In our solar system, Venus, Mars, and the earth are rocky planets that have atmospheres. It has recently been discovered that the moon and Mercury also have atmospheres, though their atmospheres are very diffuse. The planets farther from the sun, Jupiter, Saturn, Neptune, and Uranus, are called gas giants because they are made almost entirely of hydrogen, helium and trace amounts of other gases. These planets have colorful and turbulent atmospheres.
  What are some of the factors that determine what a planet's atmosphere will be like?

• Planetary gravity,
• temperature, and
• the amount of gaseous material present all have effects.

Planetary Gravity and Density

     Gravity causes objects to be pulled towards each other. A gas molecule has no measurable effect on a planet, but a planet pulls nearby gas molecules toward itself. The more the mass of the planet, the more gravitational pull it exerts on molecules of gas. This pull makes the larger atoms, such as nitrogen and oxygen, heavier than the tiny hydrogen atoms, and keeps most of them near the planet's surface.

     As we saw in The Layers of Earth's Atmosphere, 99% of the mass of the earth's atmosphere is located in the thirty miles above the surface of the planet. Even there, the closer that the atoms are to the surface, the more thickly they are crowded together. How closely they are crowded is referred to as atmospheric density. The more atoms you have in a given space, the more dense the atmosphere is.

Density refers to how tightly the atoms are packed together.

How Many Atoms?

     Density is affected by gravity, and also by how many atoms there are to pack together. Suppose that you were given a cubic yard of the earth's sea level atmosphere and asked to count the atoms in it. It would take practically forever! That cube of air would have many billions of atoms in it. Now, for comparison, let's think about a cubic yard of the moon's atmosphere. The atmosphere of the moon is difficult to detect and very diffuse when compared with the earth's atmosphere. There just aren't many atoms to work with, comparatively speaking. So would there be hundreds of atoms to count? Even thousands? Maybe only one? Counting these atoms might actually be possible!

Why do some planets have more atoms in their atmospheres than others?

     Planets start out with different amounts of atmospheric gases. There are many factors that could influence this, some related to what the planet is made of, some related to the level of volcanic activity, some related to asteroid impacts, etc. You can probably give your planet as dense an atmosphere as you wish. On earth, the elevation above 20,000 feet is called the Death Zone, because there is very little oxygen up there. Parts of the brain begin to die above that height. We cannot live there permanently, though short visits by mountain climbers are not necessarily lethal. Perhaps animals and plants could evolve to live in this thin atmosphere, but there must be an upper limit. A related problem is that dangerous solar radiation is filtered out by the atmosphere. Where the air is less dense, more radiation gets through.

      Our "sister planet", Venus, has an atmosphere 100 times as dense as earth. Earth life forms would be squashed by the pressure there.   However, animals can live at even greater pressures in the bottom of our oceans, so perhaps they could evolve on such a world if the temperatures were favorable. (Venus is much too hot.)

     As planets cool, rocks and dust and elements with high melting points solidify. Gases, which have lower melting points, remain gaseous until the temperature falls to the point at which they, too, become liquid and perhaps even solid. As we saw in The States of Matter atoms and molecules in a gaseous state are moving rapidly and freely, and bumping into each other. The hotter they get, the faster they move. If they are moving fast enough, they can reach escape velocity, the point at which they have enough momentum to escape from the planet's gravitational field altogether. If they are moving away from the planet, they can disappear into space. As the molecules leave, the planet's atmosphere becomes just a tiny bit less. Over a long period of time, the atmosphere can slowly drift away.

     Atoms are especially liable to leave a small planet or moon. Small, low mass moons and planets have low gravity to begin with, so the atoms are not held to the surface very strongly. Heat makes matters worse. Mercury is a small planet, close to the sun. It has a day time high temperature of 350 degrees Centigrade, which would make the atmospheric atoms move rapidly. In addition, particles from the sun, the solar wind collide with the atmospheric atoms, speeding them up further and perhaps changing their direction of movement. Mercury has only about a third of the gravity of the earth, so it is easier for atoms to escape from Mercury. It is not surprising that Mercury has no atmosphere to speak of.