An Asteroid Impact May Explain Our Lopsided Moon

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OhOur moon is oddly unbalanced – the near and far sides are quite different.

The side we see has a thinner crust than its neighbor, about 25 miles thick, while the crust on the far side is almost 40 miles thick. The side we see is also covered with immense plains of basalt rock formed by ancient lava flows, while the far side is lighter and dotted with craters, as we discovered in 1959 from images from the Soviet Union’s Luna 3 space probe.

Scientists aren’t sure how the two faces acquired such different features, but they have offered a few possible explanations. Some teams attribute it to the early days of the Moon, when it was still an ocean of magma. At the time, the far side could have been much colder than the near side: it was experiencing intense heat from our young neighboring planet, which was also made of sizzling magma at the time. This temperature difference may have sent more crusting crystals toward the far side.

The researchers also suggested that an asteroid impact may have triggered the moon’s imbalance. Today, samples of lunar rocks and soil appear to confirm this theory.

Read more: “The violent birth of the Moon”

The Chinese Chang’e-6 mission collected specimens from the South Pole-Aitken basin on the far side of the Moon, the first samples from this hemisphere ever brought back to Earth. The basin covers almost a quarter of the Moon’s surface. With a diameter of more than 1,550 miles, it’s roughly equivalent to the distance between Waco, Texas, and Washington, DC. It is also incredibly deep, about 6 miles on average. In fact, the South Pole-Aitken basin is the largest impact crater ever discovered in the solar system.

Given the dizzying depth of the basin, researchers wondered whether the accident had changed the composition of the Moon’s interior. A team from the Chinese Academy of Sciences took a close look at four tiny pieces of basalt rocks from the far side, which contain materials from the lunar mantle, and compared them with samples from the near side of the Moon collected during the previous Apollo and Chang’e-5 missions.

They measured the samples’ potassium and iron isotopes, which are atoms of the same element with varying atomic masses due to their distinct numbers of neutrons. Scientists found that potassium isotopes in samples from the far side were much heavier than those from the near side, while iron isotopes were only a little heavier, according to results reported in a report. Proceedings of the National Academy of Sciences paper. Although volcanic activity explains the differences in iron isotopes, the potassium results suggest another process is at play.

When potassium is heated and turned into vapor, the lighter atoms evaporate and the heavier isotopes remain. Scientists believe that these heavy isotopes of potassium indicate that “the [asteroid] “The collision caused the interior of the moon to boil,” according to a statement. This may have directed potassium and iron toward the moon’s nearer side, causing the volcanic activity that now marks its surface.

To confirm this theory, scientists need to examine additional pieces of moon dust. But their preliminary results suggest that “large-scale impacts are key factors in shaping mantle and crustal compositions,” the authors write.

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Main image: NASA/JPL-Caltech

• Molly Glick

  Published on January 13, 2026

  Molly Glick is the newsletter editor of Nautilus.