Gravity and Atmosphere
Gravity is what keeps a planet's gaseous atmosphere from
spreading out into space away from the planet. If we compare the gravitational
pull of each planet in our solar system we would find them to be different.
This is because a planet's gravity is related to its mass. Usually the greater
a planet's mass, the greater the gravitational pull. Let's look at the Earth
Jupiter's mass is 318 times greater than Earth's mass. That means Jupiter has a stronger gravitational pull than the Earth does. If an object on Earth weighs 100 pounds, place that same object on Jupiter and it will weigh 234 pounds. As the planet rotates on its axis it exerts a centrifugal effect on the atmosphere. Let's say that you are wearing a pair of lightweight, loose fitting sunglasses. If you spin around quickly in one place, these sunglasses will fly off your nose and move outward away from your face. This is a centrifugal effect. The air molecules around a planet do the same thing, especially if they are lightweight. Molecules in a gas are in constant motion zipping around and bouncing off each other. Lightweight gases such as hydrogen and helium move faster than medium-weight gases like nitrogen and oxygen. The heavy gases like carbon dioxide move at a slower rate of speed than the other two. 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 even though they move at high speeds. A small planet like Earth or Mars cannot hold onto lightweight gases. The moon is so small that its gravity is not even strong enough to hold onto the heavy gases like carbon dioxide, that means 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. Mars has only one-third the gravity of Earth, but rotates very close to the same speed as Earth. This means that when Mars was forming it could not hold onto nitrogen and oxygen gases. This left Mars with only the heavy gases in its atmosphere, mostly carbon dioxide.
Table of Contents