From the article...
Planetary magnetic fields are created by massive molten metal currents within the planet's core. A flowing current creates a magnetic field, even when the current is massive volumes of charged liquid metal moving under the influence of temperature gradients (convection) - in fact, especially then. But magnetic analysis of Martian sites by Berkeley researchers show that the red planet's protective field was switched off half a billion years ago, and now some scientists say they know why.
John Hopkins University scientists have calculated that a period of massive asteroid impacts, known to have happened around the same time, could not only have massively impacted on the surface Deep Impact-style (with all the atmospheric alteration and great-big-crater-making that entails) but added enough energy to the planet to heat up the outer layers of the planet.
Without the huge temperature difference between the core and mantle, the mega-magnetic dynamo convection currents would be switched off - and unable to start up again when things cooled down. Remember, planetary core behavior is still carrying on from when the planets first formed - as far as they're concerned the whole "crust" thing and all life as we know it is just a cooling scum on the surface. If you break something from back then you just don't have the juice to start it up again.
Without the magnetic field Mars is defenseless against the radiation that constantly pours in from space (never mind the Fantastic Four, the only superpower cosmic rays'll give you is decomposition). Earth is thought to have survived the same space-bombing because of our superior size, with our dynamo maybe stuttering a little but - very importantly - not stopping.
Link: Article
"Did Mars's Magnetic Field Die With a Whimper or a Bang?"
Zala
ScienceNOW
30 April 2009
Link: ScienceNow Article
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