Fresh look at Apollo moon rocks solves decades-old mystery about the moon’s magnetic field
For decades, a debate has raged over whether the Moon ever had a strong magnetic field or whether it was always weak. Now, a new analysis of Apollo-era lunar rocks suggests that the moon magnetic field could be mostly weak, despite brief bursts of strong activity – potentially solving the mystery for good.
The research, published Thursday (Feb. 26) in the journal Nature Geoscience, shows that the Moon’s magnetic field intensified for brief periods early in its history, about 3.5 to 4 billion years ago, but that for most of the Moon’s 4.5 billion-year history the magnetic field was weak.
“For very short periods of time – no more than 5,000 years, but perhaps as short as a few decades – the melting of titanium-rich rocks at the boundary of the Moon’s core and mantle resulted in the generation of a very strong field,” explained the lead author. Claire Nicholsassociate professor of geology of planetary processes at the University of Oxford, said in a statement.
A long debate
The debate over the Moon’s magnetic field arises from a limited sample of lunar rocks. Six Apollo missions have landed the moon between 1969 and 1972, in areas roughly around the lunar equator. These missions landed in roughly the same location, in areas with similar rock types, the researchers said.
It was easier for astronauts to land their small craft on large, flat basalt areas called Maria, which are ancient lava plains formed after ancient meteorite crashes melted the original rock there. These Apollo landing zones are rich in titanium basalts.
The new research compared the amount of titanium in lunar samples to the magnetized strength of the rocks. The scientists found that rock samples containing less than 6% titanium had weak magnetic fields, and that magnetic fields were stronger in rocks containing higher concentrations of titanium.
This suggests that the formation of high-titanium rocks and the generation of a strong lunar magnetic field are linked, according to the release. Researchers believe both were caused by the melting of titanium-rich material deep within the Moon, which temporarily generated a very strong magnetic field.
A limited sample
Apollo moon rocks are an important part of Earth’s lunar inventory. Auction house Christie’s suggests that about 1,433 pounds (650 kilograms) of moon rocks are on our planet. comes from meteorites. Of this inventory, the Apollo archives represent approximately 842 pounds (382 kg), according to NASA.
Many Apollo rocks rich in titanium have nevertheless been analyzed by scientists, giving the impression that strong magnetism has been present on the Moon for a long time, according to the Oxford press release. But this seemed strange to other scientists, who argued that the small size of the Moon’s core – only one-seventh of its radius – could not have allowed the Moon to create a powerful field for long periods of its history.
The researchers confirmed the sampling bias by running models, which showed that a random set of lunar samples analyzed by scientists would contain only a few rocks containing a strong magnetic field. The hope is that the NASA-led project Artemis Astronaut missions will land in a wider variety of locations, collecting samples that show a range of the Moon’s 4.5 billion years of history.
“If we were aliens exploring Earth and we had landed here only six times, we would likely have a similar sampling bias, especially if we selected a flat surface to land on,” co-author of the study. Jon Wadeassociate professor of planetary materials at Oxford, said in the release. “It was only by chance that the Apollo missions focused so much on the Moon’s marine region. Had they landed elsewhere, we would likely have concluded that the Moon had only a weak magnetic field and would completely miss this important part of early lunar history.”
