Air slips off Mars, atom by atom. Its pressure is so low that the chapter compares it with air above a stack of 4 Everests on Earth. You could not breathe either sky.
The Sun blows a stream of charged particles called the solar wind. Moving liquid metal deep inside Earth makes a magnetic field that bends much of this stream around us. A compass needle follows that field.
It sounds like a lid that keeps our air in. But measurements give the story a turn: Earth leaks air too. Having a field does not always mean a planet loses less gas.
Mars is smaller. It sheds its inner heat more easily, and its weaker gravity has less grip on escaping air. Old magnetic rocks show it once had a field across the planet. Its loss of air took more than a shield switching off.
Scientists still argue about when Earth's field began. An old crystal can pick up a younger magnetic mark. The early sky poses another puzzle: almost no oxygen to breathe, but enough trapped warmth for liquid water under a fainter Sun. We cannot yet measure that whole blanket of gas from the scraps of rock left behind.
