Why are the Moon's two faces so different?

In 1959 the Soviet probe Luna 3 looped behind the Moon and radioed home the first photographs of the hemisphere no human had ever seen. Astronomers expected more of the familiar Moon. They got a different world. The near side is a third covered in dark plains of frozen lava, the maria that make the face in the full Moon. The far side has almost none, one or two percent, and gravity mapping later showed its crust runs some twenty kilometres thicker. One body, two faces, and no settled reason.

Why it matters

The Moon is the calibration standard for the whole solar system: crater counts on its dated surfaces set the clocks used to age every other planet’s terrain. It is also the nearest example of a planetary split personality, with Mars carrying a hemispheric divide of its own. And the asymmetry decided lunar history. Nearly all the Moon’s volcanism happened on the near side, above a patch of crust strangely enriched in radioactive, heat-producing elements. Explaining the two faces means explaining how the Moon worked.

What has been tried

Ideas span from the violent to the quiet. A second, smaller moon may have drifted in and pancaked onto the far side, thickening its crust. The infant Earth, still glowing from the giant impact that made the Moon, may have kept the near side molten longer so the far side’s crust froze first and thickest. Or the largest confirmed impact in the solar system, the South Pole-Aitken basin on the far side, may have driven a wave of heat through the interior that swept the heat-producing elements to the opposite hemisphere. In 2024 China’s Chang’e 6 returned the first far-side samples, and they sharpened everything: far-side lava erupted as recently as 2.8 billion years ago from a mantle source that is strikingly depleted and unusually dry, its potassium isotopes carrying what looks like the chemical scar of a giant impact.

Where the edge is

No single account yet produces all four asymmetries at once: the crust thickness, the concentration of heat-producing elements, the lopsided volcanism, and the dry, depleted far-side mantle. The samples come from one landing site, so the driest-mantle result may be local rather than hemispheric. And the field is honestly split on whether the dichotomy was stamped in by an impact or baked in as the magma ocean froze.

What would count as an answer

One history of the young Moon that yields both faces from a single sequence of events, with predictions sharp enough to test, and samples from more of the far side to test them on.