Lunar meteorites exhibit a fascinating magnetic property, but it is not a field generated today. These rocks possess a stable, fossilized magnetization known as remanence, which acts as a memory of a distant past when the Moon was magnetically active. This ancient magnetic record is a scientific puzzle, as the Moon currently lacks the global magnetic field. Studying this preserved magnetism allows researchers to peer back into the thermal and geological history of our nearest celestial neighbor.
Defining Lunar Meteorites
A lunar meteorite is a fragment of the Moon’s surface ejected into space by a high-velocity impact from an asteroid or comet. After traveling through space, the rock eventually enters Earth’s atmosphere and is discovered on the ground. Authentication involves comparing their mineralogy and chemical signatures to samples returned by the Apollo missions.
Scientists analyze the rock’s composition, looking for a distinctive ratio of iron oxide to manganese oxide (FeO/MnO), which is significantly higher in lunar rocks than in those from Mars or the asteroid belt. The meteorites represent a wide variety of lunar rock types, including feldspathic breccias from the bright lunar highlands and mare basalts from the darker plains. Because they are launched from random locations across the Moon, these meteorites provide a broader geological context than the specific sites visited by the Apollo missions.
Remanent Magnetism in Lunar Samples
The magnetism found in lunar meteorites is called remanent magnetization. This is distinct from induced magnetism, which is a temporary magnetic response to an external field. The remanence is locked into the rock’s structure, indicating the rock cooled or formed in the presence of a strong magnetic field billions of years ago.
This magnetic memory is carried by tiny grains of metallic iron and iron-nickel alloys embedded within the rock matrix. These ferromagnetic minerals align themselves with the ambient magnetic field as the rock cools past the Curie temperature. Once cooled, the alignment is fixed, preserving the direction and intensity of that ancient magnetic field long after the field itself has vanished. The presence of this stable magnetization is the primary evidence that the Moon was not always magnetically inert.
The Lunar Dynamo
The source of the powerful ancient magnetic field recorded in the rocks is attributed to the Lunar Dynamo. A planetary dynamo is generated by the convection of electrically conductive fluid—such as molten iron—within a core. This movement creates a magnetic field that extends outward into space.
Evidence suggests the lunar dynamo was active for a substantial period in the Moon’s early history. It likely initiated around 4.3 billion years ago and was intense between 3.9 and 3.5 billion years ago, with a field strength comparable to Earth’s present-day magnetic field. While the Moon’s small core size makes a conventional dynamo difficult to sustain, proposed power sources include mechanical stirring from Earth’s gravitational effects (precession) and core crystallization.
Following this initial high-field epoch, the magnetic field significantly weakened, though it may have persisted in a low-intensity state until 1.0 billion years ago. As the Moon’s molten core cooled and solidified, the fluid motion necessary to power the dynamo ceased, causing the global magnetic field to collapse. The magnetic signals preserved in the lunar meteorites are snapshots across this long-lived, yet extinct, magnetic history.
Testing Magnetic History on Earth
To study this remnant magnetic signal, scientists must first isolate the ancient field from contamination acquired over the rock’s history. Lunar meteorites can pick up magnetic noise from the impact that launched them, exposure to Earth’s magnetic field, or handling by modern magnets. This unwanted modern signal is called viscous remanent magnetization.
Researchers use Superconducting Quantum Interference Device (SQUID) magnetometers to measure the weak magnetic moment of the samples. The rock is then subjected to controlled demagnetization, either through alternating magnetic fields or step-wise heating, to progressively erase the less stable, younger magnetization. This meticulous process aims to reveal the Characteristic Remanent Magnetization, the original, stable magnetic signature locked in the rock billions of years ago.