Scientists think that billions of years ago, the atmosphere of Mars was much denser and warm enough to form rivers, lakes, and perhaps even oceans of water. As the planet cooled and lost its global magnetic field, the solar wind and solar storms eroded away to space a significant amount of the planet’s atmosphere, turning Mars into the cold, arid desert we see today.
Did it ever really have much of a "global magnetic field", to help protect against solar wind?
It probably did, but the core solidified and lost its magnetism................