The envelope of gas surrounding the Earth is divided into four distinct layers by thermal characteristics (temperature changes), chemical composition, movement, and density. The troposphere starts at the Earth's surface and extends to about 15 kilometers (9 miles) high. The stratosphere starts just above the troposphere and extends to 50 kilometers (31 miles) high. The mesosphere starts just above the stratosphere and extends to 85 kilometers (53 miles) high. The thermosphere starts just above the mesosphere and extends to 600 kilometers (375 miles) high.