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From geological records in rocks and minerals we know that the magnetic field of Earth changed its polarity multiple times in the history. See Geomagnetic reversal.
Was a similar process of a planetary magnetic field reversal observed on other planets?
I suppose that the record could be in rocks directly found on the planet (i.e. Mars), on its moons or in asteroids.
There are such evidence in the case of Mars. Observations from Mars Global Surveyor show evidences of crustal magnetization. In particular, this magnetization has extensive, east-west trending linear features in Terra Cimmeria and Terra Sirenum. These are probably reminiscent of magnetic features associated with a reversing dipole.
There are no such evidence in the case of Venus: Venus has no intrinsic magnetic field today (contrary to the common belief, its inability to maintain a dynamo activity is either due to its chemical and physical conditions unable to form a solid core, either du to an early complete core solidification; it is not due to its slow rotation), and could not have maintained a remanent crustal magnetization, because its crust would have been to hot during its dynamo activity (above the Curie point).
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