A graphical version of this slide is also available. In the text only version presented here, * denotes multiplication, / denotes division, ^ denotes exponentiation, exp denotes the exponential function. The flow variables: temperature in degrees Farenheit is given by T, density in slugs per cubic foot by r, pressure in pounds per square foot by p, and altitude in feet by h.

The Martian atmosphere is an extremely thin sheet of gas, principally carbon dioxide, that extends from the surface of Mars to the edge of space. The Martian atmosphere is less dense than the Earth's atmosphere, but there are many similarities. Gravity holds the atmosphere to the Martian surface. And within the atmosphere, very complex chemical, thermodynamic, and fluid dynamics effects occur. The atmosphere is not uniform; fluid properties are constantly changing with time and place producing weather on Mars just like on Earth.

These variations extend upward from the surface of Mars. The sun heats the surface, and some of this heat goes into heating the gas near the surface. The heated gas is then diffused or convected up through the atmosphere. Thus, the gas temperature will be highest near the surface and decreases as altitude increases. The speed of sound depends on the temperature and also decreases with increasing altitude. As with the Earth, the pressure in the atmosphere decreases with altitude. The density of the atmosphere depends on both the temperature and the pressure through the equation of state and also decreases with increasing altitude.

Aerodynamic forces directly depend on the gas density. To help aircraft designers, it is useful to define a mathematical model of the atmosphere to capture the effects of altitude. The model shown here was developed from measurements of the Martian atmosphere made by the Mars Global Surveyor in April 1996. The information on the Martian atmosphere was gathered by Jonathon Donadee of Canfield (Ohio) Middle School during a cyber-mentoring program in 1999. The data was curve fit to produce equations by Dave Hiltner of St. John's Jesuit High School as part of a shadowing program in May 1999. The curve fits are given for English units. These curve fits are also available in metric units. The model has two parts.

The first part covers the lower atmosphere which extends from the surface to 22,960 feet. In this layer temperature decreases with altitude:

Temperature: T = -25.68 - .000548 * h

Pressure: p = 14.62 * exp (-.00003 * h)

The second part covers the upper atmosphere, which extends upwards from 22,960 feet. In this layer the temperature also decreases:

Temperature: T = -10.34 - .001217 * h

Pressure: p = 14.62 * exp (-.00003 * h)

In each part, the pressure and temperature are curve fit; and the density is derived from the equation of state. r = p / (1149 * [T + 459.7])

An interactive Java applet for this slide is also available. With the applet, you can change altitude and see the effects on pressure and temperature. You can also compare the Martian atmosphere to the atmosphere on Earth.

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