For decades, a debate has raged about whether the moon has ever had a strong magnetic field, or whether it was always weak. Now, a new analysis of Apollo-era lunar rocks suggests the moon’s magnetic field may be mostly faint, 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 strengthened for brief periods in its early history, about 3.5 billion to 4 billion years ago, but for most of the moon’s 4.5 billion-year history, the magnetic field was weak.
“For very short periods—no more than 5,000 years, but possibly as short as a few decades—melting of titanium-rich rocks at the Moon’s core-mantle boundary resulted in the generation of a very strong field,” lead author Claire Nicholsan associate professor of the geology of planetary processes at the University of Oxford, said in a statement.
A long debate
The debate about the moon’s magnetic field stems from a limited selection of lunar rocks. Six Apollo missions landed on the moon between 1969 and 1972, in zones roughly around the lunar equator. Those missions landed in roughly the same place, in zones with similar types of rocks, the researchers said.
It was easiest for the astronauts to land their small craft on large, flat basaltic areas called maria, which are ancient lava plains formed by ancient meteorite crashes that melted the original rock there. These Apollo landing areas are rich in titanium basalts.
The new research mapped the amount of titanium content in lunar samples against how strongly magnetized the rocks were. The researchers found that rock samples that were less than 6% titanium had weak magnetic fields and that the magnetic fields were stronger in rocks with higher titanium concentrations.
This suggests that the formation of high-titanium rocks and the generation of a strong lunar magnetic field are related, according to the statement. The researchers believe both were caused by the melting of titanium-rich material deep inside the moon, which temporarily generated a very strong magnetic field.
A limited selection
Apollo moon rocks make up a significant portion of Earth’s lunar inventory. Christie’s auction house suggests that around 1,433 pounds (650 kilograms) of moon rocks on our planet comes from meteorites. Of that inventory, the Apollo archive accounts for about 842 pounds (382 kg), according to NASA.
However, many of the titanium-rich Apollo rocks have been analyzed by scientists, creating the impression that strong magnetism was present on the moon for a long time, according to the Oxford statement. But it seemed strange to other scientists, who argued that the small size of the moon’s core – only one-seventh of its radius – could not allow the moon to create a strong 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 have only a few rocks containing a strong magnetic field. The hope is that NASA led Artemis Astronaut missions will land at a wider variety of locations, collecting samples that reveal a range of the moon’s 4.5 billion-year history.
“If we were aliens exploring Earth, and had landed here only six times, we would probably have a similar sampling bias – especially if we chose a flat surface to land on,” co-author of the study Jon Wadean associate professor of planetary materials at Oxford, said in the statement. “It was only by chance that the Apollo missions focused so much on the jump region of the Moon—if they had landed elsewhere, we probably would have concluded that the Moon only ever had a weak magnetic field and missed this important part of early lunar history.”
