WHAT NEED TO PRODUCE A STRONG MAGNETIC FIELD?

A magnetic field can be produced by circulating electric charges.

To produce a magnetic field a planet needs :

1.The liquid material in a planet's interior that has some electrical charge that is free to move about.

2.Rapid rotation of the planet to get the conducting material moving about. The faster the planet rotates, the more the material gets stirred up, the stronger the magnetic field.

If the liquid interior becomes solid or if planet's rotation  slows down the magnetic field will weaken. 

Why Venus  does not have a global magnetic field? - It spins very slowly, little convection of its molten interior. According to Magellan and Express spacecrafts data Venus should have a iron- nickel liquid core, which does not circulate. The only magnetic field Venus may have comes from the interaction between Venus's upper atmosphere and the solar wind, which causes electric currents to flow, thus is created a weak magnetic field. Mars spins quickly, but has a solid core, and has a weak magnetic field.  Earth has a  liquid iron-nickel core and spins quickly and has a strong magnetic field.  The four gas giants have  extremely strong magnetic fields. Jupiter and Saturn have a large amount of hydrogen  gravitationally super- compressed, liquid hydrogen. Some of this hydrogen is in a state that some of the electrons are squeezed out of atoms and free to move around. A moving charge will produce a magnetic field. A planetary magnetic field forms a shield protecting the planet's surface from energetic, charged particles coming from the Sun and other places. The Sun is constantly sending charged particles, called the solar wind into the solar system. The magnetic field prevents the planet's atmosphere from blowing away. 

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