gravity and atmospheric pressure?

[lemur]

I was wondering whether it would be possible to generalize about a relationship between gravity and atmospheric pressure in any gravity-well. I was trying to think of the parameters that would affect atmospheric pressure and came up with basically just the amount of heat driving the molecules and the amount of gravity compressing them. I also wondered if gravity helps determine pressure by how fast particles accelerate in free-fall, or if this is negligible because of their small mass. I wondered if there is maybe a layer of atmosphere in any gas-giant that corresponds with pressure at sea-level, and whether this would also correspond gravitationally due to the particular altitude. E.g. might there be an altitude/layer of Jupiter's atmosphere where pressure is 1 atmosphere and gravity is 1G? Also, does the chemical composition (i.e. type/mass of particles) of the atmospheric make a difference?

[Moontanman]

You have most of it, gravity and temperature are important factors in holding onto an atmosphere but the size of the planet would matter as well, if you took a planet 3/4 the size of the earth but with the same mass it could not hold onto as extensive an atmosphere as the Earth does now and the Earth couldn't hold onto as extensive an atmosphere as a planet 5/4 the size of the Earth could with the same mass. Saturn's moon Titan is a good example it has a huge atmosphere several times deeper than earths atmosphere and with a higher pressure and density. If Titans mass was in a smaller body it could not hold onto such an extensive atmosphere. the gravitational potential falls off slower from a large planet than a small planet if there masses are the same.

Edited September 28, 2010 by Moontanman

[emrekanca]

if you took a planet 3/4 the size of the earth but with the same mass it could not hold onto as extensive an atmosphere as the Earth does now

 

I think you' re wrong.The formula is F=k₀.m₁.m₂/r² therefore if radius is smaller the planet will hold a thicker atmosphere with the same mass.But the total radius including atmospher would be smaller.

[lemur]

You have most of it, gravity and temperature are important factors in holding onto an atmosphere but the size of the planet would matter as well, if you took a planet 3/4 the size of the earth but with the same mass it could not hold onto as extensive an atmosphere as the Earth does now and the Earth couldn't hold onto as extensive an atmosphere as a planet 5/4 the size of the Earth could with the same mass. Saturn's moon Titan is a good example it has a huge atmosphere several times deeper than earths atmosphere and with a higher pressure and density. If Titans mass was in a smaller body it could not hold onto such an extensive atmosphere. the gravitational potential falls off slower from a large planet than a small planet if there masses are the same.

Good answer. So a more gradual gradient of gravitation (talk about alliteration) is better for hold atmosphere and less dense planets have a more gradual gradient? What if you think about it purely in terms of gravitational level as a sub-section of any gravity-well, regardless of depth. In other words, instead of looking at the planet as a whole in terms of mass and size, I'm wondering if you could just look at any region of the universe with gravitation of @1G and have comparable atmospheric pressure. So, for example, a certain region of Jupiter's atmosphere at a certain altitude might correspond with another altitude in Neptune's atmosphere, etc.

 

I was also thinking about this for stars, themselves. Obviously there must be some distance from the sun (solar altitude) that has 1G gravitation, but of course planetary gravitation dominates gas-distribution around the sun. However, there may be some other star somewhere that is surrounded by a gaseous atmosphere that does not coagulate into orbiting planets. In that case, could such a star's atmosphere exhibit similar pressure and even composition as Earth's atmosphere?

 

The last issue related to this that I find interesting is how gravity and atmospheric composition could interact to form stratified layers of atmosphere with differing composition. For example, we think of Jupiter as consisting almost exclusively of hydrogen, but wouldn't the hydrogen all float to the top of Jupiter's atmosphere. In that case, if there was nitrogen, oxygen, etc. wouldn't those gasses sink below the hydrogen? I find it interesting that there could be a layer of Earth-like atmosphere somewhere deep within Jupiter or Saturn and I wonder what the characteristics of that layer would be in terms of pressure, temperature, altitude above "sea level," etc.

 

 

[D H]

I was wondering whether it would be possible to generalize about a relationship between gravity and atmospheric pressure in any gravity-well.

Google the terms "hydrostatic equilibrium" and "lapse rate".

 

 

 

[Moontanman]

Good answer. So a more gradual gradient of gravitation (talk about alliteration) is better for hold atmosphere and less dense planets have a more gradual gradient? What if you think about it purely in terms of gravitational level as a sub-section of any gravity-well, regardless of depth. In other words, instead of looking at the planet as a whole in terms of mass and size, I'm wondering if you could just look at any region of the universe with gravitation of @1G and have comparable atmospheric pressure. So, for example, a certain region of Jupiter's atmosphere at a certain altitude might correspond with another altitude in Neptune's atmosphere, etc.

 

 

Actually no you cannot use the idea of G to predict the atmospheric pressure, Titan has far less than one G but more than 50% higher pressure and more density than the Earth and actually a more massive atmosphere.

 

 

 

I was also thinking about this for stars, themselves. Obviously there must be some distance from the sun (solar altitude) that has 1G gravitation, but of course planetary gravitation dominates gas-distribution around the sun. However, there may be some other star somewhere that is surrounded by a gaseous atmosphere that does not coagulate into orbiting planets. In that case, could such a star's atmosphere exhibit similar pressure and even composition as Earth's atmosphere?

