Atmosphere of Earth and balloons, and too low to permit orbital spacecraft. The mesosphere is mainly accessed by sounding rockets and rocket-powered aircraft. The stratosphere is the second-lowest layer of Earth's atmosphere. It lies above the troposphere and is separated from it by the tropopause. This layer extends from the top of the troposphere at roughly above Earth's surface to the stratopause at an altitude of about . The atmospheric pressure at the top of the stratosphere is roughly 1/1000 the pressure at sea level. It contains the ozone layer, which is the part of Earth's atmosphere that contains relatively high concentrations of
Period 2 element can form an allotrope known as ozone. Ozone is a triatomic gas even more reactive than oxygen. Unlike regular diatomic oxygen, ozone is a toxic material generally considered a pollutant. In the upper atmosphere, some oxygen forms ozone which has the property of absorbing dangerous ultraviolet rays within the ozone layer. Land life was impossible before the formation of an ozone layer. Fluorine is the chemical element with atomic number 9. It occurs naturally in its only stable form F. Fluorine is a pale-yellow, diatomic gas under normal conditions and down to very low temperatures. Short one electron of the