Gravity versus atmospheric pressure

[Mitchz95]

Is it hypothetically possible to thicken the atmosphere of a low-gravity world like Mars to the point that it feels like 1g on the surface? Just curious.

[kurja]

The two are unrelated. You could have a 100 kPa pressure but how would that make you feel like you were in 1g when you're not? It wouldn't.

[YNM]

Air pressure doesn't give you any feel of gravity. Look at ISS - fully pressurised yet still zero-g. In fact, it's the other way around - more gravity is needed to retain more atmosphere (but let's not get into how to make a Mars with Earth's atmosphere).

[mikegarrison]

30 minutes ago, kurja said:

The two are unrelated. You could have a 100 kPa pressure but how would that make you feel like you were in 1g when you're not? It wouldn't.

Well, they aren't "unrelated". Gravity is why an atmosphere has pressure.

I assume the question was whether you could raise the Martian atmospheric pressure to equal that of Earth at the surface. I'm pretty sure that if you had enough nitrogen and oxygen available you could. (Other gasses would work too, but ideally for us you would have the same 78/22 ratio that we have here. You would have to have quite a bit more than we have here, though, because the atmospheric pressure at the surface is the weight of all the air above it, and on Mars that weight would be lower.

In the long run, that atmosphere wouldn't stay there. With no protection from the solar wind and a light gravity, the sun would strip it off. But that would take a while.

[kurja]

1 hour ago, mikegarrison said:

Well, they aren't "unrelated". Gravity is why an atmosphere has pressure.

 

Of course. You can have either with or without the other though. Well I guess a 100kPa atmosphere of any composition couldn't naturally occur on some tiny body but the way the question was phrased is somewhat odd. Like asking if it would make you feel like you had pants if you had two shirts ,)

Edited March 15, 2016 by kurja

[Mitchz95]

The reason I asked is that I always see quotes that you would be crushed like a soda can on Venus. I guess I was equating that pressure to gravity.

[AngelLestat]

8 hours ago, Mitchz95 said:

Is it hypothetically possible to thicken the atmosphere of a low-gravity world like Mars to the point that it feels like 1g on the surface? Just curious.

no, is not possible, because all the mass of the atmosphere above you it will produce a gravity field in opposite direction which is closer than the gravity field caused by the atmosphere of the other side of the planet, but we are talking of gases, so distances here are larger.
In the same way that if you travel to the earth center, each time the gravity is lower, it would not be much difference here.
And the pressure and mass of the atmosphere should be huge to add some significant gravity field.

Just imagine saturn gravity and going down, the only thing that rise is pressure, gravity will be lower, and I guess it does not matter if it has a solid or very dense core, because at that level gases are also highly compress which increase its density.

 

---------------IGnore, you already explain your question-----------------------

Edited March 15, 2016 by AngelLestat

[wumpus]

Of course, you go the other way around.  A kerbal like creature could claim that you could "land" on Earth, and due to the oceans experience buoyancy to any gravity 1>=your choice>=0.

Note that to have an oxygen atmosphere at breathable levels, you will probably need pretty close to 1g gravity (you probably only need somewhere between Denver and Leadville air, or 1 and 2 miles of elevation.  Don't know the metric equivilant, but it has to be lower than Everest (i.e. the death zone).

[sevenperforce]

1 hour ago, Mitchz95 said:

The reason I asked is that I always see quotes that you would be crushed like a soda can on Venus. I guess I was equating that pressure to gravity.

Nah, the atmospheric pressure on Venus crushes you, not the gravity. The gravity on Venus is lower than Earth's.

When you crush a soda can with your foot, you're not changing gravity for it; you're simply crushing it. Same with Venus; the very thick atmosphere is heavy enough to crush you.

2 minutes ago, wumpus said:

Note that to have an oxygen atmosphere at breathable levels, you will probably need pretty close to 1g gravity (you probably only need somewhere between Denver and Leadville air, or 1 and 2 miles of elevation.  Don't know the metric equivilant, but it has to be lower than Everest (i.e. the death zone).

