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Figure 6.17 Internal structure of the Moon.

Below the crust is the Moon's mantle. The Moon's mantle probably makes up most of its interior. Unlike the Earth's mantle, however, the Moon's mantle is only partially molten.

The mantle is solid down to a depth of about 800 to 1000 km. The composition of the upper mantle may be deduced from the composition of the mare lavas, which came from these regions. Below about 1000 km the mantle becomes partially molten. Evidence for this came mainly from seismic data collected when a large meteorite weighing about 1 tonne hit the far side of the Moon in July 1972. At the centre of the Moon there may be a small iron-rich core perhaps only 800 km in diameter, but its existence is uncertain.

Analysis of Moon rocks shows no evidence for formation in a different part of the solar system from Earth. There is some evidence to support the theory that the Moon may have been once part of the Earth. The bulk density of the Moon is close to that of the silicate mantle of the Earth; however the bulk composition of the Moon is different from that of Earth's mantle. The Moon as a whole contains a higher proportion of iron and a lower proportion of magnesium than the Earth's mantle. The Moon also has a lower proportion of lead but a higher proportion of calcium, aluminium and uranium than Earth.

Information from the Moon rocks supports an 'impact theory' for the formation of the Moon. In this theory a large object, about the size of Mars, hit Earth in its first 100 million years of life. This collision ejected a lot of rock material from the Earth's surface into orbit, forming a 'debris ring'. This ring gradually condensed into the Moon. Such an impact could also have tipped the Earth off its axis and so created the seasons.

The surface

There are two main types of terrain on the Moon, the heavily cratered and very old highlands or terrae, and the relatively smooth and younger plains or maria (singular mare). These can be identified by the naked eye. Most of the surface is covered with regolith, a mixture of fine dust and rocky debris produced by meteor impacts. This layer ranges in thickness from 1 to 20 metres. Unlike Earth's soil, which has decayed biological matter in it, the Moon's regolith does not contain any biological matter.

The maria make up about 16 per cent of the Moon's surface and are sometimes called 'seas' even though they contain no water (mare means 'sea' in Latin). Maria are huge impact basins that have been covered by molten lava. Most maria exist on the side of the Moon facing Earth. The more prominent maria are Mare Tranquillitatis (Sea of Tranquillity), Mare Nubium (Sea of Clouds), Mare Nectaris (Sea of Nectar) and Mare Serenitatis (Sea of Serenity). The largest mare, Mare Imbrium (Sea of Showers), is circular and measures 1100 km in diameter. Like most maria, it is 2 to 5 km below the average lunar elevation.

Rocks brought back to Earth from the maria are solidified lava (mainly basalt), which suggests that the Moon's surface was once molten. These rocks have a composition similar to those found in volcanic rocks on Hawaii or Iceland - they contain heavy elements like iron, manganese and titanium. The molten lava has come from inside the Moon and has risen to the surface through large impact fractures in the crust.

Figure 6.18 Moon rock. (Photo: J. Wilkinson)

There is also some tectonic activity in the maria caused by the weight of basalts pushing on the crust. At the edges of the maria, the basalts are stretched, causing fracturing and faulting. In the interior of maria, the basalts are compressed, resulting in folding that produces wrinkle ridges. Most maria also contain small craters and occasional cracks (lava tunnels or channels) called rilles. In the highlands, tectonic activity has produced small scarps.

Lunar probes have shown that the far side of the Moon contains one prominent mare, Mare Moscoviense, and is heavily cratered. The cratered area on the far side is 4 to 5 km higher than the average lunar elevation.

The Moon's surface is covered with meteorite impact craters that vary in size from tiny pits to huge craters hundreds of kilometres in diameter. Virtually all the craters are round and the result of meteorite impact. Some of the craters have rays or streaks extending outwards from their centre, while others have raised peaks at their centre. These peaks occur because the impact compresses the crater floor so much that afterwards the crater rebounds and pushes the peak upwards. As the peak goes up, the crater walls collapse and form terraces. One of the most striking craters with a central peak is Copernicus. Copernicus crater is about 92 km across and 800 million years old. Rays are often formed when material is ejected and scattered across the surface during large impacts. The most striking crater with rays is Tycho, which formed about 109 million years ago.

Most of the craters on the near side are named after famous figures in the history of science, such as Tycho, Copernicus and Kepler. Other craters bear the names of philosophers such as Plato and Archimedes. Features on the far side have modern names such as Apollo, Gagarin and Korolev, with a distinctly Russian bias, since the first images of the far side were obtained by Luna 3. The largest impact basin (crater) is the Aitken basin, 2500 km wide and 12 km deep, at the south pole on the far side. The Imbrium basin, about 1800 km wide, and the Crisium basin, about 1100 km wide, are both found on the near side. The Orientale basin, 1300 km wide, located on the western limb, is a splendid example of a multi-ring crater.

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