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Chang’e-6 lunar samples returned from the farside of the moon in 2024 challenge the long-held scientific theory of the Late Heavy Bombardment, according to a study published July 23 in Science Advances. Planetary scientists previously believed the solar system experienced a sudden, cataclysmic spike in asteroid impacts roughly 3.9 billion years ago. However, the analysis of 28 impact melt bits from the lunar farside reveals a longer, more gradual period of regular meteorite strikes rather than a single violent event.
Challenging the Late Heavy Bombardment Theory
For decades, standard models of solar system evolution relied heavily on moon rocks gathered during NASA’s Apollo missions and the Soviet Union’s Luna missions in the 1960s and 1970s. Argon isotope measurements from those historical samples consistently dated around 3.9 billion years old. This clustering led researchers to establish the Late Heavy Bombardment hypothesis, which fundamentally shaped theories regarding planetary formation and the emergence of life on Earth.
However, those early missions landed exclusively on the nearside of the moon, within vast plains of solidified lava known as mare. According to Wanfeng Zhang, an engineer at the Guangzhou Institute of Geochemistry in China, a massive impact that carved out the Mare Imbrium basin roughly 3.92 billion years ago spread debris across much of the nearside. This catastrophic event effectively masked the true impact history of the moon by contaminating the collection sites with ejecta from a single localized cataclysm.
Analysis of Chang’e-6 Farside Samples
To bypass the distortion caused by the Mare Imbrium basin, researchers examined pristine material recovered from the moon’s unexplored farside by China’s Chang’e-6 mission. The team analyzed 28 bits of impact melt—rocks liquefied by the extreme heat of hypervelocity impacts—retrieved from the farside’s Apollo impact basin.
By measuring the radioactive decay of argon isotopes within the melt fragments to determine when the rocks last cooled, Zhang and colleagues mapped ages ranging from 4.33 billion to 1.13 billion years old. The resulting age distribution shows no sharp statistical cluster around 3.9 billion years ago. Instead, the data point toward a steady, extended timeline of meteorite bombardments.
“The lunar farside bombardment was not dominated by a cataclysmic spike,” Zhang states.
Scientific Debate and Verification Challenges
Independent researchers emphasize that interpreting impact cadence from a single landing site remains challenging. Planetary scientist Simone Marchi of the Southwest Research Institute in Boulder, Colorado, notes that while earlier datasets are indeed affected by the Mare Imbrium bias, acquiring additional samples from diverse lunar regions is necessary to solidify the revised timeline.
Other specialists raise technical questions regarding the classification of the analyzed samples. The samples reportedly lack specific morphological indicators typically associated with impact melt generation.
Despite these methodological debates, Harrison notes that the accumulation of new geochemical data is altering the broader consensus among planetary scientists. “The mood in the room has changed,” Harrison says. “The paradigm is shifting, or has shifted.”
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