Meteorites are fragments of asteroids and other bodies that fall to Earth; the main types (stony, iron and stony-iron) are thought to sample the interiors of broken-up rocky bodies, so they give evidence for the composition of the Earth's deep interior; iron meteorites support an iron-rich core and stony meteorites a silicate mantle, because we cannot sample the deep Earth directly.

What this dot point is asking

Eduqas wants you to explain what meteorites are, to name the three main types (stony, iron and stony-iron) and what each is made of, and to explain how they give evidence for the composition of the Earth's deep interior. The key argument is that we cannot sample the deep Earth directly, so meteorites, thought to be fragments of broken-up rocky bodies like the early Earth, act as samples of the kind of material the interior is made of: iron meteorites support an iron-rich core and stony meteorites a silicate mantle.

The answer

What meteorites are

A meteorite is a piece of rock or metal from space that survives its fall through the atmosphere and lands on the Earth's surface. Most meteorites are fragments of asteroids, and a few come from the Moon or Mars. Many asteroids are the broken-up remains of small rocky bodies that formed early in the Solar System, at the same time as the Earth and from the same kind of material.

The three main types

Meteorites are classified by what they are made of:

• Stony meteorites are made mainly of silicate minerals (rock). They are the most common type and resemble the rocks of the Earth's mantle.
• Iron meteorites are made mainly of iron and nickel metal. They are dense and metallic.
• Stony-iron meteorites are a mixture of silicate minerals and iron-nickel metal.

Why the types matter: sampling a broken planet

Early in the Solar System, bodies large enough to heat up differentiated: the dense metal sank to form a metallic core and the lighter silicates formed a rocky mantle around it, exactly as the Earth did. When such a body was later smashed apart by collisions, its fragments sampled different depths:

• fragments from the metallic centre became iron meteorites;
• fragments from the rocky outer part became stony meteorites;
• fragments from the boundary between the two became stony-iron meteorites.

So the three meteorite types are thought to be samples of the core, mantle and core-mantle boundary of broken-up planet-like bodies.

Evidence for the Earth's interior

This is the link Eduqas is after. We cannot drill to the deep Earth: even the deepest boreholes barely scratch the crust, and the core lies thousands of kilometres down. Meteorites give us actual samples of the kind of material that makes up a differentiated rocky body like the Earth:

• Iron meteorites show that broken bodies had iron-nickel metallic cores, supporting the idea that the Earth has an iron-rich (iron-nickel) core.
• Stony meteorites show silicate outer layers, supporting a silicate mantle around that core.

Combined with other evidence (the Earth's overall density and the behaviour of earthquake waves), meteorites help confirm the layered, iron-core, silicate-mantle structure of the Earth.

Examples in context

Example 1. Iron meteorites in museums. The large iron meteorites on display are slices of the metallic cores of asteroids. Their iron-nickel composition is direct evidence of the metal-rich centres that differentiated bodies, including the Earth, possess.

Example 2. Meteorites and the age of the Earth. Because meteorites formed at the same time as the rest of the Solar System, radiometric dating of them gives the age of the Earth (about 4.5 billion years), which we cannot get from Earth rocks alone because the oldest crust has been recycled.

Try this

Q1. Name the type of meteorite made mainly of iron and nickel. [1 mark]

• Cue. An iron meteorite.

Q2. Explain why meteorites are useful evidence for the Earth's deep interior. [2 marks]

• Cue. We cannot sample the deep Earth directly, and meteorites are fragments of differentiated bodies, so they provide samples of core-like (iron) and mantle-like (silicate) material.

Q3. State what an iron meteorite suggests about the composition of the Earth's core. [1 mark]

• Cue. That the core is iron-rich (made of iron and nickel metal).