What factors determine the pressure of a planet's atmosphere? For example: Mars has an atmospheric pressure that's much lower than Earth's. Well, o.k., that's a simplistic statement. Go high enough up in Earth's atmosphere and no doubt you can find a spot where the "atmospheric pressure" is actually far lower than it is on Mars, say at the very deepest depths of Valles Marineris. So I'm not even sure how to make intelligent comparisons here. "Sea level" is a fine and convenient point of reference on Earth, but what do you use as a suitable comparison on a planet like Mars? High points on Mars like Olympus Mons make Mount Everest look like a pimple, and low points like Valles Marineris make the Marianas Trench look like an insignificant scratch. So what's a workable point of comparison? I have no idea so I'll leave that to others. But what I'm really interested in it this - could Mars ever possibly have an atmospheric pressure comparable to Earth's at sea level ANYWHERE on it's surface? Or is it the case that that's just not feasible? That the gravity of Mars just isn't strong enough to allow that. That if Mars was to miraculously acquire an atmosphere as thick as Earth's, it'd just boil off, and quickly thin back out to where it is now. And that even if that didn't happen, having an atmosphere as thick as Earth's still wouldn't do the trick anyway. The atmospheric pressure would STILL be much lower than on Earth because even with the same amount of it's mass tied up in it's atmosphere, with the lower gravity of Mars, the atmosphere, even with a comparable mass, would still weigh less, thus lowering the atmospheric pressure. For all I know, in order to have an atmospheric pressure comparable to Earth's, Mars might require an atmosphere piled up for THOUSANDS of miles above its surface before it ever came anywhere close to Earth's sea level pressures, and that there isn't enough of anything at all on Mars that has the slightest chance of becoming atmosphere to do that. I also realize that gravity isn't the only thing to consider. In fact for all I know it might not even be a significant factor. There's also the very distinct possibility that, in my ignorance, I might even be asking a question that can't be answered. The answer to my question might well involve SO many factors, interacting in SO many complex ways, and involve SO many things we just don't know about Mars yet, that this question is best filed under "God only knows, and he ain't sayin."