Multiple choice

There is no atmosphere on the moon because _____.

  1. it is closer to the earth

  2. it revolves around the earth

  3. it gets light from the sun

  4. on the moon the escape velocity of gas molecules is less than their root mean square velocity
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The Moon has no atmosphere because its escape velocity (2.38 km/s) is much lower than the root mean square velocity of gas molecules at lunar surface temperatures. Light gas molecules (like hydrogen and helium) easily exceed escape velocity and escape into space over geological time. Higher mass molecules might be retained, but the Moon lacks significant sources for these gases.

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