Atmosphere of Earth (see the temperature section, below). Because the general pattern of the temperature/altitude profile is constant and measurable by means of instrumented balloon soundings, the temperature behavior provides a useful metric to distinguish atmospheric layers. In this way, Earth's atmosphere can be divided (called atmospheric stratification) into five main layers. Excluding the exosphere, the atmosphere has four primary layers, which are the troposphere, stratosphere, mesosphere, and thermosphere. From highest to lowest, the five main layers are: The exosphere is the outermost layer of Earth's atmosphere (i.e. the upper limit of the atmosphere). It extends from the exobase, which is located at
Opus: The Day We Found Earth their planet of origin to live among the stars. By then, Earth has become a mythical planet and the belief in its existence has become a religion called "Earthology". On the other hand, the human gene pool has deteriorated after millennia of genetic engineering, and the only way to revert this damage is to trace back to the oldest sample of genes left on their long forgotten home planet. Project Earth was then founded and a spaceship named OPUS was launched in order to search for Earth and save the failing human race. The story follows a childlike robot Emeth,