The Chang’e 6 spacecraft with its sample collection arm extended was photographed on the Moon by a small rover deployed by the lander. Credit: CNSA
China’s lunar exploration program continues to reap impressive scientific results. Their methodical progression of lunar orbiters, landers, and sample return missions shows a coherent national commitment to exploring and understanding the Moon in the 21st century. Unlike the first American and Russian lunar probes at the dawn of the space age, which often ended with spectacular fireballs in the sky, China’s lunar program has benefited from the maturity of spacecraft and rocket technology. The Chang’e series of spacecraft (named after the mythological Chinese goddess of the Moon) have achieved spectacular successes and provided insights into the Moon’s geological past.
In a historic first, the Chang’e 6 sample return mission earlier this year returned 4.27 pounds (1,935 kilograms) of soil and rock samples from the Moon’s far side. This first sampling of the far face revealed intriguing evidence of long-term volcanism on the Moon. Previous Apollo missions and the Russian Luna mission and China’s uncrewed Chang’e 5 mission returned samples from the Moon’s near side.
The exciting results of the analysis of the first samples returned from the hidden side were announced by QWL Zhang and others in an article published on November 15 by the prestigious scientific journal Nature. The Chinese team studied 108 basalt fragments from two small samples of lunar soil collected by the robotic arm on the Chang’e 6 lander on June 2, 2024 and returned to Earth later that month.
By studying the decay of isotopes in the samples, scientists found that most of the samples were about 2.8 billion years old. But surprisingly, a fragment of basalt formed about 4.2 billion years ago.
The hidden enigma
When comparing the near and far sides of the Moon, one wonders whether they are from the same world. Vast slabs of basalt, the dark volcanic rock that creates the caricature of “man on the moon,” cover just 18 percent of the moon’s surface. The near side hosts 93% of the basalt fields while only 7% are found on the far side. The recovery of the far-side basalt from the Chang’e 6 mission represents an exciting opportunity to study a rare part of the far-side of the Moon.
Before Chang’e 6’s daring landing, the chemistry of the far face could only be deduced from remote sensing from an orbiting spacecraft. Since NASA’s Lunar Orbiter missions mapped the far side in the late 1960s, scientists have understood that the visible side of the Moon differs markedly from the visible side. The far side lacks large seas like the near side, contains larger basins, and the far side’s crust is thicker.
Chang’e 6 was aimed at the southern area of the Basaltic Sea within the Apollo Basin, which in turn lies in the northeastern part of the South Pole-Aitkin Basin which extends across much of the southern hemisphere of the far side . The basalt within the Apollo basin lies 3.4 miles (5.5 kilometers) below the mean lunar elevation and represents one of the lowest points on the Moon. This low elevation allowed ancient volcanism to breach the normally thicker outer-side crust and present rare samples of far-side basalt.
Putting the Chang’e 6 sample return results into modern lunar perspective, planetary geologist and Brown University professor emeritus James Head told CNN in June, “The enigmatic visible side of the Moon is so different from the visible side of the Moon in so many ways, that without returned samples, lunar scientists cannot fully understand the moon as an entire planetary body. The samples returned from Chang’e 6 will make great strides in solving these problems.”
A key discovery was Chang’e 6’s recovery of a fragment of high-aluminum basalt that dates back 4.2 billion years and is the oldest known sample of KREEP basalt, or lightly radioactive basalt containing a mineral compound of potassium (K), rare earth elements (REE), and phosphorus (P). Previously, KREEP basalts were only detected in Oceanus Procellarum and Mare Imbrium on the near side.
To explain why KREEP is so localized on the visible side, some astronomers have suggested that another, smaller Moon formed after the proto-Earth collided with a Mars-sized object about 4.5 billion years ago. It is widely accepted that our current Moon formed from debris from the collision. But the “great splat” hypothesis held that the second, smaller moon soon morphed into the current larger moon, and its material created the thicker crust on the far side as the impact shock pushed radioactive material onto the surface. surface of the Procellarum basin which now cradles the current Oceanus Procellarum and Mare Imbrium. The discovery of KREEP basalts on the far side of the Moon requires a reevaluation of the idea that the impact of a second, smaller Moon thickened the far-side crust and moved KREEP material toward the visible side. If the KREEP basalts are indeed native to both the visible and visible sides, the second moon hypothesis could join the growing list of now-disproven ideas about lunar evolution.
Part of the challenge is understanding both the physical extent and time periods of volcanism on the Moon. The Apollo, Russian Luna and Chang’e 5 samples showed that nearby volcanism occurred as early as 4 billion years ago and as long as 120 million years ago. But to date only Chang’e 6 has sampled farside basalts.
Other basalt samples recovered from Chang’e 6 are 2.8 billion years old and lack KREEP. The implication is that over the 1.4 billion years between the eruption of the distant KREEP-rich sample and the subsequent KREEP-poor samples there was a fundamental change in the source of the magma fueling the eruptions. The smaller a planetary body is, the faster it cools after its igneous formation. As heat-inducing radioactive elements are depleted, other magma-producing heat sources must be present. This calls into question why a small body like the Moon has remained hot internally for almost its entire existence.
More data coming soon
A sample from a single location on the far side is far from definitive evidence for any idea about the strange dichotomy between the near and far sides of the Moon. But the Chang’e 6 samples add another piece of data that will contribute to the final understanding of the Moon’s evolution. Because impact scattering can distribute basalt samples thousands of kilometers from the source, care must be taken to ensure that a local basalt sample is native to the location sampled. Fortunately, the 4.2 billion-year-old KREEP-rich basalt sample from Chang’e 6 provides further clues to its original location. The sample displays a pristine magmatic structure and does not possess any impact-induced fractures, leading to high certainty that the ancient KREEP-rich basalt sample indeed originated within the Apollo Basin.
NASA’s Commercial Lunar Payload Services (CLPS) program promises to deliver more scientific instruments to the Moon in the near future, with three missions planned for external exploration. These missions promise to deliver more data points on the far side of the Moon; this should allow scientists to formulate more refined ideas about the evolution of the hidden face that can be tested by subsequent missions
The Moon slowly reveals its secrets, but science is persistent and soon international explorations will solve the riddle of the Moon’s enigmatic far side.