 

I would have to say no, the internal heat of the star would prevent this and the gravity of the star would compress it's substance to far greater density with a cut off just like we see in our sun... Oxygen would not be found as anything other than compounds and them only if the temp was low enough. Now is possible for gas to accumulate in a ring around a star or planet (see Larry Niven's book, "The Integral Trees" as a fictional example)

 

The last issue related to this that I find interesting is how gravity and atmospheric composition could interact to form stratified layers of atmosphere with differing composition. For example, we think of Jupiter as consisting almost exclusively of hydrogen, but wouldn't the hydrogen all float to the top of Jupiter's atmosphere. In that case, if there was nitrogen, oxygen, etc. wouldn't those gasses sink below the hydrogen? I find it interesting that there could be a layer of Earth-like atmosphere somewhere deep within Jupiter or Saturn and I wonder what the characteristics of that layer would be in terms of pressure, temperature, altitude above "sea level," etc.

 

Hydrogen and Helium make up almost all of Jupiter, the other elements are diluted to the point of being trace elements. Jupiter is made up mostly of liquid metallic hydrogen not hydrogen gas. Gasses like oxygen would never be found in their molecular form, they are always found as compounds. Earth like pressure could only be found high in Jupiter's atmosphere not deep inside it...

[lemur]

Nice post. Thanks.

 

 

Actually no you cannot use the idea of G to predict the atmospheric pressure, Titan has far less than one G but more than 50% higher pressure and more density than the Earth and actually a more massive atmosphere.

This confuses me. What is compressing the atmosphere so dense if there is less gravity?

 

 

 

Hydrogen and Helium make up almost all of Jupiter, the other elements are diluted to the point of being trace elements. Jupiter is made up mostly of liquid metallic hydrogen not hydrogen gas. Gasses like oxygen would never be found in their molecular form, they are always found as compounds. Earth like pressure could only be found high in Jupiter's atmosphere not deep inside it...

How would you figure out what the altitude was exactly and how much gravitation there is at that level? Is that possible without taking a scale there to measure it on site?

Edited September 29, 2010 by lemur

[Mr Skeptic]

Some places just have more atmosphere. It's like going 10 ft below water vs 20 ft below water. Gravity is essentially just as strong, there's just twice the water above you resulting in twice the pressure. For atmosphere extra pressure translates to extra density as well.

[lemur]

Some places just have more atmosphere. It's like going 10 ft below water vs 20 ft below water. Gravity is essentially just as strong, there's just twice the water above you resulting in twice the pressure. For atmosphere extra pressure translates to extra density as well.

 

So the pressure/gravity relationship is affected by the composition and mass of the atmosphere above you? So you could hypothetically find the 1G altitude area of Jupiter's atmosphere but then so much gas would still be above you that atmospheric pressure could be much higher than on Earth at sea-level? Then, if you went up to search for Earth sea-level atmosphere the gravity level might be 0.5G or something?

[Mr Skeptic]

Something like that. In any case, the atmospheric pressure will drop more quickly than gravity does as you go higher up, and could depend on the quantity, composition, and temperature of the atmosphere as well as the gravity. A pretty good example is the moon, which does not have 1/7th the atmosphere of earth, because it lost almost all its atmosphere.

[lemur]

Something like that. In any case, the atmospheric pressure will drop more quickly than gravity does as you go higher up, and could depend on the quantity, composition, and temperature of the atmosphere as well as the gravity. A pretty good example is the moon, which does not have 1/7th the atmosphere of earth, because it lost almost all its atmosphere.

That's why I mentioned Jupiter instead of the moon as a possible example. More than anything I was interested in identifying Earth-commonalities in intermediary layers of the atmospheres of larger-atmosphered planets. The gas-ring around a star idea is also interesting. I was also curious about the buoyancy of particular elements/compounds within an atmosphere. I thought that maybe there could be some reason that nitrogen, oxygen, and CO2 co-mingle in Earth's atmosphere that could also occur in a particular layer of another atmosphere, sandwiched between other layers, perhaps.

[doberman211]

So quick question, what is the atmospheric pressure to G ratio? how much KPa would one need to get to 1 G? And can one simply add the answer to say titan's .14G? I am attempting to calculate just how much Earthlike Titan is.

[steevey]

You have most of it, gravity and temperature are important factors in holding onto an atmosphere but the size of the planet would matter as well, if you took a planet 3/4 the size of the earth but with the same mass it could not hold onto as extensive an atmosphere as the Earth does now and the Earth couldn't hold onto as extensive an atmosphere as a planet 5/4 the size of the Earth could with the same mass. Saturn's moon Titan is a good example it has a huge atmosphere several times deeper than earths atmosphere and with a higher pressure and density. If Titans mass was in a smaller body it could not hold onto such an extensive atmosphere. the gravitational potential falls off slower from a large planet than a small planet if there masses are the same.

 

If the gravity of a planet smaller than Earth is much stronger than Earth, why can't great amounts of atmosphere exist if the gravity can hold it in place? There's even atmospheres of iron gas predicted to be around neutron stars. Are you saying it requires too much potential energy? Cause if a planet was small and had a large atmospheric mass, the atmosphere would extend further into space than on Earth, so would the atmosphere just have so much poential energy then that it would just leave the planet?

Edited February 25, 2011 by steevey