Metals from space

Iron meteorites are natural alloys of iron and nickel. Their structure, strength and crystal patterns show metallic bonding and slow cooling on a scale no laboratory can match.

Metallic bonding

In a metal, positive ions are arranged in a regular lattice surrounded by a 'sea' of delocalised electrons. The strong attraction between ions and electrons gives metals high melting points. The electrons can move, so metals conduct electricity and heat. Layers of ions can slide, so pure metals are malleable.

In an alloy, atoms of different sizes distort the layers, making it harder for them to slide. Alloys are therefore usually harder than pure metals. Meteoritic iron, an alloy containing nickel, is tough enough that it was hammered into tools, beads and blades thousands of years before people could smelt iron.

Patterns from slow cooling

Iron-nickel metal can exist in two crystal forms: taenite, which is richer in nickel, and kamacite, which has less nickel. In an asteroid's core, the metal crystallised as taenite. As the core cooled, kamacite slowly grew as plates inside it, along particular directions in the crystal.

When a slice of an iron meteorite is polished and lightly etched with dilute acid in a laboratory, these plates show up as a criss-cross of bands called the Widmanstätten pattern. It only forms with extremely slow cooling, from a few to thousands of degrees per million years, impossible to reproduce in a factory. A Widmanstätten pattern is therefore strong evidence of a meteorite.

Weathering

Iron-nickel metal rusts. Iron meteorites that land in damp places slowly oxidise, and chlorine from the soil speeds the process up. Museums store them in dry conditions to stop them crumbling.

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