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Some Meteorites Are Mysteriously Magnetic, And We Finally Know Why
SPACE
07 August 2023
ByBRIAN KOBERLEIN, UNIVERSE TODAY
Metal Asteroid In Space
Illustration of the metallic asteroid Psyche. (Peter
Rubin/NASA/JPL-Caltech/ASU)
One of the striking things about iron meteorites is that they are often
magnetic. The magnetism isn't strong, but it holds information about
their origin.

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

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

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

Some meteorites are magnetic but shouldn't have formed in a strong
magnetic field. Iron meteorites are typically categorized by chemical
composition, such as their ratio of nickel to iron.

One type, known as IVA, is known to be fragments of smaller asteroids.
Small asteroids don't have strong magnetic fields, so IVA meteorites
shouldn't be magnetic, but many of them are. There's a new study showing
how that's possible.

Small asteroids form through what is known as the rubble pile method.
Small chunks of iron-rich rock aggregate over time, building up to
become an asteroid.

For a body to generate a strong magnetic field, there needs to be liquid
iron to create a dynamo effect, and since small asteroids don't
experience this, they can't have magnetic fields. Or can they?

Asteroids are also subject to collisions over time. It's these
collisions which break off fragments that become the meteorites we find
on Earth. But the authors show that impacts can create a magnetic dynamo
within an asteroid.

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

When a cold rubble core is surrounded by a molten layer, the core is
heated up. Lighter elements evaporate out of the core and migrate toward
the surface, which churns the layers to generate convection.

The convection of iron generates a magnetic field, which imprints itself
on parts of the asteroid. Later collision then creates magnetic
fragments, some of which reach Earth.

So the magnetism of IVA meteorites comes not from the original formation
of their parent asteroid, but rather from later collisions that stirred
up their core.

Knowing this, researchers can gain a better understanding of the history
of our solar system, and how things such as planetary drift might have
triggered more frequent asteroid collisions.

Yet another reason not to look for meteorites with hand magnets. The
very act of finding a meteorite could also erase the history of its
collisions.

This article was originally published by Universe Today. Read the
original article.