Rocks that China’s Chang’e-6 mission carried back from the Moon’s far side in 2024 are now being used to explain why the two lunar hemispheres look so different. A team led by Professor Hejiu Hui of Nanjing University concludes that after the Moon became tidally locked to Earth, Earth’s pull heated the near side more strongly, and that this extra heat steered the movement of the Moon’s early magma ocean. The result appears in National Science Review, in a paper that compares the far-side samples against Apollo material from the near side.
The tech news worth your time: 400,000+ professionals from Google, Apple and OpenAI read Techpresso, a daily email with the most interesting tech news and insights.
We may earn a commission if you sign up.
Chang’e-6 and the first far-side samples
The Moon keeps one face toward Earth, and that face differs sharply from the hidden one in crust thickness and in where KREEP-rich material sits. For decades the only samples available came from the near side, through the Apollo missions and China’s Chang’e-5, so any theory of the asymmetry had to lean on orbital observations of the far side rather than rocks in the laboratory. That gap is what makes a first batch of far-side material so valuable to lunar scientists.
Chang’e-6 changed that. Its lander touched down on June 1, 2024, in the southern mare of Apollo Basin inside the South Pole-Aitken basin, collected about 1.9 kilograms of material, and returned it to Inner Mongolia on June 25, 2024. It was the first mission to bring back samples from the far side. The Nanjing University group, analyzed three kinds of rock clasts from it, namely anorthosites, Mg-suite rocks and mare basalts, and compared them with near-side samples from Apollo and Chang’e-5.
Magnesium-rich crust and the missing KREEP
The comparison turned up a pattern. According to the Science China Press summary on ScienceDaily, Chang’e-6 anorthosites show higher peak magnesium numbers than comparable Apollo samples, indicating that the far side’s crust contains more magnesium-rich material. That agrees with earlier remote observations of the hemisphere from orbit.
The Mg-suite rocks tell a related story. Their magnesium numbers are similar to the Apollo counterparts, but the Chang’e-6 samples carry extremely low concentrations of rare earth elements and lower thorium-to-samarium ratios. That points to KREEP, the potassium, rare earth and phosphorus-rich residue of the last magma ocean melt, being scarce on the far side. The authors say that could mean the far-side Mg-suite magmas formed without much KREEP-rich input, or that the KREEP layer developed unevenly.
The basalts add a counterweight. Their mantle sources resemble those of Apollo low-titanium basalts, and the mantle layers look broadly similar between hemispheres, so the split lies in the crust and the final products of the magma ocean rather than deep inside.
Earth’s tidal heating and the circulating magma ocean
The mechanism rests on the Moon’s orientation. After the Moon became tidally locked to Earth, one face always pointed toward the planet, and tidal heating was stronger on that near side. In the team’s model this drove horizontal circulation in the magma ocean, moving hot magma toward the far side and carrying magnesium-rich anorthosite with it. The result was a thicker far-side crust, while KREEP and ilmenite-bearing cumulates concentrated on the near side.
In the National Science Review paper, Lunar farside-nearside asymmetry formation during magma ocean solidification, Hui, the corresponding author, and 20 co-authors frame the process as lateral thermal convection in a long-lived lunar magma ocean. The far side, by comparison, developed only relatively thin layers enriched in KREEP. Put simply, the model has the near side collecting the heat-producing, KREEP-rich leftovers of the magma ocean while the far side was built up from the hotter, magnesium-rich material that circulation delivered, which would leave the two hemispheres with different crust and different late-stage products from the same original ocean. It also fits the sample data, since far-side Mg-suite rocks show little sign of the late KREEP-rich residue that Apollo counterparts carry.
One landing site and the limits of the evidence
Several caveats come with the idea. Chang’e-6 sampled a single far-side location, and the authors note that one site may not represent an entire hemisphere, so more far-side samples are needed to test the conclusions. The authors argue that convection in the magma ocean fits this sample set better than impacts or other proposed causes, but a model built from one landing site and a handful of rock fragments is a hypothesis to be tested rather than a settled history. The Chang’e-6 basalts themselves are about 2.8 billion years old, much younger than the magma ocean they are being used to reconstruct, so the inference runs through several steps.
What the work does offer is something earlier models lacked: measured far-side chemistry to test against. The next samples, from other far-side regions, will show whether the magnesium-rich crust and the shortage of KREEP hold across the hemisphere or belong to one corner of it.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
The tech news worth your time: Subscribe to Techpresso →
We may earn a commission if you sign up.
More from Morning Overview
- Hurricane Hunter radar shows four warning signs that a tropical cyclone is about to strengthen, a University of Miami study found
- Tropical Storm Rachel is dumping up to 12 inches on four Mexican states on its way to major hurricane strength
- The FTC says Lens.com doubled the price shoppers saw in Google ads
- Common allergy, bladder and sleep pills tied to sharply higher dementia odds
