Studying the chemistry of meteorites can help answer fundamental questions about the Solar System such as:
- How old is the Solar System?
- What sort of materials did the Solar System form from?
- What were the chemical and physical conditions like in the early Solar System?
- How do asteroids and planets form and evolve?
Meteorites are pieces of rock and metal that originate on large extraterrestrial bodies. At some point in their history they were detached from their parent planet or asteroid and fell to Earth. Thousands of tonnes of extraterrestrial material falls to Earth every year but only a small fraction of this consists of pieces large enough to recover. Many meteorites are recovered from deserts or ice fields as here they are easy to distinguish from their surroundings. Most meteorites found come from asteroids but a few are from the Moon and Mars.
Meteorites differ in chemical and physical composition and can be grouped into three broad categories: stony, iron and stony-iron. There is great variation within all these groups which means lots of information about the solar system can be gathered from different meteorites. We use cosmochemistry to understand the origin and evolution of these materials. Cosmochemistry involves analysing the different chemical species in extraterrestrial samples to understand the chemical processes that have taken place. We can measure the abundances of chemical species to understand what sort of material the meteorite formed from and how it formed. We can also use radioactive material found in meteorites to calculate how old they are. Knowing how old different materials are allows us to construct a timeline of the processes and events that occurred during the history of the Solar System.
Stony meteorites are composed mainly of silicate minerals. Silicate minerals are the largest group of rock-forming minerals and make up most of the Earth’s crust. Stony meteorites are further divided into two main groups: chondrites and achondrites.
Chondrites have never experienced large-scale melting and therefore retain some of the most primitive Solar System material. They have bulk chemical compositions similar to the Sun and are thought to be most representative of the material the solar system formed from. Inclusions of silicate material, rich in calcium and aluminium are found in many (but not all) chondrites. The ages of these inclusions have been calculated and shown to be ~4.56 billion years old. This material is thought to be the oldest in the Solar System.
Achondrites have experienced large-scale melting events and include meteorites from the Moon and Mars, as well as asteroids. If rocks reach temperatures high enough they can melt and produce molten rock called magma. Achondritic meteorites are igneous rocks; this means they crystallised (solidified) from magma. As magma cools minerals of different compositions crystallise from the melted material. The less dense material tend to remain close to the surface whilst the denser material (containing heavy metals such as Fe) sinks. This layering process is known as differentiation and is thought to have produced the outer crusts, inner mantles and metal cores of large asteroids and rocky planets such as Mercury, Venus, Earth and Mars.
Iron meteorites are though to come from the cores of differentiated asteroids. They consist mainly of iron-nickel metal. The different minerals in the metal can form a distinctive pattern known as Widmanstatten texture (shown in the picture above).
Stony-iron meteorites consist of a mix of silicate and metal material. It is not yet known whether they originated near the core-mantle boundary or if mantle and core material was mixed together during impact events. Both groups can provide information on how asteroids and planets form and evolve.
All images courtesy NASA/JPL-Caltech.