Not really. You can have a breathable oxygen atmosphere as long as you have the right partial pressure of oxygen; the overall atmospheric pressure doesn't need to be particularly close to Earth's. Of course, having a higher proportion of oxygen in the atmosphere makes fires a bit more...fiery.

And you don't need 1 g to have an atmospheric pressure equal to Earth's, either; you just need a thicker atmosphere.

 

[Mitchz95]

19 minutes ago, sevenperforce said:

Nah, the atmospheric pressure on Venus crushes you, not the gravity. The gravity on Venus is lower than Earth's.

When you crush a soda can with your foot, you're not changing gravity for it; you're simply crushing it. Same with Venus; the very thick atmosphere is heavy enough to crush you.

But my foot is still pushing down the can, is it not? How is that different from the can being pulled down by gravity? If I was stepping on the can on Mars, it would take more mass to crush it than if I was on Earth.

Sorry if these are stupid questions. It just popped into my head last night.

[sevenperforce]

Just now, Mitchz95 said:

But my foot is still pushing down the can, is it not? How is that different from the can being pulled down by gravity? If I was stepping on the can on Mars, it would take more mass to crush it than if I was on Earth.

Sorry if these are stupid questions. It just popped into my head last night.

That's okay, it's not a stupid question.

I happen to have a soda can on my desk right now. Gravity is definitely pulling down on it, right? But the aluminum walls are strong enough to hold up under their own weight. If I want to crush the soda can, I will need to make the force pulling down on it greater than the strength of the aluminum walls.

One way to do that is to increase the gravity. If Earth was fifty times as heavy but still the same size, the gravitational force on the can would be fifty times greater and the walls would collapse.

There's an easier way to increase the force, though. If I place an object on top of the soda can that is much more massive than the can, then all that weight will also push down on the can (due to gravity) and it will collapse.

If I'm doing the same thing on a world with lower gravity, I will need to add more mass to produce the same amount of force. But that's easy enough to do; just keep stacking stuff on top.

Venus has lower gravity than Earth, but its atmosphere has a lot more "stuff" in it, so it weighs much more. 

 

[Mitchz95]

7 minutes ago, sevenperforce said:

That's okay, it's not a stupid question.

I happen to have a soda can on my desk right now. Gravity is definitely pulling down on it, right? But the aluminum walls are strong enough to hold up under their own weight. If I want to crush the soda can, I will need to make the force pulling down on it greater than the strength of the aluminum walls.

One way to do that is to increase the gravity. If Earth was fifty times as heavy but still the same size, the gravitational force on the can would be fifty times greater and the walls would collapse.

There's an easier way to increase the force, though. If I place an object on top of the soda can that is much more massive than the can, then all that weight will also push down on the can (due to gravity) and it will collapse.

If I'm doing the same thing on a world with lower gravity, I will need to add more mass to produce the same amount of force. But that's easy enough to do; just keep stacking stuff on top.

Venus has lower gravity than Earth, but its atmosphere has a lot more "stuff" in it, so it weighs much more. 

 

But doesn't the added downwards force add to the perceived gravity? It still sounds to me like it should make no difference whether gravity is pulling it down, or something heavy is pushing it down. Whether you turn up the gravity fifty times, or put something fifty times as heavy on top, the end result is the same.

Edited March 15, 2016 by Mitchz95

[sevenperforce]

2 minutes ago, Mitchz95 said:

But doesn't the added downwards force add to the object's perceived gravity? It still sounds to me like it should make no difference whether gravity is pulling it down, or something heavy is pushing it down. Whether you turn up the gravity fifty times, or put something fifty times as heavy on top, the end result is the same.

Ah, "perceived gravity". I see what you're asking.

Gravity pulls us down. Atmospheric pressure, on the other hand, pushes against us from all directions, including up.

If you're crushed by atmospheric pressure, you implode; if you're crushed by gravity, you splat.

[AngelLestat]

2 minutes ago, sevenperforce said:

Ah, "perceived gravity". I see what you're asking.

