Astronomy quiz question: molecule escape from atmosphere (2)

Astronomy 10 Quiz 4, Spring Semester 2008
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal Q4.1

[3.0 points.] Which one of the following statements best explains why atoms and molecules are able to escape from the exosphere, but would be unable to escape from the lower atmosphere of a planet?
(A) Atoms and molecules are more massive in the lower atmosphere.
(B) Atoms and molecules move faster in the lower atmosphere.
(C) Atoms and molecules are more likely to encounter collisions in the lower atmosphere.
(D) Atoms and molecules have a higher temperature in the lower atmosphere.
(E) (None of the above choices (A)-(D), as atoms and molecules are able to escape from the lower atmosphere of a planet.)

Correct answer: (C).

The average speed of a molecule/atom in the atmosphere depends on the mass of the molecule/atom, and on the temperature. If this speed is greater than approximately one-sixth of the escape velocity of a planet (which depends on the mass of the planet), then the atmosphere will eventually leak out over several billion years. However, molecules/atoms may only escape from the exosphere (the upper atmosphere); no matter how fast their average speed is in the lower level, frequent collisions there prevent escape into space. This is the "mosh pit effect"--you can't leave the mosh pit if you are in the middle of the moshers, but once you can make your way to the edge, escape is possible (provided you don't get pulled back in).

Student responses
Section 4160
(A) : 3 students
(B) : 1 student
(C) : 23 students
(D) : 1 student
(E) : 3 students

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