What’s the Moon made of – Cheese

This article explores what the Moon is made of - not cheese, in case you were wondering. The complex nature of its composition is quite amazing, as is its similarity to the Earth’s chemical makeup.

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Like most stuff in the Solar System (that isn’t the Sun) the Moon is made of rock and lots of it. Aside from the weird stuff that happens under Jupiter’s enormous gravity rock kind of behaves relatively similar for most things in the Solar System. It clumps together and squashes under enourmous pressure that melts the inside and the heavier elements tend to make their way to the middle of the object. The Moon is fairly unremarkable in its make up and in this short article we’ll have a look at what it is made of – and I can assure you it is not cheese – yet.

The surface of the Moon is basically dust and boulders. This was caused by the continual pounding of the surface over billions of years of impacts from meteorites. The surface has been smashed down into the grey regolith that covers the entire surface of the Moon and caused the fine grain dust that annoyed astronauts so much when they went there. There’s no atmosphere on the Moon so nothing to impede even the smallest meteorite so even tiny grains of dust impact the surface, whereas on Earth they would burn up in the atmosphere. The regolith varies in depth from about 2 metres in the Maria regions to about 20m in the older highlands. Underneath this regolith is another layer called the megaregolith, which is basically the fractured bedrock of the Moon. Fractured because the huge number of impacts over the life of the Moon, this bedrock froms the bulk of the crust which extends some 50km (though there’s plenty of variability is how thick scientists think the crust is) on the near side and considerably thicker on the dark side. There is some evidence of past volcanology on the Moon but there haven’t been any active volcanoes for millions of years. The crust of the Moon contains silicon, oxygen, magnesium, iron, calcium and aluminium and lunar soil is mostly oxygen, silicon and iron with smaller amounts of the other elements mentioned.

The dark side of the Moon (Credit: NASA, LRO)

The surface has the characteristic highlands of a lighter colour and the darker coloured Maria lowlands. The difference is because the darker areas are slightly younger than the highlands due to the early volcanic activity on the Moon. An observer would notice that the highlands are more heavily marked with craters than the darker lowlands indicating the difference in age. The striking features of the lunar surface are the huge numbers of craters. These are the result of billions of years of bombardment of comets and meteorites, there were from different periods in the early Solar Systems including some times with increased bombardments leading to what has been a relatively stable lower frequency of strikes over the last couple of billion years. The impact craters are an area of study all of their own and over the years have turned up some fascinating things about impact craters and the mechanisms that occur when they form. One of the most interesting things is that a circular crater forms even when the meteorite strikes at angles as low as 5 degrees. This is due to the huge amount of energy that is released as the meteorite is often travelling in excess of 15km/s. The strike causes a wave in the surface of the Moon and then the energy ejects the material at the site of the impact, another process involving waves bouncing around leaves the characteristic mountains that are often seen in the middle of lunar craters.

Some of the different surface features of the Moon (Credit: NASA, Apollo 15)

Underneath the crust is probably an area of molten iron that leads to a solid iron core. Lunargeologists think that this core might be about 450km in radius which is quite small for a body the size of the Moon and indicates the lower amount of metal in the Moon as compared to the Earth. Other than the missing iron, both the Moon and the Earth are very similar chemically. There are small perturbations in the orbit of the Moon and these might be explained by an amount of the core being still molten. The Moon also has an extremely weak magnetic field, probably from residual magnetisation of the crust from some long dead geodynamo that may have crated a stronger magnetic field in the Moon’s past. The make up of the Moon is supported by the theories of how it formed, in that it doesn’t have much in its composition from the inner regions of the Earth – the more heavier metals, indicating that the impact that formed the Moon probably involved mainly the outer layers of the Earth.

Unfortunately for those who have designs on the Moon for the extraction of metals it might not be a very lucrative prospect but there may be enough material that could be mined to support the building of a Moon base or even the construction of spacecraft. The only cheese that the Moon has will be what we fly to it – possibly a good 1Kg block of Colby from the supermarket.