There's less water in the mantle beneath the farside of the moon than in the mantle on the nearside, according to studies of the precious lunar samples brought back to Earth by China's Chang'e 6 mission.
Furthermore, the findings bolster the theory that the moon formed when a giant, Mars -size protoplanet slammed into the young Earth about 4.5 billion years ago and sent a cascade of debris into orbit that coalesced into the powdery gray orb we see today.
"We believe the new result is in line with the giant impact hypothesis of the moon," HU Sen, a professor at the Institute of Geology and Geophysics at the Chinese Academy of Sciences, told Massima.
Chang'e 6 arrived at the South Pole–Aitken (SPA) basin , located on the farside of the moon, in June 2024. The lander's robotic drill and scoop collected 4.27 lbs (1.935 kilograms) of basaltic regolith from the SPA basin before returning it to Earth. Chang'e 6's treasure trove of lunar material is the first sample to be collected from the moon’s farside.
As such, it has some questions to answer.
Among them is whether its water content is compatible with the giant impact theory of the moon's formation. Modeling of the giant impact scenario suggests the mantle on the farside of the moon should have less water than the nearside does.
One line of evidence for this comes from the relative abundances of thorium on the lunar surface. Magma is formed from melt within the mantle, and the areas of the mantle that produce such melt are referred to as the "mantle source." When magma is formed in the mantle source, thorium, like water, remains in the melt rather than producing crystalline minerals. Therefore, the abundance of thorium on the surface can act as a proxy for water.
The nearside, covered in ancient deep impacts and volcanic flows, has regions that are rich in thorium, as we would expect. Few areas on the farside, with its dearth of volcanism, have a high abundance of thorium. However, the SPA basin is an exception: as a giant impact basin, its floor is flooded by now-solid lava that welled up through the wound in the moon's surface from the mantle below, like blood oozing from a cut.
As one of the few impact basins on the farside, the SPA basin is one of the few farside locations with thorium on the surface, where the impact that formed the basin dug deep enough into the mantle to bring the melt to the surface.
Therefore, the reasoning follows, if the SPA basin contains lower levels of thorium, this might indicate that the mantle source also has reduced amounts of water. The analysis of basaltic rocks obtained from the SPA basin by the Chang'e 6 mission reveals a mantle source with a water content ranging from merely 1 to 1.5 μg.g⁻¹ (micrograms—millionths of a gram—per gram of sample).
Hu indicated that the modeling outcomes indicate the lunar mantle could contain a maximum of less than 10 μg.g⁻¹ of water. He added that their estimation of 1–1.5 μg.g⁻¹ aligns well with these predictive models.
This estimation is significantly lower compared to the average water content found in the nearside mantle. According to research from Apollo mission specimens, this can be as high as 200 μg.g⁻¹, varying with the specific area on the lunar nearside.
"If the moon’s far side contains less water within its interior compared to the near side, this suggests that the water in the lunar mantle might be unevenly distributed," explained Hu.
This discrepancy, or dichotomy, regarding the variation in water presence within the lunar mantle on the Moon’s far side as opposed to its near side is merely another point to incorporate into the extensive list of discrepancies. differences Among the two halves of the Moon. Topology, volcanic activity, surface age, mineral composition, and the previously mentioned variation in thorium content are some discrepancies that any theory aiming to describe the creation and later development of the lunar body must account for.
"We infer that such a dichotomic distribution of water in the moon may be the result of the massive impact event forming the SPA basin," said Hu. The SPA basin is huge, spanning 1,600 miles (2,500 kilometers ) and is one of the very largest examples of its kind in the solar system — Its impact on the remainder of the moon, especially the far side, would have been immense.
Hu suggested another possibility: the water within the lunar mantle might show a gradual distribution. For example, the deeper layers of the moon’s mantle, which formed earlier, could contain less water compared to the more superficial parts that developed later.
It's certain that one aspect remains clear: Even though the moon's far side is as arid as can be, it persists in captivating us endlessly.
The examination of the Chang'e 6 specimens was detailed in the journal on April 9. Nature .
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