http://www.physics.sjsu.edu/facstaff/becker/physics51/mag_field.htm
Comparative planetology: Magnetic fields.
The Earth has a magnetic field:
http://www.physics.sjsu.edu/facstaff/becker/physics51/mag_field.htm
This magnetic field keeps the most energetic Solar radiation
away from the Earth because charged Solar wind particles
cannot cross magnetic field lines.
(The Earth's magnetic field is caused by a dynamo
effect in the Earth's iron core because the metallic
core is spinning.)
http://spaceweb.oulu.fi/projects/cluster/
This additionally
has the effect of causing ...
what?
Venus has a very, very weak magnetic field despite having
a similar-sized core to the Earth. Why? Because Venus'
rotation is very, very slow -- so the dynamo effect can't
really take hold.
Mars does not currently have a magnetic field (further
evidence that the planet is now dormant; Mars' core
is quite a bit smaller than the Earth's and hence
both slowed down and cooled/froze faster). However,
it does have magnetized surface rocks!
http://mgs-mager.gsfc.nasa.gov/publications/grl_28_connerney/grl_28_connerney.html
This implies that at some time in the past, Mars' surface
was protected from the Sun's most harmful radiation.
Implications for astrobiology ....