Why does Mars not have much of an atmosphere?

 

Is it because it is 1/3 the size of Earth and with 1/3 the gravity it can keep only heavier gases?

 

I noted that Earth's gravity is not strong enough to keep free hydrogen and helium from floating off into space, but how many gases would Mars lose with so much less gravity?

 

Just curious.

Work out the escape velocity of say nitrogen and then work out the thermal energy of a nitrogen atom on the surface of mars if thernal > escape it can't be held by gravity....