Pressure is defined as the atmosphere pressure at ground level. The moon has a low escape velocity and its surface is essentially a vacuum. Mars has a thin carbon dioxide atmosphere because its medium escape velocity is too low to hold lighter gases for geologic time periods. Vulcanism and comet strikes add to the atmosphere of a body, while gases slowly leak especially if they're hot or if the body has a low escape velocity. The pressure information for mars was obtained from Nine Planets - Mars.

Spacecraft are slowed by high pressure and therefore dense atmospheres and so are sometimes carried above part of the atmosphere by another vehicle. Pressure is used with molecular weight and temperature to calculate the density of the atmosphere.

density = molecular weight * pressure / 8314 ( J * K / kmol ) / temperature
 

This is used in atmospheric spacecraft, multi stage spacecraft, and spacecraft cost.
 
 

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