A myth deeply engrained into popular culture is that the Moon is made of green cheese. The Apollo missions showed beyond reasonable doubt that the moon is made of rock and not, sadly, of one of our favourite dairy products. That being said, data brought back from Apollo and the missions that followed it actually showed that cheese and our natural satellite do share two important characteristics:

Sound speed: Apparently, the speed of sound waves within the shallow lunar subsurface is much slower than that in solid rock and, in fact, close to the sound speed in cheese. This is because these near-surface rock layers have been pulverised by aeons of meteoritic bombardment and have become porous. The more porous the material, the harder it is for sound waves to propagate through it. Whereas the sound speed in basalt, a common volcanic rock on both Earth and the Moon, is 5.8 km/s, the figure for the lunar near-surface layer is 1.2 km/s which compares well with the sound speed in Cheddar (1.7 km/s).

Holes: The moon is literally peppered with enormous bowl- or saucer-shaped impact craters but it also features smaller holes reminiscent of those found in many types of cheese eg in Emmental. These lunar pits as they are called, have been caused by ancient rivers of lava carving tunnels just beneath the lunar surface more than a billion years ago. The same process is responsible for lava tubes and skylights found near Earth’s volcanoes. Once the lava drains from these tunnels, the ceiling would collapse revealing the empty space underneath.

Top Left: A lunar pit in Mare Tranquilitatis. Inset, Right: Oblique view of the pit opening, showing the overlying rock layers. Bottom Left: A skylight at El Malpais National Monument, New Mexico, USA

The existence of lunar pits was unknown to the Apollo astronauts and scientists; the first such pit was found by the Japanese Kaguya spaceraft about 10 years ago and orbiting probes have now found more than 200 pits all across the Moon. Most are some tens to a few hundred metres wide, no larger than a city block. They are attractive destinations for human exploration because they offer natural shelter from space hazards such as meteoroid strikes, temperature extremes and solar radiation storms.


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