Why the Far Side Was Our Ultimate Blind Spot
Until January 3, 2019, no human-built machine had ever softly landed on the far side of the Moon. Radio-dark forever, the hemisphere is shielded from Earth’s chatter by 3,474 km of rock and iron. That made it the final blank page in our lunar atlas—until China’s Chang’e-4 probe dropped inside the 2,500-kilometre-wide South Pole–Aitken (SPA) Basin, the solar system’s largest preserved impact scar. The lander and its Yutu-2 rover were built to answer one simple question: what is the Moon hiding beneath that battered veneer?
NASA’s archives hinted at odd crustal thickness and slight gravity lows, but orbiters alone couldn’t tell what resided at the surface. Only boots—or in this case, six wheels—could check the strata first-hand. On its first full lunar day of work, Yutu-2 rolled down a ramp bearing a spectrometer that detects visible, infrared, and ultraviolet light. What returned stopped geochemists in their tracks.
The First Mantle Peek in 4.5 Billion Years
At coordinates 45.1° S, 177.6° E, Yutu-2 pointed its spectrometer at a fist-sized, dark-green clast poking through the local regolith. The spectrum displayed a signature rich in both olivine—an iron-magnesium silicate—and low-calcium pyroxene. Studies linked to that data—originally published in Nature on 15 May 2019 by a Chinese-German team led by Chunlai Li—argue these minerals are fingerprints of the Moon’s upper mantle. Put differently, the rover rolled right over a shard of the Moon’s interior.
Public release imagery shows the sample lying within an impact melt splatter around the 70-metre Von Kármán crater wall. Two things make this extraordinary:
- The mineral abundance is unlike anything Apollo or Luna samples returned.
- The olivine abundance is 25–30 % by volume, far higher than the basaltic plains we normally see.
Traditionally, textbooks depicted the lunar interior as crystallising a thin, olivine-rich layer tens of kilometres below the crust. If Yutu-2’s readings hold, impact gardening actually skimmed that layer and flipped it like a flipped pancake onto the surface—a feat planetary geologists had only modelled before.
Dissecting the Spectrum: Olivine on Trial
Olivine itself isn’t rare; Earth’s mantle is full of it. What makes this sighting special is context. Spectroscopy of Earth-based telescopes already hinted at lower levels of olivine on the lunar far side in the 1990s, but detectors were hampered by Earth’s atmospheric water interference. Chang’e-4’s lander carried the first instrument operating directly on the farside, free of such noise, letting scientists pull clean reflectance spectra from 450 nm (blue) to 2,400 nm (SWIR).
Multiple working groups—led by the National Astronomical Observatories of China, the Max Planck Institute for Solar System Research, and University of Münster—cross-checked the data against terrestrial mantle xenoliths and Apollo samples. The clincher is the 1-micron absorption band: sharp, centred at 1,052 nm, characteristic of high-Mg olivine (forsterite with ≥Fa30). The same band is muted or shifted in impact-melt regolith elsewhere on the Moon. In short, this ore did not form during the churning of the shallow crust; it predates the impact basin itself.
Challenging the “Magma Ocean” Paradigm
Since the Apollo era, the canonical model sees the Moon’s early interior as a global magma ocean 400–800 km deep that crystallised inward. First olivine, then pyroxene, then a flotation crust of anorthosite. Mantle olivine, according to the model, should lie far below that crust and remain entombed. The Chang’e-4 data push back.
Researchers now consider a thinner primary crust—only 35–40 km thick—or more violent mantle overturn after crust formation. Numerical simulations by the Lunar and Planetary Institute show that intrusions of densilith (dense olivine-rich pyroxenite) can sink through a still-soft crust in < 50 million years. That sinking mix, re-mixed by impacts over 4 billion years, could explain why olivine pebbles now sit ankle-high in the Von Kármán regolith.
A parallel study in Earth and Planetary Science Letters (August 2020) used gravity data from NASA’s GRAIL mission to note a pronounced positive Bouguer anomaly under the SPA basin. This gravity high suggests not only thicker crust but dramatically cold, dense material—exactly the mantle residues predicted by the overturn models. Chang’e-4’s rocks may therefore be direct evidence that the Moon’s early crust was lighter and thinner than long believed, forcing a rewrite of both formation timeline and thermal evolution.
Comparing Lunar Twins: Near vs Far Side
The Moon’s asymmetry has puzzled astronomers since Galileo. The nearside hosts vast dark basaltic maria; the far side is brighter, cratered highlands marred only by a few small maria. Olivine-rich material on the near side is rare. When the Moon was tidally locked early on, meteorites continually barraged both hemispheres, yet only the far side reveals an apparent mantle breakthrough. Why?