Gravity pulls us down. Atmospheric pressure, on the other hand, pushes against us from all directions, including up.

If you're crushed by atmospheric pressure, you implode; if you're crushed by gravity, you splat.

You would not implode in case the air you are breathing is part of the same atmosphere (all directions as you said), our bodies are incompressible.  Only gases can change its density by pressure, unless we are on pressures higher than millions of bar.

The only problem with pressure comes from the fact that nitrogen becomes narcotic in our blood under certain pressure.
If we ignore that fact, we would be ok no matter how thin or thick would be our atmosphere..  
1000 bar or  0.1 bar.

The problem with forces on our body only matter if they are not uniform..  a gravity field produce uniform forces, so in free fall nothing happen.
Pressure produce uniform forces..  but if something hit you as a stone, you are receiving a force just in one part of your body and is not uniform.. so you get hurt.

[Mitchz95]

8 minutes ago, sevenperforce said:

Ah, "perceived gravity". I see what you're asking.

Gravity pulls us down. Atmospheric pressure, on the other hand, pushes against us from all directions, including up.

If you're crushed by atmospheric pressure, you implode; if you're crushed by gravity, you splat.

Ah, now I understand. I guess I was thinking that since the atmosphere is pulled down by gravity, it would be like a blanket that further weighted down anything on the surface.

Thank you.

[sevenperforce]

10 minutes ago, AngelLestat said:

You would not implode in case the air you are breathing is part of the same atmosphere (all directions as you said), our bodies are incompressible.  Only gases can change its density by pressure, unless we are on pressures higher than millions of bar.

The only problem with pressure comes from the fact that nitrogen becomes narcotic in our blood under certain pressure.
If we ignore that fact, we would be ok no matter how thin or thick would be our atmosphere..  
1000 bar or  0.1 bar.

At low pressure you have extreme embolisms and the moisture in your mucus membranes starts to boil away. At high pressure your eardrums rupture and it becomes impossible to expel air from your lungs.

[AngelLestat]

4 minutes ago, sevenperforce said:

At low pressure you have extreme embolisms and the moisture in your mucus membranes starts to boil away. At high pressure your eardrums rupture and it becomes impossible to expel air from your lungs.

You did not read what I said..  if you remove the issues of gases (like nitrogen, or in the case of higher pressure oxygen) in our blood stream "embolisms", then you dont have problem with high pressure.
About low pressure, I mention 0.1 bar, this is higher than the Armstrong limit.

[sevenperforce]

1 minute ago, AngelLestat said:

You did not read what I said..  if you remove the issues of gases (like nitrogen, or in the case of higher pressure oxygen) in our blood stream "embolisms", then you dont have problem with high pressure.
About low pressure, I mention 0.1 bar, this is higher than the Armstrong limit.

Gas embolisms are not the only type of embolisms. And high pressure will still rupture your ear drums and make it impossible for you to breathe.

You mentioned 0.1 bar, but you said "we would be ok no matter how thin or thick would be our atmosphere" without limiting it to the range; it seemed like 0.1 bar and 1000 bar were just examples. If you really meant "we could survive a range of pressures between 0.1 bar and 1000 bar" then that is a different claim.

[Bill Phil]

If that was how it works, then you'd feel a few tons of force here on Earth.

The pressure pushes in all directions.

10 hours ago, YNM said:

Air pressure doesn't give you any feel of gravity. Look at ISS - fully pressurised yet still zero-g. In fact, it's the other way around - more gravity is needed to retain more atmosphere (but let's not get into how to make a Mars with Earth's atmosphere).

What about Titan?

[AngelLestat]

Just now, sevenperforce said:

Gas embolisms are not the only type of embolisms. And high pressure will still rupture your ear drums and make it impossible for you to breathe.

You mentioned 0.1 bar, but you said "we would be ok no matter how thin or thick would be our atmosphere" without limiting it to the range; it seemed like 0.1 bar and 1000 bar were just examples. If you really meant "we could survive a range of pressures between 0.1 bar and 1000 bar" then that is a different claim.

