Using data from twin spacecraft named Ebb and Flow, they found a 2 to 3 percent difference in the moon mantle’s ability to deform on each side.

Ryan Park and colleagues analysed data from the NASA GRAIL mission to map the Moon’s gravitational response to its orbit around Earth, which can offer insights into the satellite’s internal structure.

Park and colleagues then modelled the Moon’s structure and determined that the two to three percent figures can be explained by a difference in mantle temperature of 100–200 kelvin between the two hemispheres, in which the nearside mantle is warmer than the farside. They hypothesize that this thermal difference could be sustained by radioactive decay of thorium and titanium within the Moon’s nearside, which could be a remnant of the volcanic activity that formed the nearside surface 3–4 billion years ago.

The authors note that the methods used to probe the Moon’s interior could be used to measure differences in the structure of other planetary bodies such as Mars, Enceladus and Ganymede, specifically because they do not rely on a spacecraft landing on the surface.