The striking difference between the two faces of the Moon

The data show that there are fundamental differences between the “near side” of the Moon, which faces Earth, and the “far side,” which remains in the background. The near side consists of vast plains covered with lava, while the far side is more rugged and structurally more rigid.

The research provides strong support for theories that volcanic activity two to three billion years ago led to an uneven distribution of radioactive elements in the Moon's interior.

"We found that the near side of the Moon is stretched more than the far side," said Ryan Park, director of the Solar System Dynamics Group, adding that this difference in flexibility suggests that the interior structure is fundamentally different.

The gravity data were collected by two spacecraft, Ebb and Flow, of the GRAIL mission that orbited the Moon in 2011–2012. With this data, scientists created the most detailed gravity map of the Moon to date.

According to Park, the results were initially so surprising that the calculations were repeated several times, a process that took 10 years in total.

The new model also shows how the Moon stretches due to Earth's gravity. Just as the Moon affects tides on Earth, the Earth exerts a similar force on the Moon.

The same technique was previously applied to the asteroid Vesta, which revealed that Vesta may have a smaller or non-existent core than previously thought. It also found that Jupiter's volcanic moon Io may have a magma ocean.

“Gravity gives us a unique perspective on the interior of a celestial body without needing the surface,” said Ryan Park, emphasizing how valuable this method is for planetary exploration.