In 2021 research, scientists say they can show the moon hasn’t had a magnetic field for at least the last 4 billion years—chipping away at a longtime argument over whether the moon ever had a magnetic field at all. Their evidence comes via specimens gathered during the Apollo missions decades ago.
How do you tell if the moon had a magnetic field at different points in time? The first step is to find samples that are the right age, in this case up to 4 billion years old. The second step is to decide if those samples would have “recorded” magnetic activity circling around them.
You can do a version of this yourself if you paint cars for a living, where magnets help to “align” the flakes in beautiful metallic paint. You can also buy magnetically activated nail polish that appears to change color based on how you magnetize it as it dries.
So scientists used samples gathered from the Apollo missions decades ago, made of the right kind of material to register magnetic activity, like the car paint or nail polish. The Apollo samples, “formed at ∼3.9, 3.6, 3.3, and 3.2 billion years ago,” don’t show any evidence of core dynamo activity—the telltale behavior indicating the presence of a magnetic field. (A dynamo is a spinning electrical generator, like the spinning, iron core of the Earth.)
There’s a second step to the research, too. That’s for scientists to show that the moon’s surface shows evidence the moon has been consistently blasted by solar winds—something the magnetic field would protect against. The surface of the moon is relatively stable with little movement, so this evidence may be undisturbed for billions of years. “We predict that these deep lunar soils represent a rich volatile reservoir, reflecting a ∼4-Ga-old history of ion transport that can be explored by new missions such as Artemis,” the researchers write. “Together, these data indicate that the Moon lacked any long-lived dynamo after ∼4 Ga,” the researchers conclude.
To understand the overall importance of this question, we can compare the moon to Earth’s healthy and functional magnetic field. The core of the Earth acts as an electromagnetic dynamo, spinning and generating a very large version of a magnetic field. This magnetic field helps protect the Earth from being bombarded by solar winds and cosmic radiation.
What does it mean that the moon doesn’t have a magnetic field? Scientists study this question because it reveals information about how the moon formed as well as its progression over time. “Determining the presence or absence of a past long-lived lunar magnetic field is crucial for understanding how the Moon’s interior and surface evolved,” the researchers write.
We can compare the moon to Mars in this case—Mars has very weak localized magnetic fields today, but scientists know the planet did have a dynamo core in the past, like a smaller Earth that burned out its lifetime a great deal earlier. If the moon had similar evidence, scientists could tell it was also a smaller version of Earth. That’s why it’s so important to keep studying the materials from the moon that are available to us.
Caroline Delbert is a writer, avid reader, and contributing editor at Pop Mech. She's also an enthusiast of just about everything. Her favorite topics include nuclear energy, cosmology, math of everyday things, and the philosophy of it all.