Keeping an Atmosphere

Thermal speed versus escape speed

An interactive lesson with 6 steps, about 5 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• Gas speed $v_{\text{rms}} = \sqrt{3kT/m}$: light and hot means fast.
• A planet keeps a gas if its rms speed is below about 1/6 of escape velocity.
• Earth lost its hydrogen; cold Titan and the giant planets kept theirs.

Steps

1. A race against gravity
2. Nitrogen on Earth
3. Losing hydrogen
4. The airless Moon
5. Hydrogen’s speed
6. Heavier is slower