One of the striking things about iron meteorites is that they are often magnetic. Magnetism is not strong, but it gives information about their origin.

This is why astronomers discourage meteorite hunters from using magnets to distinguish meteorites from the surrounding rock, because hand magnets can erase a meteorite's magnetic history, an important scientific record.

Magnetic meteorites occur because they are formed in the presence of a magnetic field. The iron grains in the meteorite are aligned along the external magnetic field, which gives the meteorite its magnetism.

For example, the Martian meteorite known as Black Beauty got its magnetism from the strong magnetic field of young Mars.

Some meteorites are magnetic but must not have formed in a strong magnetic field. Iron meteorites are typically categorised according to their chemical composition, such as the ratio of nickel to iron.

One type, known as IVA, is known to be fragments of smaller asteroids. Small asteroids do not have strong magnetic fields, so IVA meteorites should not be magnetic, but many of them are. There is a new study that shows how this is possible.

Small asteroids are formed by what is known as the rubble pile method. Small pieces of iron-rich rock accumulate over time to become an asteroid.

In order for an object to generate a strong magnetic field, there must be liquid iron to create a dynamo effect, and since small asteroids do not experience this, they cannot have magnetic fields. Or can they?

Asteroids are also subject to collisions over time. The fragments that break off in these collisions become the meteorites we find on Earth. But the authors show that collisions can create a magnetic dynamo inside an asteroid.

If the colliding object is not large enough to shatter the asteroid, but large enough to melt a layer of material near the surface, then a chain of events could occur.

When a cold rubble core is surrounded by a molten layer, the core heats up. Lighter elements vaporise from the core and migrate towards the surface, which churns the layers and creates convection.

The convection of iron creates a magnetic field, and this field leaves its imprint on parts of the asteroid. The collision then creates magnetic fragments, some of which reach Earth.

The magnetism of the IVA meteorites is therefore not due to the original formation of their parent asteroids, but to later collisions that stirred up their cores.

Knowing this, researchers can better understand the history of our solar system and how events such as planetary drift may have triggered more frequent asteroid collisions.

Another reason not to search for meteorites with hand magnets. The act of finding a meteorite could also erase the history of its collisions.

 

Source: https://www.sciencealert.com/