Raymond Despins, age 11, of Calgary, Alberta, Canada, for his question:

Did the Earth always have some atmospheric gases?

The past history of our planet is shrouded in perhaps five billion years of unrecorded history. At present, scientists do not have enough evidence to prove exactly how it formed and changed. But they have enough to suggest a likely theory. Most experts suspect that it condensed from an enormous cosmic cloud, along with the sun and its sister planets.

If this is so, the gaseous unborn earth contained all the heavier elements that later formed the solid planet. The atmosphere around the newly formed earth must have been much deeper and denser than it now is. Scientists suspect that it contained vast quantities of hydrogen, helium and other light gases that later boiled away into space, leaving more or less its present percentage of nitrogen. Through the ages, fumes and gaseous vapors were added to the atmosphere and when life appeared, the plant world added quantities of oxygen. Perhaps the present ratio of atmospheric gases was established many millions of years ago. But changes have occurred all the time and are expected to continue.