The Moon's far side: what it hides

Real geological differences

When the Soviet Luna 3 sent the first images of the far side in 1959, the surprise was immediate. The two sides are quite different:

• Thicker crust. On the far side, the crust averages 50 km in thickness, against about 30 km on the near side.
• Almost no maria. The maria (dark lava plains) cover about 31% of the visible side, but only around 1% of the far side.
• Far more craters. The surface is dominated by highlands saturated with old impacts.

Why this asymmetry? Hypotheses compete:

1. Uneven heating during formation. If the Moon formed close to Earth, the side facing here would have been heated by the still-hot young planet's radiation, melting material and generating a thinner crust more prone to later volcanism.
2. Giant impact on the northern hemisphere of the far side. Some simulations show that a large early impact could have thickened the crust on the opposite side.

The question is still open.