Astronomy quiz question: molecule escape from atmosphere

Astronomy 10 Quiz 4, Summer Session 2007
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal Q4.1

[3.0 points.] Which one of the following statements best explains why it is not possible for molecules near sea level to escape from the Earth's atmosphere into space?
(A) Speeds are too slow.
(B) Frequent collisions with other molecules.
(C) Low temperatures.
(D) Infrared radiation trapped by the greenhouse effect.
(E) Tectonic plate motion and subduction.

Correct answer: (B).

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 8027
(A) : 3 students
(B) : 7 students
(C) : 1 student
(D) : 0 students
(E) : 2 students

Sinfest, by Tatsuya Ishida
January 25, 2001 (excerpt)

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