Robert Donnelly, age 12, of Staten Island, New York, for his question:

Where is the exosphere?

A sphere, of course, is a global shape and in the word "exosphere" the "sphere" is our global planet. The "exo" is something outside, way outside the solid globe. It is a part of the gaseous atmosphere, the part that is most remote from the land and sea on the surface of the solid planet. The atmosphere itself is a sphere arranged in layers, each with its own characteristic density, temperature and other features.

The exosphere is the top or outer layer of the atmosphere. Its base merges with the ionosphere at about 300 miles above the planet's surface. The thin, thin gases of the outer sphere reach farther and farther upward and finally merge with the fine fragments of interplanetary space. These upper gases of the earth are exposed to solar and cosmic radiation. The temperature reaches seething degrees. Most nuclear particles in the exosphere are in atomic form and molecules of the gaseous air are few and very far between.