The atmosphere is a thin layer of air surrounding the Earth. What keeps the…

Any body with mass exerts a gravitational pull on other masses, and this pull acts throughout the surrounding space, growing weaker as the mass of either body decreases or the distance between them increases. Earth's atmosphere is a huge collection of gas molecules extending outward from the surface; for these molecules to stay near the planet instead of dispersing into space, some sufficiently strong attractive force must act on them at essentially every altitude they occupy.

Earth's own mass is enormous compared with the mass of a single rock, ocean, or even the Moon, and this mass acts on the air molecules from very close range. As a result, Earth's overall gravitational pull is strong enough to keep the bulk of the atmosphere bound to the planet as a layer around it.

Compare the other options by their own value:

• The Sun's heat increases the kinetic energy and average speed of air molecules; molecules that gain enough speed can exceed the escape velocity for their altitude and leave the atmosphere over time.

• Rocks and oceans are surface features that together make up only a tiny fraction of Earth's total mass; a gravitational pull's strength scales with the mass of the object producing it, so the pull each of these produces on its own is correspondingly tiny.

• The Moon's mass is about one-eighty-first of Earth's, and it lies roughly 384,000 km away; since a gravitational pull's strength depends on both the mass of the body and the distance to it, a body this much smaller and farther produces a correspondingly small pull on Earth's air.

So it is Earth's own gravitational pull, arising from the planet's full mass acting at close range, that keeps the atmosphere around Earth, not solar heat, the surface rocks and oceans on their own, or the Moon's comparatively small and distant pull.