Without the ozone layer life could not exist on the Earth's surface. Separating the mesosphere from the stratosphere is another isothermal layer called the stratopause . Above the mesosphere is another isothermal layer called the mesopause . The last atmospheric layer, as defined by vertical temperature change, has an altitude greater than 90 kilometers and is called the thermosphere . The thermosphere is the hottest layer in the atmosphere.
Heat is generated from the absorption of solar radiation by oxygen molecules.
Temperatures in this layer can reach 1300 to 1800 degrees Celsius.