Atmospheric Flight

Gravity and Atmosphere

How is it possible for planets to have an atmosphere in the first place?

Gravity is what keeps a planet's gaseous atmosphere from dispersing away from the planet and out into space. Each planet has a different gravitational attraction, proportional to the mass of the planet. There is also a centrifugal effect on the atmosphere from the rotation of the planet on its axis. Molecules in a gas have a random, molecular motion where all the gas molecules are zipping around and bouncing off each other. For a certain temperature, different kinds of gases move with a different average velocity. Lightweight gases such as hydrogen and helium have faster velocities, while medium-weight gases like nitrogen and oxygen have medium speed velocities, and heavy gases like carbon dioxide have slower velocities. To visualize this choose this link to an animation of molecular motion.

A large planet like Jupiter with a strong gravitational pull is able to hold the light gases despite their high velocities, but a small planet like Earth or Mars cannot. The moon is so small that its gravity is not even strong enough to hold onto the heavy gases like carbon dioxide, therefore it has no atmosphere.

The gravity on Earth, however, is strong enough to hold onto gases like nitrogen and oxygen. So when the Earth was forming, the lightweight gases of hydrogen and helium escaped into space, leaving behind the heavier gases of nitrogen and oxygen. Since Mars has only one-third the gravity of Earth, Mars could not hold onto nitrogen and oxygen gases. This left Mars with only the heavy gases in its atmosphere, mostly carbon dioxide.