antipode site.
“There are large parts of lunar magnetism that are still unexplained. But the majority of the strong magnetic fields that are measured by orbiting spacecraft can be explained by this process — especially on the farside of the Moon,” Narrett said in a press release.
Working in concert
But questions remain about whether this process resulted in sufficient magnetic field strength to create the magnetism that still exists today, almost four billion years later. Narrett’s team believes the answer is yes — if another simultaneous mechanism worked with the arriving plasma wave.
The force of a basin-forming asteroid impact would also have sent seismic shock waves radiating through the Moon’s interior, which would refocus from below the surface at the impact’s antipodal site. Such a hammer blow from within the Moon has been previously postulated as the source of regional surface magnetism, but the simultaneous arrival of both internal seismic waves and an external plasma cloud had not been previously investigated.
The ballistic ejecta from a lunar basin-forming impact took up to four hours to arrive at the impact antipode site. But the plasma wave arrived much sooner, at about the same time the impact’s seismic waves that have transited the Moon’s interior and concentrated at the antipode. The seismic shock would have been powerful enough to unsettle the spin of electrons in the atoms of the rocks there. This set the stage for the temporary magnetic field spike to reorient the electrons and magnetize the rock.
“For several decades, there’s been sort of a conundrum over the Moon’s magnetism — is it from impacts or is it from a dynamo? And here we’re saying, it’s a little bit of both,” said Oran.“And it’s a testable hypothesis, which is nice.”
Some of the mysterious magnetic regions on the Moon are near the south pole, where the Artemis manned landings are planned. Soon, Artemis will return a treasure trove of lunar samples and potentially provide ground truth for this fascinating possible mechanism for the Moon’s strange magnetism.
Related: Will Lunar Vertex solve the mystery of lunar swirls?
