When you think about it, Earth is a closed system. The water you’re drinking is part of the same water cycle as the water the dinosaur’s drank. The ground you’re walking on has been there for millennia. As rocks are worn away by weathering and soil erodes, the resulting materials make their way into the cycle that forms landmasses. Even the elements in the air we breathe are part of a cycle. Everything plays its part and the system as a whole continues to function. There’s just one question. Why isn’t our atmosphere sucked into space?
The upper layers of our atmosphere are right there, adjoining the vacuum of space. Why doesn’t the vacuum of space suck up our air? After all, a vacuum in a container, when punctured, quickly fills with air. Here’s the thing. The vacuum in the container is not sucking the air into the container. The air outside the container is rushing into the empty space. It does that because of the air pressure outside the container. The closer to Earth you are, the greater the air pressure. Since there is no pressure in the container, the air has greater pressure and moves in to fill the space. For the same reason, the vacuum of space does not suck up all the air from our atmosphere. And since the pressure at the highest levels of our atmosphere, the ones that are adjacent to the first layers of space, is so low, nothing is rushing in to fill that vacuum.
This leads to another question: Why doesn’t the air in our atmosphere escape into space? This does occur, in minute amounts, but for the most part this does not happen because the molecules in our atmosphere are not moving quickly enough. They have to move at escape velocity, or 11,179.365 m/s (36,677.7 ft/s). That’s far too fast for molecules to move when they’re bumping into one another and subject to the force of gravity. Newton’s First Law of Motion states that an object at rest will remain at rest and an object in motion will remain in motion in a straight line unless acted upon by another force. In this case, the force is gravity. It’s not that the molecules wouldn’t make their way into the vacuum of space, it’s that they can’t make their way because they cannot reach escape velocity.