# Atmospheric Pressure determined by?

1. Mar 26, 2005

### MonstersFromTheId

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."

2. Mar 27, 2005

### SpaceTiger

Staff Emeritus
Keep in mind that planets are very round, so the mountains and valleys all look like pimples when viewed from a distance, even on Mars. The variations in pressure between the high and low points of Mars are about an order of magnitude (the scale height of the atmosphere is 11 km and Mount Olympus is 27 km), while the average on Mars and Earth differs by a factor of about 200. Any way you slice it, Mars has a lower pressure, but my guess is that the numbers they usually quote are based on the surfaces "average" distance from the planet center.

It's a complicated question. If left alone, then all atmospheres will boil off eventually (assuming no feedback), it's just a matter of timescale. Lighter molecules burn off more quickly than heavy ones because at a given temperature, they're moving faster. I don't know how familiar with physics you are, but what effectively happens is that the high-velocity end of the Maxwell-Boltzmann distribution will have velocities in excess of the escape velocity, so they can't stay bound to the planet. There will always be some parts of the distribution in excess of the escape speed, but for heavier molecules it's a much smaller fraction, so it will take longer for them to burn off.

This would all be relatively simple, except that you can have many feedback mechanisms. That is, even if the molecules escape on short timescales, if they're being regenerated by some process (vulcanism, for example), then you can maintain an atmosphere. So yes, in theory, if Mars where expelling gas and energy into the atmosphere, it could have a much higher pressure, but there would have to be some process maintaining it.

Hope this helps.