Nathan Mitkoff, age 12, of Utica, N.Y.,for his question:

WHY DOESN'T THE EARTH'S ATMOSPHERE ESCAPE?

If the moon ever had an atmosphere like ours, it escaped and went off into space. This is because the moon's weaker gravity is not strong enough to hold a mass of gases above its surface. The planet earth is much more massive and its surface gravity is six times greater than the moon's. It is strong enough to capture and hold an atmosphere weighing 5 quadrillion tons. In round figures, this is five with a tail of 15 zeros.

As it reaches outward from the surface, the earth's gravity grows weaker with distance. This is one reason why the upper layers of the atmosphere are thinner, at the very top, perhaps 1,000 miles above the surface, most likely a few of its gaseous molecules do get enough speed energy to escape into space. But the bulk of our atmosphere cannot escape the earth's gravity.