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# Atmospheric Retention Lab — Instructor Resources

## Description

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.

## Simplifications

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.