When you look at photos from Apollo missions you may feel a mix of pride and wonder. When University of Oxford geologist Claire Nichols gazes at them, she feels something else: frustration.
There are tantalizing rocks just out of frame.
“You see in the background of some of the photographs amazing cliff faces where you can see exactly how everything is layered,” Nichols tells Inverse. Because of limited time on the missions, “they didn’t get any samples” from these incredible strata.
For a geologist studying the Earth’s and Moon’s now-defunct magnetic field, such rock formations could be the proverbial golden nugget — offering a clear picture of just what type of magnetic field the Moon possessed before it mysteriously switched off around 900 million years ago.
Most of the Moon rocks brought back by Apollo crews were ejected from their origins by meteor impacts, Nichols says. Without knowing their original orientation as they cooled in lava flows billions of years ago, the magnetic particles inside can’t reveal much about the Moon’s ancient magnetic field.
But that doesn’t mean they don’t contain any clues.
In a new paper published Monday in the journal Nature Astronomy, Nichols and her study co-authors detail how they were able to combine a couple of fortuitous Apollo-era samples with newer NASA imaging of the Moon’s surface with the help of someone who was there — Apollo 17 astronaut and geologist Harrison Schmitt. Schmit was the first scientist and one of the last men to walk on the Moon
In turn, the team gained new insights into the strength and shape of the lunar magnetic field.
The big reveal? It may have looked a lot like Earth’s.
What is a magnetic field?
The interaction between the movement of materials within a planet (like molten rock and lava) and a planet’s rotation creates a planetary magnetic field. For example, the molten material within Earth’s outer core possesses electrical currents. Its movement produces the magnetic field, which extends from the interior of the planet to space, deflects cosmic radiation and particles emitted by the Sun, and causes fantastic-looking polar auroras.
Other planets have magnetic fields of various strengths. Earth’s magnetic field is weaker than those belonging to Jupiter, Saturn, Uranus, and Neptune.
Today, while the Moon doesn’t have an internal magnetic field, it has some magnetic spots. A scientific point of debate is to what extent the Moon’s magnetic field once was — and what connection it had to Earth.
How the discovery was made — Nichols and colleagues determined the angle — in other words, the steepness — of the Moon’s ancient magnetic field from rock samples extracted from an Apollo 17 landing site and compared it to predictions based on assumptions of what the Moon’s magnetic field should have looked like.
They found the field angle matched exactly what they predicted it should be if the Moon’s magnetic field looked like the Earth’s — one with a North and South pole.
“Because the Moon rotates so slowly and is tidally locked to the Earth, the magnetic field should actually be highly complicated,” Nichols says. It should have a field with multiple poles. However, “our study shows they looked more like a bar magnet.”
“That's why this sample was really cool.”
These findings don’t entirely exclude multi-polar field shapes, Nichols adds, but they suggest the Moon had a field that looked a lot like Earth’s — a result the study team didn’t expect.
It’s actually surprising the Moon had a magnetic field at all because the lunar core is much smaller than Earth’s, Nichols explains.
But the paper’s findings show the Moon’s field not only resembled Earth’s — it may have also been similar in strength. The study team determined the Moon's magnetic field may have averaged around 50 microTeslas in strength, whereas Earth’s field averages around 25 to 65 microTeslas.
“Which is kind of nuts,” Nichols says.
Digging into the details — Because most Apollo samples were not taken directly from rock formations, past studies had tried to infer the original orientation by examining the direction of magnetic particles in the rocks and assuming they aligned with the direction of lava flow on the lunar surface.
Nichols and her colleagues tried this too — with limited success.
“It's a clever approach and it was a really cool thing to try,” she says. “But it doesn't necessarily work for these samples.”
Two major innovations pointed them to another approach.
- The high-resolution images of the Apollo 17 landing site taken by NASA’s Lunar Reconnaissance Orbiter
- The years of work done by her co-author and Apollo 17 astronaut Harrison Schmitt
Schmitt realized some of the rocks the mission had collected had not been impact ejecta, but rocks that had slumped down from a crater wall. By analyzing both orbiter and Apollo era photos, the rocks’ original orientations could be calculated.
“We don't know how it's rotated since it slumped, but you can see kind of the layering that implies the horizontal lava flows within it and it hasn't moved much from its original position,” Nichols says. “That's why this sample was really cool.”
Why does the Moon’s magnetic field matter?
The Moon’s magnetic field is important because it provides the nearest comparison to Earth for understanding how, why, and when planetary bodies generate magnetic fields — and what that means for life.
“For me, at least it's the habitability question. How common is this? How many ways are there to drive a field?” Nichols says.
“We think that magnetic fields are pretty important for having a habitable planet, you know, they provide shielding in the atmosphere.”
Given that the Moon shouldn’t have had such a strong magnetic field based on previous theories, a better understanding of our natural satellite could help scientists determine what happened to Mars’s magnetic field — and what to expect on other planets as we continue to explore space.
And because their findings can’t rule out other lunar magnetic field shapes, or explain just how the Moon was generating such a strong field — and then stopped — Nichols and colleagues see this paper as posing more questions than it answers.
For instance, this study is compatible with theories of polar wander, where the Moon might have shifted its axis of spin to compensate for changes in surface density due to ancient volcanism. However, more research is needed to know for certain.
Advice for future lunar astronauts
One way to learn a lot more about the Moon’s magnetic field, and rule out competition theories definitively, would be to get more samples, and samples with orientation information attached.
This is why Nichols and other researchers are looking forward to NASA’s Artemis missions, which could return humans to the lunar surface by 2024.
“Really we just need oriented samples,” Nichols says. “When we have oriented samples, then we can answer a lot more questions.”
What should the Artemis astronauts remember while on their return to the Moon? Pay attention to which rocks they pick up.
Abstract: Palaeomagnetic studies of Apollo samples indicate that the Moon generated a magnetic field for at least 2 billion years1,2. However, the geometry of the lunar magnetic field is still largely unknown because the original orientations of essentially all Apollo samples have not been well constrained. Determining the direction of the lunar magnetic field over time could elucidate the mechanism by which the lunar dynamo was powered and whether the Moon experienced true polar wander. Here we present measurements of the lunar magnetic field 3.7 billion years ago as recorded by Apollo 17 mare basalts 75035 and 75055. We find that 75035 and 75055 record a mean palaeointensity of ~50 μT. Furthermore, we could infer from the magnetization direction of 75055 and the layering of its parent boulder that the inclination of the magnetic field at the time was 34 ± 10°. Our recovered inclination is consistent with, but does not require, a selenocentric axial dipole (SAD) field geometry: a dipole in the centre of the Moon and aligned along the spin axis. Additionally, although true polar wander is not required by our data, true polar wander paths inferred from some independent studies of lunar hydrogen deposits and crustal magnetic anomalies are consistent with our measured paleoinclination.
Editor’s note 10/2: This article originally described Harrison Schmitt as the last man to walk on the Moon. Instead, it was Eugene Cernan.