Katherine Carey, age 12, of Albany, New York, for her question:
What keeps the atmosphere around the earth?
Suppose the atmosphere got fed up with our messy pollution and decided to leave home. We would be stranded with no breathable oxygen and exposed to destructive solar radiations. There would be no clouds, no rain or snow and surface water would evaporate under the blazing sun. Naturally, all life on land and in the seas would come to a standstill. Fortunately our atmosphere cannot desert us because the face of gravity hugs it close around the globe.
When we say that gravity prevents our atmosphere from escaping, naturally we wonder about the moon. Astronauts assure us that the lunar gravity is about one sixth as strong as the earth's, yet the moon has no atmosphere worth mentioning. What's more, scientists say that if we gave the moon an atmosphere, its gases would escape and get lost in space.
Obviously the answer has something to do with the strength of gravita¬tional forces. The moon's gravity is too weak to hold gases above its solid surface. The earth's gravity is strong enough to hug down a gaseous atmosphere weighing five quadrillion tons which is the figure five with a tail of fifteen zeros. It takes stupendous force to hold the earth and its atmosphere together and it works because it is a two way pulling power.
The force of gravity is a built in quality of matter and every parti¬cle has its infinitesimal quota. It pulls, or attracts, other particles and other particles pull back. This silent, invisible gravitation is a cosmic glue that helps to hold the pieces of the universe together.
The earth's quota of gravity depends on its mass, which is the amount of matter packed into its size, or volume. It acts as if it pulls from a point 4,000 miles down in the center of the planet. From there it extends upward, gaining strength as it goes. It pulls at objects on and above the surface and with their lesser quotas of gravity they pull back.
This two way pulling power of surface gravity is measured in weight. The earth and its atmosphere hug each other with a force equal to five quadrillion tons. However, as gravity extends above the surface, it dimin¬ishes with distance at a set rate.
At the surface, 18 ounces weigh a pound. At 8,000 miles above the surface, the same pound weighs only four ounces. The force of gravity between the earth and its filmy air decreases as we go higher above the surface. This is one reason why the densest layer of the atmosphere is at the bottom and the upper gases grow thinner and thinner.
The earth is massive enough to hold its shell of gases, steadily through the ages. It is 81 times more massive than the moon because the moon is smaller and also made of less dense materials. The earth's surface gravity is six times greater than the moon's. Also, during the hot lunar day, gases would expand and become very light. Hence, the gravity between the moon and its gases would be too weak to hold a lunar atmosphere.