Mars was once a warmer world of rivers, lakes and a thicker atmosphere, but after its internal dynamo died and the planet lost the magnetic shield that helps protect an atmosphere, the solar wind stripped much of its air away over billions of years, leaving the cold desert we see today
The familiar version goes like this. Early Mars was a warmer world with rivers and lakes and a thick atmosphere. Its internal dynamo died, t
A planetary magnetic field deflects some of the solar wind, but it can also open channels along which charged particles escape, and in some configurations a field may speed up loss rather than prevent it. Venus is the standing complication. It has no internally generated magnetic field and sits closer to the Sun, yet it holds a dense atmosphere. The match in timing between Mars losing its dynamo and losing its air is suggestive, and the protective reading is the leading one, but the causal claim is not established. Some researchers have argued that a longer-lived dynamo might even have helped Mars lose water rather than save it.
