The Moon is actually tens of millions of years older than previously believed.

According to the most widely accepted hypothesis regarding the formation of the Moon, the satellite originated from debris resulting from a collision between Earth and a yet-to-form planet — Theia — roughly the size of Mars. This theory is increasingly questioned, and alternative scenarios are emerging. Fortunately, more data is being accumulated that allows for the testing of various hypotheses. To establish the "date" of the potential collision, scientists are analyzing crystallized rocks from the primordial magma ocean of the Moon.

Analysis of lunar surface rocks and inclusions yields varying results. On one hand, the age of many rocks and inclusions is estimated at 4.35 billion years, while on the other, there are inclusions dated at 4.51 billion years. Currently, researchers rely on rock analysis and favor the theory that the satellite formed later, but the young Moon hypothesis faces several significant challenges.

Most computer models of planet formation indicate that a satellite like the Moon should have formed earlier. Thermal evolution modeling of such a body also revealed that the magma ocean cooled slowly. Finally, there remains the question of the "old" inclusions that are 4.51 billion years old.

If we accept that the Moon formed 4.51 billion years ago, we need to explain the abundance of younger rocks. According to one theory, a complete remelting of the planet was triggered by a collision that created the South Pole-Aitken basin, but this is not supported by computer models.

The authors of a new study proposed an alternative hypothesis — tidal heating of the Moon around 4.35 billion years ago. In this case, the remelting of magma is not linked to the formation of the satellite or a collision. The findings are published in the journal Nature.

The Moon's orbit around Earth has changed significantly throughout its evolution. At a minimum, according to computer modeling results, the satellite should have formed at a distance of up to four Earth radii, while today it is on average at a distance of 60 radii. As the Moon changed its position, the balance of gravitational forces from Earth and the Sun also shifted. These changes provoked tidal heating and the melting of the satellite's magma.

According to the researchers, during this prolonged episode of tidal heating, the Moon's mantle underwent several cycles of remelting. However, a complete magma ocean never formed. Magma melted in localized areas and erupted to the surface, similar to Io, one of Jupiter's moons.

The tidal heating hypothesis explains the young age of the rocks, the ancient inclusions, and the lack of craters on the surface. Today, there are about 50 impact basins on our satellite, whereas there should be significantly more. Researchers estimate that around 400,000 planetesimals larger than 10 kilometers in diameter remained in the vicinity of Earth after the dispersal of the protoplanetary disk.