Life on Earth, as we know it, is only possible because Earth has a magnetic field that protects us from solar wind.  The magnetic field is the reason we have an atmosphere and vast amounts of liquid water.  But our magnetic field is not constant.  It’s polarity and strength change over time.  Compasses show that the Magnetic North Pole drifts around the Arctic over time.  Mars lost its magnetic field around 4 billion years ago. Why?  What was so different in the early formation of Earth and Mars that Earth maintained its protective shield?