without it, Earth's atmosphere would have been, as on Mars, stripped away by solar wind long before the present epoch.

Another theory, however, suggests Mars did once experience its own iron catastrophe and was once shielded by a magnetosphere. By this theory Mars has simply cooled faster than the Earth, gradually solidifying its dynamic iron center, hence shutting down its magnetosphere. The finding of signs of liquid water once existing on Mars suggests that it once had its own magnetic shield to keep the water in the atmosphere of