Once your ears equal pressure as always does each time you change of altitude, then the problem is solve.
You can get tired at higher pressures to breath, that will also depend on the oxygen concentration, but you dont need to breath so frequent at higher pressure because there is much more oxygen in each cm3 of air.
I am not really sure what other issues you might have other than embolism at higher pressure, that is why I put 1000 bar as upper limit.
But the main point, is that you dont implode, because you are breathing air at the same pressure..  Is similar of what happens with fishes, they can live at extreme pressures even if their bodies are ultra delicates and light.   

Just now, Bill Phil said:

If that was how it works, then you'd feel a few tons of force here on Earth.

The pressure pushes in all directions.

we feel 12 tons by m2 in earth, which is equal to 1,2 kg by cm2.
This is why I hate of some sensational tv scientist, when they said.. if you are in venus surface, it will be like to had 100 cars above you..

[fredinno]

3 hours ago, AngelLestat said:

no, is not possible, because all the mass of the atmosphere above you it will produce a gravity field in opposite direction which is closer than the gravity field caused by the atmosphere of the other side of the planet, but we are talking of gases, so distances here are larger.
In the same way that if you travel to the earth center, each time the gravity is lower, it would not be much difference here.
And the pressure and mass of the atmosphere should be huge to add some significant gravity field.

Just imagine saturn gravity and going down, the only thing that rise is pressure, gravity will be lower, and I guess it does not matter if it has a solid or very dense core, because at that level gases are also highly compress which increase its density.

 

---------------IGnore, you already explain your question-----------------------

No, Saturn' gravity increases as you go down as it gets more dense as you do so.

[wumpus]

2 hours ago, sevenperforce said:

Nah, the atmospheric pressure on Venus crushes you, not the gravity. The gravity on Venus is lower than Earth's.

When you crush a soda can with your foot, you're not changing gravity for it; you're simply crushing it. Same with Venus; the very thick atmosphere is heavy enough to crush you.

Not really. You can have a breathable oxygen atmosphere as long as you have the right partial pressure of oxygen; the overall atmospheric pressure doesn't need to be particularly close to Earth's. Of course, having a higher proportion of oxygen in the atmosphere makes fires a bit more...fiery.

And you don't need 1 g to have an atmospheric pressure equal to Earth's, either; you just need a thicker atmosphere.

 

The catch is that oxygen isn't going to stick around [at the right partial pressure] on a planet with much less than 1g gravity (assuming that nitrogen is pretty much the limit here).  Of course, oxygen isn't going to stick around on *any* planet (due to how it loves to oxydize things), you need plants or similar producing it on a regular basis.  If the oxygen is slipping away from gravity, there had better be some enormous source of oxygen the "plants" are getting it from.

The takeaway here is that you could probably get away with short-term (millenia) teraforming of low g planets, but don't expect a breathable atmosphere on a low-g planet to happen naturally.

[AngelLestat]

7 hours ago, fredinno said:

No, Saturn' gravity increases as you go down as it gets more dense as you do so.

It could be, but there is no info about that because we dont know how density change with altitude, if it has jumps where density increase a lot over certain layers then yeah, it could rise, but is not sure. 
And when I said that the gravity can lower proportional to the height, we should always start from what we call surface gravity for gas giants, which is the 1 bar pressure level.
 

[Mitchz95]

10 hours ago, Bill Phil said:

What about Titan?

From what I've read, the reason Titan and Pluto have atmospheres is that they're farther away from the Sun, so it's harder for the air to get hot enough to reach escape velocity.

[YNM]

@ Bill Phill : Titan is a mixture of low-temperature environtment and fairly high molecular mass gasses. I once read somewhere about if you have gasses moving with average kinetic energy (another way to say average temperature) a sixth of escape velocity energy (which should contain the mass of the gas particles), you'll see the gasses escaping from the planet's atmosphere. I haven't searched any additional proof to back that up though.