Josebh Paul Traywick, aged 13, of Orangeburg, S.C. for his question:
Why doesn't our air rush in to fill the vacuum of outer space? ..
This is a very thoughtful question. We know treat air rushes in to
fill an empty space. Just as water flows in to fill up the cupful you take from a pail. Denser air even rushes in to fill an area of thinner, lighter air. This is what causes the winds.
We know too that the earth with its blanket of air travels along through empty space. And the empty space beyond the atmosphere is almost a vacuum. So it is thoughtful to wonder why our air does not rush to fill the vast reaches of outer space. How come our wonderful old world can keep her cozy blanket of atmosphere?
The experts have figured this out ‑ down to an atom. Everything is hugged to the face of the earth by gravity. And it is gravity which holds the blanket of atmosphere above us. You pit your strength against gravity every time you toss a ball through the air. You give some of your energy to the ball. The ball uses the energy to pull against gravity. The faster it starts, the further it travels through the air. It falls, or is pulled back by gravity, when its speed‑making energy is used up.
To escape the pull of the earth a moving object must have great speed. Experts figure that a space rocket must take off at 7 miles a second. ‑It must travel at 25,000 an hour to break the power of earth's gravity. We call this the escape velocity ‑ the speed necessary to escape the earth’s gravity.
Every atom and molecule in the atmosphere must obey these laws of gravity. A particle could escape the upper atmosphere if it reached a speed of 6.98 miles per second. Air molecules whip up speed from heat or from collisions, one with another.
The air at the top of the atmosphere is very thin and rare. Few collisions take place between the molecules. Air molecules at 70 degrees Fahrenheit travel at about 500 yards a second ‑ way below escape velocity. So few, if any, atoms of air ever get up enough speed energy to escape the mull of gravity. The old earth, then, has plenty of pulling power to hall her blanket of air from rushing off to fill the vacuum of surrounding space.
A planet's gravity, of course, depends upon its mass, or weight. Mars, smaller than earth, has less gravity than earth. Its air molecules can escape at a speed of 3.2 miles per second. Therefore its air is thinner than ours. The velocity of escape from giant Jupiter is 39 miles a second.
This same rule applies to suns and satellites. And, air molecule can escape our little moon at 1.5 miles a second. The moon's daytime temperature often reaches 200 degrees Fahrenheit. So, if there was ever air on the moon, it has long since whipped up enough speed energy to escape. But to escape the massive sun, a particle of air must travel at 380 miles a second. So, the son has a dense, deep atmosphere, the moon has none at all.