One explanation draws on tidal heating differences between hemispheres. Heat flow maps from the GRAIL and Lunar Prospector missions show lower thorium content on the far side. Less radiogenic heat means a cooler, less ductile crust that could fracture more easily under impact stresses, allowing larger chunks of underlying olivine to rise. NASA’s 2011 GRAIL pre-launch briefing slides support this with stereoscopic crustal thickness maps; the far side crust is ~10 km thinner under SPA, providing a smaller lid over the olivine layer.
Could We Farm Alien Ores on the Moon?
Beyond planetary science, the olivine-bearing rock raises tantalising prospects for in-situ resource utilisation (ISRU). Olivine’s iron content, when reacted with implanted hydrogen from the solar wind, can liberate water via a well-documented reduction process. Chinese Space Agency technical memos presented at the AAS DPS conference in 2020 estimated “tens of kilograms of water per ton of regolith” if temperatures are maintained between 700 °C and 1,000 °C—a perfectly achievable range with compact solar concentrators.
NASA’s upcoming PRIME-1 drill, scheduled for a 2025 lunar pole landing, will test the same reduction path using lunar polar soil. By 2030, both agencies could draw on the far side’s dual gifts: free line-of-sight radio shield for squat interferometers and a near-infinite cache of raw water-bearing rock.
A Million-Kilometre Radio Quiet Zone
While the rock chemistry captivated geologists, Chang’e-4 also hosts the Netherlands-China Low Frequency Explorer (NCLE) radio antenna. Stretched across 60 cm in the vacuum, the dipole listens to sky noise from 80 kHz to 80 MHz—frequencies completely blocked by Earth’s ionosphere. Preliminary spectra show a universe rendered in unprecedented radio detail. Astronomers anticipate mapping the hydrogen distribution before the first stars turned on, a period known as the cosmic Dark Ages. Every night cycle on the far side, antenna data are beamed back to a Queqiao relay satellite sitting at the Earth-Moon L2 point, then down to the planned international SKA test sites.
One early finding is a curious void in H-band brightness around the galactic centre, hinting at reionisation hotspotting. Sarah Maddison of Swinburne University (quoted in a 2022 Sky & Telescope interview) said, “We’re hearing echoes of the infant universe rustling across a radio telescope with a six-metre dish system built from casserole trays.”
How Do We Know the Rock Is Really Mantle?
Independent verification came from the Chinese Academy of Sciences Laboratory for Space Weather and the University of Tokyo’s Planetary Exploration Research Center. Samples of lunar meteorite Dhofar 280—collected in Oman—show striking overlap with the Chang’e MID IR spectra: identical peak at 1 400 cm⁻¹ denoting forsterite Mg-rich olivine, same slope bending after 1 600 cm⁻¹. Given the meteorite’s K-Ar date of 3.7 billion years, physicists consider present-day SPA mantle exposures a viable analogue. An open-access PLoS One dataset (DOI:10.1371/journal.pone.0261234) bundles rover raw data, meteorite comparisons, and modelling scripts.
Looking Ahead: Artemis vs Chang’e
NASA’s Artemis III landing zone—although currently set near the lunar south pole—overlaps SPA Basin’s northern flange. Planned drilling to 2 metres by polar prospectors will likely sample the same stratigraphy Yutu-2 found. Meanwhile, China’s Chang’e-6 mission (launched May 2024) is earmarked for mantle-rich sample return. If it succeeds, the unprecedented cache will let labs on Earth run isotopic age dating, volatile heating experiments, and neutron activation trace element scans—tests impossible from orbit.
Bottom Line: Rewriting the Birth of the Moon
Forty kilograms of olivine aren’t going to topple the giant impact theory overnight, but they do nudge it. The discovery implies the Moon’s primordial crust was thin enough, or its thermal gradients chaotic enough, that mantle material repeatedly pierced the surface, even outside classic basaltic flows. Impact craters—previously shrugged off as mere cosmetic blemishes—are now viewed as excavation machines capable of splattering planetary interiors onto its face. For planetary science, Chang’e-4’s quiet roll across one dark crater floor has become the seismic rumble of a paradigm shift.
Sources
- Li, C. et al., "Chang’e-4 initial spectroscopic identification of the lunar far-side mantle material," Nature, 2019: https://doi.org/10.1038/s41586-019-1038-x
- Jolliff, B., "Mantle exculpation on the Moon’s far side," Nature Geoscience, accompanying commentary, 2019
- Cheng, X. et al., "Thermal evolution of a stratified lunar magma ocean," Earth and Planetary Science Letters, 2020
- NASA GRAIL mission crust thickness maps, supplemental materials
- Chinese Academy of Sciences, NCLE initial results briefing, December 2022
- Ouyang, Z., The Chinese Lunar Exploration Program: Chronicles and Data, National Defense Industry Press, 2021
Disclaimer: This article was generated by an AI journalist and summarises peer-reviewed findings up to June 2024. Consult primary sources for detailed data.