Earth is the only planet known to have an atmosphere enabling surface liquid water. Ample evidence suggests that Mars once also had such a habitable atmosphere, however, Mars no longer does.

Earth’s main magnetic field prevents the solar wind from directly interacting with our habitable atmosphere. Mars’ crustal magnetic fields suggest that the planet once also had a main magnetic field, however, again, Mars no longer does. Without a main magnetic field, the solar wind directly interacts with Mars’ atmosphere. 

Does an intrinsic planetary magnetic field protect a planet’s atmosphere from space weather and stellar winds? Or does a planet's gravity matter more? Within our solar system and beyond, in what ways do stellar winds drive atmospheric escape?