The moon is sometimes called "two-faced" because the surface of its side perpetually facing away from Earth looks so different than its side always facing our planet. And the differences run deeper than that, as shown by an analysis of rock and soil retrieved in 2024 by China's Chang'e-6 robotic lunar spacecraft.
Scientists said the chemical makeup of the minerals in the material obtained from a location on the moon's farside showed it formed from lava within the lunar mantle about 60 miles (100 km) under the surface some 2.8 billion years ago, crystallizing at a temperature of about 2,000 degrees Fahrenheit (1,100 degrees Celsius). They compared that to data on previously studied samples of rock that crystallized in the nearside mantle.
It turns out that the Chang'e-6 sample, the only one ever gotten from the farside, formed in the lunar interior at a temperature about 180 degrees Fahrenheit (100 degrees Celsius) cooler than the 33 samples previously retrieved from the nearside during NASA Apollo missions and by a Chinese spacecraft in 2020. The researchers said they believe this difference between the two sides persists to this day.
"Our results demonstrate the existence of thermal asymmetry between the nearside and farside mantle," said geoscientist Yang Li of University College London and Peking University, who led the study published in the journal Nature Geoscience.
"This takes us one step closer toward understanding the dichotomy of the moon. Specifically, the moon has a dramatic difference for the two sides at its surface, such as volcanism, crust thickness and topography," Li added.
The farside possesses a thicker crust - the outermost layer of the planet - and is more mountainous and cratered. It appears that this side was less volcanic in the past than the nearside, so it has fewer dark patches of basalt, a type of rock formed from lava long ago. The surface of the nearside is smoother and mostly covered in dark volcanic plains.
The moon, like Earth, formed about 4.5 billion years ago. Volcanism on the moon, Earth and other planetary bodies involves the eruption of molten rock from the mantle - the layer just under the crust - onto the surface. The Chang'e-6 landing site in the South Pole-Aitken Basin, an impact crater, is an area with the thinnest crust on the moon, helpful for finding evidence concerning volcanism.
Chang'e-6 used a scoop and drill to obtain material after arriving on the lunar surface in June 2024, then returned it to Earth, landing in the Chinese region of Inner Mongolia.
Because of Earth's gravitational pull, the moon always shows the same side to our planet - tidally locked, in scientific terms. But the differences in temperature in the interior may not have anything to do with this.
The researchers hypothesize that the interior of the moon's farside may be cooler than the nearside as a result of having a smaller amount of elements such as uranium, thorium and potassium that release heat due to radioactive decay.
Some scientists have hypothesized that this uneven distribution of such elements within the moon may have been caused when a massive asteroid or some other celestial body smashed into the farside early in lunar history. This could have disturbed the moon's interior and shifted denser material containing more heat-producing elements to the lunar nearside.
There are also hypotheses that the moon early on may have smashed into and merged with a second, smaller moon orbiting Earth. As a result, its current internal differences would reflect the thermal differences between the two moonlets before they collided.
"Understanding the origin of this lunar dichotomy is essential for reconstructing the moon's formation history, thermal evolution and crustal development, and may have implications for understanding the origin and evolution of other planets," Li said.
