How does Earth keep its atmosphere?
Earth’s atmosphere is only 1/1,200,000 the mass of Earth itself. So it is a very thin skin surrounding our planet. How does Earth hold on to this thin skin of atmosphere?
The answer is gravity – the same force that keeps us anchored to Earth.
And yet, although you might not realize it or think about it, Earth does continually lose some of its atmosphere to space. This loss occurs in the upper atmosphere, over billion-year time scales.
Molecules in our atmosphere are constantly moving, spurred on by energizing sunlight. Some move quickly enough to escape the grip of Earth’s gravity. The escape velocity for planet Earth is a little over 11 kilometers per second – about 25 thousand miles an hour. If Earth were much less massive – say, as massive as Mars – gravity’s grip would be weaker. That’s one reason why Mars lost most of its original atmosphere.
In the vicinity of our heavier Earth, where gravity is stronger than on Mars, not all particles are equally likely to escape. Light ones, like hydrogen and helium, typically move faster than heavier ones, like oxygen and nitrogen. The light atoms are more likely to reach escape velocity and escape to space. That’s why light molecules are rare in our atmosphere, in contrast to their abundance in the universe at large.
Still, all in all, Earth’s atmosphere is here to stay. And that’s a good thing because our atmosphere protects life on Earth in many ways. It absorbs harmful ultraviolet radiation from the sun, helps keep Earth’s surface warm via the greenhouse effect, and reduces temperature extremes between day and night. Yay atmosphere! It keeps Earth livable.
So, thanks to gravity, although some of Earth’s atmosphere is escaping to space, most is staying here.