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Computer Drawing of the equations used to model the Martian
 atmosphere in Metric Units.

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.

Variations in atmospheric properties extend upward from the surface of Mars. The sun heats the surface and some of this heat goes into warming the gas near the surface. The heated gas is then diffused or convected up through the atmosphere. Thus, the gas temperature is highest near the surface and decreases as we increase altitude. 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 spacecraft 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 metric units. These curve fits are also available in English units.

The model has two zones with separate curve fits for the lower atmosphere and the upper atmosphere. The lower atmosphere runs from the surface of Mars to 7,000 meters. In the lower atmosphere, the temperature decreases linearly and the pressure decreases exponentially. The rate of temperature decrease is called the lapse rate. For the temperature T and the pressure p, the metric units curve fits for the lower atmosphere are:

T = -31 - 0.000998 * h

p = .699 * exp(-0.00009 * h)

where the temperature is given in Celsius degrees, the pressure in kilo-Pascals,and h is the altitude in meters. The upper stratosphere model is used for altitudes above 7,000 meters. In the upper atmosphere the temperature decreases linearly and the pressure decreases exponentially. The metric units curve fits for the upper atmosphere are:

T = -23.4 - 0.00222 * h

p = .699 * exp(-0.00009 * h)

In each zone the density r is derived from the equation of state.

r = p / [.1921 * (T + 273.1)]

This is the atmosphere model used in the FoilSim simulator. The model is also used in the interactive atmosphere simulation program. 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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Editor: Nancy Hall
NASA Official: Nancy Hall
Last Updated: May 05 2015

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