This is an artist's concept comparing the present day magnetic fields on Earth and Mars. Earth's magnetic field is generated by an active dynamo -- a hot core of molten metal. The magnetic field surrounds Earth and is considered global (left image). The various Martian magnetic fields do not encompass the entire planet and are local (right image). The Martian dynamo is extinct, and its magnetic fields are "fossil" remnants of its ancient, global magnetic field.