A buried rock on the Moon's far side points to a magnetic field 4.2 billion years old

Buried roughly six miles beneath the Moon's far side lies a mass of ancient rock about 37 miles wide — denser than the crust around it, strongly magnetized, and, according to a new study, a relic of a time when the Moon generated its own magnetic field. The finding, published September 23 in the journal Science Advances, offers the clearest orbital evidence yet that the Moon once ran an internal dynamo, and it reframes a scientific argument that has persisted since the Apollo era.

The rock sits beneath a region on the Moon's far side called Dewar — the half that never faces Earth — where a bright, winding surface marking about 16 miles across, known as the Dewar swirl, had already drawn scientific attention. A team led by Xi Yang, a Ph.D. student at ETH Zurich, and lecturer Anna Mittelholz of the same institution, along with colleagues at the German Aerospace Center and TU Berlin, focused on Dewar because something unusual was happening there: one of the strongest magnetic anomalies on the far side coincided precisely with a distinct gravitational anomaly.

The Dewar region is a genuine stroke of luck: one of the strongest magnetic field anomalies on the far side of the Moon and a distinct gravity anomaly coincide spatially there.— Anna Mittelholz, Lecturer, ETH Zurich

That overlap gave the researchers something previous orbital studies lacked: the ability to model gravity and magnetism together in a single framework. Gravity measurements described the density of the buried rock; magnetic data described how strongly it was magnetized. Combined, they allowed the team to identify an actual buried structure rather than a magnetic signal that could, in theory, be produced by many different rock configurations, according to Mittelholz.

The data came from three spacecraft: NASA's Lunar Prospector, launched in 1998; Japan's Kaguya, launched in 2007; and NASA's twin GRAIL orbiters, launched in 2011. No new physical samples were analyzed.

The buried body is denser than the surrounding crust and sits beneath soil rich in iron and titanium. Once the team removed material thrown out by ancient impacts, the surface above it revealed a low mound resembling the Marius Hills volcanic complex on the Moon's near side — and the crust at Dewar is the thinnest in the area. The researchers interpret the structure as magma that rose from the Moon's interior and cooled underground before reaching the surface, forming a buried volcanic complex.

The age of the body is constrained by the geology around it. It must be younger than the enormous South Pole-Aitken basin, which formed about 4.31 billion years ago, and older than debris on top of it thrown out by a basin that formed about 4.14 billion years ago — placing its formation at roughly 4.2 billion years ago, according to earth.com.

As magma cools slowly, it locks in a record of any magnetic field present at the time. Because the rock can hold only so much iron-bearing material, the team could calculate the weakest field capable of producing the observed magnetization. Their answer: at least about 11 microtesla, and up to 22 microtesla in the most strongly magnetized part. Earth's magnetic field today measures around 50 microtesla, making the ancient lunar field at least roughly one-fifth as strong.

We have found that the magnetic field on the Moon at that time was very likely stronger than 10 microtesla. On Earth today, the magnetic field strength stands at around 50 microtesla.— Xi Yang, Ph.D. student, ETH Zurich

The team also ruled out the main alternative explanation. A passing impact — when a large asteroid strikes and briefly generates a plasma cloud that can magnetize nearby rock — is an unlikely source here. Such a field lasts only hours, while a body this size would have taken tens of millions of years to cool. Dewar also lies outside the zones opposite large impact basins where impact-magnetization effects are thought to concentrate, according to earth.com.

The debate the new study enters has roots in the 1970s, when rocks returned by Apollo astronauts showed signs of having recorded a magnetic field. For decades, most researchers agreed the Moon's dynamo was active from roughly 4.25 billion to 3.5 billion years ago. But more recent analyses of Apollo samples found no magnetic signal at all across parts of that same period, leading some researchers to question whether an early dynamo existed at all. The Apollo samples present a particular challenge: pulled from their geological context more than 50 years ago, their magnetic records can be disturbed by heating, shock, or even routine handling and storage, making two similar-looking samples capable of giving contradictory answers.

The debate stretches back to the 1970s, when the Apollo missions first returned lunar rocks that showed signs of having recorded a magnetic field.— Anna Mittelholz, Lecturer, ETH Zurich

The new study does not resolve every open question. The researchers were unable to estimate how long the active dynamo existed, and it remains unclear how the Moon's small core could have generated a field potentially as strong as some previous studies have suggested. Several proposed mechanisms for a small-core dynamo would produce only a few microtesla at the surface — well below the minimum the Dewar rock implies. The study authors say their findings shift the debate from whether the Moon had a dynamo to how it worked.

Earth and the moon had drastically different geological histories. The moon was smaller and fully molten for a long time. It also had a lot of volcanism early on, and there are many questions we cannot exactly answer if we don't know how the moon looked like back then.— Adrien Broquet, Researcher, German Aerospace Center, Berlin

The Dewar swirl itself fits neatly into the picture. The leading explanation for lunar swirls is that a local magnetic field deflects solar wind — the stream of charged particles from the Sun that slowly darkens exposed lunar soil — keeping a patch brighter than its surroundings. At Dewar, the buried rock's magnetization runs sideways, parallel to the surface, and the soil above is iron-rich. Nearby magnetized bodies with similar sideways fields have no swirl, because their surfaces lack enough iron to show the contrast. According to earth.com, that makes Dewar a particularly clean test of the swirl hypothesis.

The implications reach beyond the Moon. If the Moon sustained a dynamo for any period, that sets a lower bound on the conditions needed to run one — information relevant to understanding other small rocky and icy bodies in the solar system, Mittelholz noted. Pinning down the Moon's magnetic history more precisely, however, will require samples from the far side. A lander at Dewar that measures the ground and collects rock could replace the current minimum field estimate with a measured value and pin down the age more precisely. Upcoming missions — including Lunar Vertex, China's Chang'e 7 and 8, and Artemis — may eventually provide those observations and samples, according to earth.com.

Why it matters — Understanding whether and how the Moon ran a magnetic dynamo sets the baseline for what conditions small rocky bodies need to generate protective magnetic fields — with implications for the Moon's own history and for assessing other worlds.

Sources differ on Duration and continuity of the ancient lunar dynamo: Most researchers have held that the dynamo was active from roughly 4.25 billion to 3.5 billion years ago. (cnn.com) vs More recent analyses of Apollo samples found no magnetic signal at all for parts of that same period; some researchers question whether an early dynamo existed. (cnn.com)

Reported by cnn.com, earth.com, science.org