Criteria for Magnetosphere
Why does Earth have a magnetosphere and Venus does not?
As you know magnetospheres are believed to be caused by the currents
produced by the motion of charged particles within the planet.
Electrical currents have an allied magnetic field. While we cannot
be sure why, Venus apparently has very little magnetic field. This
could mean that the chemical substances that are needed to produce
the electrical current, or the physical requirements -such as motion
of a molten core- are not present in Venus. I do not think it is
established why Venus lacks these chemical or physical requirements,
but the data certainly indicates that it must because our probes show
that Venus has orders of magnitudes weaker magnetic fields than that
Greg (Roberto Gregorius)
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Update: June 2012