Internally generated field (approximate size) Magnetic tail extends more than 100 Earth radii behind the planet Draped interplanetary magnetic field Earth Solar wind Mars radii 4 3 2 Crustal fields can pinch off Mars flow generates an induced magnetic field, which deflects the wind. The wind and the field it contains then drape around the planet, " similar to what one might envision happening to cooked pasta noodles when thrown at a basketball, " Brain wrote in 2021. An induced magnetosphere is much smaller than an intrinsic one. Those around Venus and Mars extend to only a fraction of the planet's radius. Earth's intrinsic field, on the other hand, is about a dozen times wider than the planet itself. Nor will an induced field matter on the planet's surface - you can't navigate with it. But it will matter in the atmosphere. Sitting Ducks A planet can lose atmosphere in several ways. A blast of plasma from the Sun can tear material off or drive currents through it, heating the gas so much that the world's gravity can't hold onto it anymore. Sunlight can overheat it, sk yand tele scope .o r g * DECEMBER 2023 37 B o w s h o c khttps://skyandtelescope.org/