Soil on the far side of the Moon has preserved a "memory" of a sudden magnetic anomaly.

Stones indeed have a memory and can tell us a great deal, as paleontologists and geophysicists are well aware. When rocks are formed within a magnetosphere, they retain residual magnetism for billions of years, meaning that the history of the celestial body is "recorded" within them.

Such records were brought back by astronauts of the Apollo program half a century ago in the form of hundreds of kilograms of lunar soil. It has been established that these rocks are approximately three to four and a half billion years old, and many of them turned out to be magnetized. For instance, those that are three and a half billion years old demonstrated a magnetic field strength of at least 40 microteslas.

This is astonishing, as it is comparable to the strength of Earth's magnetic field—the planet that still has a solid, hot inner core that heats the surrounding liquid masses and causes them to mix. This mixing generates electric currents, which create the protective magnetosphere around us.

Planetologists are absolutely convinced that the Moon cannot have this internal magnetic dynamo now, nor could it have existed three and a half billion years ago: all calculations suggest that the lunar core cooled no later than 4.2 billion years ago. Thus, the Moon's own magnetic field could have only persisted for a few hundred million years. It is worth noting that the Solar System is 4.6 billion years old, and the Moon appears to have formed almost immediately after Earth.

Therefore, to explain the relatively recent magnetization of lunar rocks, various ingenious methods of generating a magnetic field for the Moon without the involvement of its core had to be "invented." Scientists suggest that it may have been generated due to Earth's gravitational influence—the Moon was much closer in the past. Alternatively, magnetism could have been caused by the oscillation of the Moon's rotational axis, which might have led to the movement of material inside.

There is even an hypothesis that as the Moon cooled unevenly, dense titanium-rich blobs gradually sank deeper into its interior, and when they reached the still-hot core, temporary electromagnetism occurred.

However, even these phenomena should have gradually diminished. Nevertheless, as Chinese scientists recently reported in an article for the journal Nature, the observed situation bears little resemblance to a gradual fading of the lunar magnetic field. Researchers compared the age and magnetization of the lunar soil collected by the Chang'e-6 interplanetary mission in June 2024.

It is worth noting that the Chinese spacecraft landed on the far side of the Moon, in the Apollo crater located in the gigantic South Pole-Aitken basin. The probe collected and sent back to Earth just over 1935 grams of soil. These are the first samples ever taken from the far side of our natural satellite. Interestingly, they turned out to be significantly younger than the Apollo samples, showing magnetization ranging from five to 21 microteslas.

According to scientists, the data reveals a picture of a sharp weakening of the Moon's magnetic field 3.1 billion years ago, followed by a sudden and equally sharp strengthening 2.8 billion years ago. Thus, less than three billion years ago, the Moon still had a magnetic field, and it was not particularly weak.

It remains to be determined how this occurred. Researchers suspect that the aforementioned precession of the rotational axis or some processes in the depths of the lunar magmatic ocean during those distant times might be involved. It is also possible that the gradual solidification of the core could have contributed to this phenomenon.