spinning molten iron core per se is not reason for Van Allen, spinning liquid is (our
sun, Jupiter, etc). and the faster it spins, the stronger the field.

It is my understanding that the earths liquid core was there before collision, and is
fueled by uranium breakdown.

PS: I am a biologist trapped in the physics department.
http://weloveteaching.com/author/author1.htm


I think it has more to do with the fact that the earth has a molten iron
core (which spins), while it is thought that Mars does not. Speculation
is that when a mars-sized planet collided with the earth ~3 billion
years ago forming the moon we have, it melted the earth. This allowed
the denser iron to sink to the middle. Since no such collision
occurred for mars (it's moon is very small, basically a captured
asteroid), it never had the opportunity to have an iron core.


PS: I may be a computer geek, but working in a physics department with
my office near the astronomers is fun. ;-)