How exactly are atmospheres held?
By gravity of course, but note that the density of the atmosphere decreases with altitude. With Earth, it drops to one half about every 5 kilometers in altitude (temperature can modify this). If you rise 5 kilometers, only half the atmosphere is above you, with only half its weight compressing the air, so density and pressure are down to 50%. Rise to 10 km, and it’s only 25%, and so on. At 100 kilometers you are down to about one part in million, and collisions between molecules become more rare, so that they rise and fall like stones tossed upwards. The “halving distance” could be more than 5 kilometers, or less. If gravity were smaller (as it is on the Moon) and temperature higher (as happens at high altitudes), it would be larger–and the atmosphere would rise higher. The somewhat early loss of the space station “Skylab” in 1977 was helped by higher sunspot activity, which caused the upper atmosphere to heat up and expand (from more solar X-rays), with more air resistance in low orb
