Key idea: Planetary atmospheres are some times composed of a number of layers. These atmospheres are said to be stratified. Atmospheric pressure decreases with altitude, but temperature will increase or decrease with altitude depending upon the layer and the atmospheric conditions.

Martian Sunset from Mars Pathfinder Site Astronomical tidbit: This panoramic photograph of the Martian sunset was taken by the Mars Pathfinder and released on August 1, 1997. Pathfinder was the first US spacecraft to set down on the surface of Mars since the Viking Landers in the late 1970's. The photograph shows the atmosphere to be colored pink at sunset. This coloring is due to rusted iron dust suspended in the Martian atmosphere. The red surface coloring of Mars is also due to oxidized iron. At certain times of the Martian year, great dust storms occur. The large amounts of dust in the Martian atmosphere during these storms absorb much of the incoming radiation from the Sun. This causes large temperature inversions in the Martian atmosphere. The Pathfinder mission was developed and operated as part of NASA's Discovery program by the Jet Propulsion Laboratory.

Planetary atmospheres can be composed of a number of layers. These atmospheres are said to be stratified. The lowest layer in the atmosphere is called the troposphere. This is where most of the mass of the atmosphere is located and where nearly all of the water vapor is located if it exists. The temperature of the troposphere generally decreases with height above the surface of a planet. If you were measuring the temperature of the troposphere while rising in a weather balloon, you would observe the temperature getting colder and colder. The temperature decreases because the troposphere is heated primarly by convection from the surface of the planet. Convective heating works best close to the surface and its effectiveness decreases as the height increases.

Sometimes, the temperature of the troposphere increases with height. This happens above ice caps for instance. In this case, the ice draws the heat out of the atmosphere and it draws more heat at lower altitudes than it does at higher altitudes. This should make sense to you if you think about how it would get colder and colder as you approached a large block of ice. When the temperature increases with height it is called a temperature inversion.

The highest point in the troposphere is called the tropopause. The height of the tropopause in Earth's atmosphere varies by latitude and by time of year, but it is at an altitude of approximately 11 kilometers.

In the Earth's atmosphere, the layer above the tropopause is called the stratosphere. Much of the ozone in our atmosphere is located in the stratosphere, and it absorbs ultraviolet radiation from the Sun. You learned about this in an earlier lesson. The ozone is heated when it absorbs the radiation, and the temperature of the stratosphere increases with height. That is because more absorption takes place high in the stratosphere (closer to the Sun) than low in the stratosphere. It is also because the density of the stratosphere decreases with height, and the thinner gas higher in the stratosphere is heated more greatly by absorption of each ultraviolet photon than the thicker gas low in the stratosphere. The top of the stratosphere is called the stratopause. In Earth's atmosphere, the height of the stratopause is about 50 kilometers.

You learned in a previous lesson that atmospheric pressure is caused by the weight of the gases and vapors in the atmosphere pressing down on the atmosphere itself and on the surface of the host body. You also learned that the pressure drops as altitude above the surface increases because there is less mass in the atmosphere above a reference height as the height increases. The air pressure in Earth's stratosphere, 20 miles or so above the surface of our planet, has decreased so much that it is comparable to the pressure on the surface of Mars due to the very thin Martian atmosphere. That pressure is approximately 100 times less than on the surface of our planet.

The layer above the Earth's stratosphere is called the mesosphere. Here the temperature once again decreases with height. The mesosphere has the lowest temperatures in Earth's atmosphere. The top of the mesosphere is called the mesopause. The height of the mesopause is approximately 85 kilometers.

The uppermost layer in our atmosphere is called the thermosphere. The temperature of this layer increases with altitude, due also to the absorption of radiation from the Sun. The density and the pressure in the thermosphere are extremely low. The pressure can be one million times less than it is on the surface of the Earth. This pressure is comparable to that on the surface of Pluto and Neptune's satellite Triton. Temperatures in the upper thermosphere are extremely high because the density of the gas there is so low and the rarefied gas is greatly heated by the absorption which occurs there.

As part of the Mars Global Surveyor mission, the MGS Radio Science Team will be obtaining thousands of profiles of temperature and pressure in the Martian atmosphere. These profiles will record the variation of temperature and pressure with height above the Martian surface. Over the course of the mission, the observations will cover the entire Martian planet and will continue for a full Martian year. The data which you will receive from the Martian atmosphere will be culled from these measurements. There will be much to learn from these Martian data!!!

Temperature inversions can take place in the troposphere when dust or other particles absorb some of the incoming radiation from the Sun. When there is such an inverion, the temperature is cool at the surface and increases with altitude. Such inversions are very common on Mars during periods of dust storms when large amounts of dust in the atmosphere absorb radiation which would otherwise reach the surface. Carl Sagan (after whom the Pathfinder landing station was named) and other scientists have been concerned that such a temperature inversion might take place in Earth's atmosphere following a nuclear war and lead to what has been called a nuclear winter.

Question: It is believed that the dinosaurs became extinct following the impact of a huge meteor on Earth and the subsequent change in Earth's climate. How do you suppose that the impact affected our climate on such a large scale???

Last updated: November 17, 1999
Joe Twicken /
Rob Wigand