Asteroids - Types

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C-type, S-type and M-type asteroids

In 1975, three astronomers – Clark Chapman, David Morrison and Ben Zellner – characterised asteroids depending on their composition by using spectroscopy, and put them into three main groups. The first group are the C-type asteroids. These are made from so-called carbonaceous materials, which are rich in carbon-based compounds. C-types are the most common, constituting around 75 percent of all asteroids in the Asteroid Belt, and are most numerous in the middle to outer regions of the Belt. They are generally coloured a ruddy, dark hue, reflecting less than 10 percent of the sunlight that falls on them, and they also contain a large amount of water molecules, but hardly any metallic elements.

The next type of asteroid is the S-type. The ‘S’ stands for the rocky silicate minerals that make up the majority of their composition – also large amounts of iron, magnesium and nickel are present, but barely any water, in contrast to the C-types. They are also a little bit brighter than C- types, and constitute 17 percent of the asteroids in the Asteroid Belt.

The third and final major type of asteroid is the M-types. Here, the ‘M’ stands for metals, as M- types contain more metallic elements than other types of asteroids. They also contain rare metals, such as platinum. Indeed, a large amount of the platinum on Earth comes from a giant impact crater called the Sudbury Basin in Canada, where a giant M-type asteroid struck the Earth 1.8 billion years ago.

The geology of asteroids can tell us a great deal, not only about conditions in the Solar System when they formed, but also what has happened to the asteroids since then. The typical composition of many smaller asteroids is similar to a loose jumble of rock and dirt. Add sufficient gravity and over time and the various types of material will begin to settle, with the largest fragments sinking to the bottom with the lighter and smallest particles filtering to the top. This is what has happened to a few asteroids, such as Ceres, Pallas and Vesta, which grew large enough for their material to become layered – all the heavy iron sank to their centres, while the lighter, less dense material was nearer the surface. This process could only happen if the asteroids were hot enough for their rock to be soft and molten, a bit like the molten lava that spews from volcanoes. The heat for this may have come from their formation, but also from radioactive rocks deep within the asteroids.

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