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Atmospheric Retention Lab — Instructor Resources


The NAAP Atmospheric Retention Lab pedagogical objectives indicate the goal for students to understand the first-step of understand atmospheric retention – the influence of gravity and temperature on atmospheric retention. A Second-Look page mentions a few of the additional factors that go into atmospheric retention beyond the scope of this lab.


Course Integration

The Atmospheric Retention Lab does not have an prerequites. However, an understanding of blackbody curves may serve as a conceptual foothold for students to understand the Maxwell distribution.



Many simplifications are made in the simulators where additional physical realism would not further this goal. A listing of these simplifications (and clarifications) for each simulator follows:

Projectile Simulator

  • No atmospheric drag is included. However, this is fairly realistic in that particles would escape from the upper atmosphere anyway.
  • No rotation of the planetary body.
  • No gravitation from any other objects. Objects fired at the escape velocity would likely encounter gravity from another object long before they got near zero velocity.

Gas Retention Simulator

  • The “particles” in this simulator are really tracer particles meaning that each one represents a large number of real particles. These tracer particles appear and disappear with over time and each new tracer that is generated represents a speed in the Boltzmann Distribution. The likelihood of a speed being generated is equal to its true relative abundance.
  • Particles may escape the chamber if their velocity when reaching a wall is greater than the escape velocity. No effort is made no calculate components of velocity perpendicular to the wall.
  • Collisions are necessary to replenish the high velocity tail of the distribution that escapes. However, collisions of the tracer particles are simulated by simply randomly redirecting a particle.
  • The replenishment of the high-velocity tail occurs independently of time. When a gas escapes from the chamber the number of particles at all speeds declines.
  • Note that pressure is not discussed. Thus, some of the gases involved could be solid at low temperatures depending upon the pressure.

Gas Retention Plot

  • The root-mean-square velocity is not used in this module. Since it only differs from the average velocity by a few percent, including it introduces more vocabulary with no real benefit.
  • Note that for each gas a region of the speed-temperature parameter space is colored where the gas would be totally retained by the body to a dashed line at 10×vavg. This coloring slowly faces out to 6×vavg.