Asteroid Crash May Have Demagnified Mars
Irene Klotz, Discovery News

July 24, 2008 -- Scientists don't know what happened on Mars that caused its magnetic field to collapse. They suspect the planet's liquid metallic core cooled, ending convective currents that spread magnetic field lines through the planet's rock and soil and out into space.
But that may just be part of the story.
A team of researchers led by Jafar Arkani-Hamed of the University of Toronto in Canada believe a large asteroid circling the planet set up a gravitational tug-of-war that got Mars' core churning. Eventually, the asteroid lost its grip and crashed into its parent planet. Mars paid a dear price as well. Without the tidal forces, the planet's core lost its momentum, killing off the magnetic field.
What remain are patches of strong magnetic imprints in the oldest parts of Mars' crust. Because the fresher surface features are magnetic-free, scientists believe Mars lost its shield about four billion years ago.
Laboratory tests and computer simulations by Arkani-Hamed and colleagues and published in the Journal of Geophysical Research last month show a large asteroid circling about 46,000 miles above Mars could have had a strong enough pull on Mars to coax its liquid core into moving. The dynamic would have lasted about 400 million years before the asteroid crashed, demagnetizing Mars.