I have read that in order to retain a gas, a planet's escape velocity must be at least six times the thermal velocity of that gas (I understand that this is just an approximation of the Maxwell distribution).

From wikipedia, the thermal velocity of hydrogen is 1754 m/s (at room temperature), and I know that Earth's escape velocity is 11200 m/s - exceeding six times the thermal velocity.

Despite this, Earth does not have an abundance of hydrogen or helium, and every graph of atmospheric escape I have seen shows both gasses requiring a much higher escape velocity.

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Is there another factor that these graphs are taking into account?

  • $\begingroup$ The first graph apparently uses a factor 10 instead of 6. Where did you get 6 from? The difference is probably how fast the gas would escape, so there will be differences at what timescale this is calculated. If you have a source for the Value 6, then we can clarify which assumptions went into that. $\endgroup$ Jul 4 at 14:03