The exosphere

   Exosphere is the outermost rim of the earth's atmosphere. The exosphere is about 250 miles above the earth's surface. The density of gases that fill the exosphere is about a million-millionth of the density at the earth's surface. Thus, moving gas particles collide infrequently. Unless they happen to collide, the gas particles in the exosphere tend to rise and then to fall back again into the exosphere. Sometimes, however, gas particles escape from the exosphere and are lost in space. Then they are no longer a part of the earth's atmosphere. The gas particles that succeed in escaping from the earth's gravitational field are generally the lighter gases: helium and hydrogen. Beyond the exosphere the earth's gravitational force is too weak to hold the gas particles captive in the atmosphere. Scientists say that the temperatures in the exosphere are very high, possibly near 2,000° F. This heat is generated by ionization and the absorption of solar radiation by cosmic dust.