Magnetic fields of the planets

[RTFishall]

Why don't Venus and Mars have magnetic fields whereas the Earth and Mercury do?

[thundercrack]

don't know for sure, but is it because they have no iron?

[RTFishall]

thanks for response thundercrack.
But isn't it odd that both Mars and Venus seem to have no iron core but Mercury does. Also, I always thought one of the reasons why Mars was so red was because of iron.... very strange.

[Teddio]

Magnetic fields are created by moving electric charges (ie. an electric current) The Earth has a current flowing in its metallic core which creates the Earth's magnetic field.
Small-scale local magnetic fields are produced in the Earth's crust by rocks with permanent magnetism such as magnetite (iron oxide)
Moving plasma, such as in the Sun, also create magnetic fields.

[heathfield]

It's considered that a solid core surrounded by a liquid mantle are necessary for the 'dynamo' effect that Tonyted mentions.
However, Mars is reckoned to have a liquid core, and Venus a solid core but no liquid mantle, so they can't generate a magnetic field.
Our Moon is another curious case, having no overall magnetic field, yet it does have many areas of high magnetic intensity.