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Asteroids Can Have Magnetic Fields

It has been found that asteroids can have magnetic fields. This was quite a surprise. Planets have magnetic fields due to a moving molten metal core. Mercury has a very strong field. The reason for this is not yet known.

Vesta is a large asteroid with a diameter of 525 kilometer. It was first identified in 1807 by Heinrich Wilheim Olbers. It is the most easily seen asteroid from Earth. Smaller asteroids hit Vesta and some of the debris falls to Earth. The fallen "rocks" can be linked to Vesta because they have the same spectral color match.

Vesta is unusual for an asteroid as it once had a crust, mantle and molten metal core. The orbiting dawn spacecraft/satellite confirms this. It is now frozen and no longer active. A meteor named Alan Hills A81001 composed of Vestan crust fell to Earth in the Arctic. When it was analysed it was found to have a weak magnetic field.  The original crust gained  a magnetic field when it cooled 3.7 billion years ago. An impact a billion years later hit the frozen crust. This melted and refroze some crust which resulted in magnetism being transferred, imprinted, on rock which became the Alan Hills meteor.
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