# Thread: How much mass does an object need to acquire an atmosphere?

1. Earth has its atmosphere because of Its mass right? So that begs the question how much mass does an object have to have in order for it to gain an atmosphere or does every object have a miniature atmosphere of their own? I suppose it would also depend on the following: The composition of the atmosphere in question e.g The gas density and whether or not other heavier objects with greater gravitational pulls are cancelling out the object in question.

On a side note: by atmosphere I don't necessarily mean it has to be thick one like the one earth has, I'm Just wondering is there a starting point at which a object begins to gravitationally attract gas or would it only depend on the two factors I stated above?

So for example, If i were floating around in the middle of space with no other bodies with a greater mass in sight to cancel out my gravitational pull and a cluster of gas floated by. Wouldn't that air be gravitationally attracted to me and become my atmosphere considering I would have a much higher mass then the gas? if so why or why not? I was also wondering when a spaceship escapes the earths atmosphere does it then take a bit of the atmosphere with it because It is heavier then the gases? Or does the gravitational pull from the earth outweigh the pull from the spaceship? If not, What else stops the spaceship from not achieving a small atmosphere of air?
If my question does not make sense, I will kindly elaborate.

2.

3. A celestial body has atmosphere because it wasn't removed by solar wind, vacuum, angular momentum etc. The moon has an atmosphere, the sun has atmosphere, even large asteroids have atmosphere, only how much it has, varies.

The weight of a gas has little to no effect on how high in the atmosphere it is, nor on how quickly it is removed. Same rules apply to a feather and a lead weight, they drop the same speed. And even though helium is lighter than argon, it also disperses at different rates, which prevents us from having to breath too much xenon and argon.

And no, if you are too small, the vacuum will keep the atmosphere too low to detect around you.

And yes, if a spaceship leaves the atmosphere it creates turbulence which throws some air into space.

4. The limit on what gases a planet can hang onto in our solar system is a combination of the sun's radiation and the planet's mass. Gases surrounding a planet absorb solar energy, and if they get heated hot enough the individual molecules pick up enough velocity they can reach escape velocity and leave the planet. This why the inner planets, Mercury through Mars, are largely rocky, while the outer planets, Jupiter through Uranus are mostly gas. The sun has heated the inner planets enough that most of their gas has escaped. Some gases are more prone to being lost due to the sun's heating than others, gases with the lightest molecules tend to be most susceptible. This is why there is little hydrogen and helium, the two lightest elements, in Earth's atmosphere but the gas giants have more hydrogen and helium than anything else. The smaller the planet or other body, the lower its escape velocity, and the less energy gas molecules need to reach escape velocity. This is why smaller bodies, like astronauts, asteroids, and all but the largest moons do not retain any atmosphere.

5. Originally Posted by danhanegan
The limit on what gases a planet can hang onto in our solar system is a combination of the sun's radiation and the planet's mass. Gases surrounding a planet absorb solar energy, and if they get heated hot enough the individual molecules pick up enough velocity they can reach escape velocity and leave the planet. This why the inner planets, Mercury through Mars, are largely rocky, while the outer planets, Jupiter through Uranus are mostly gas. The sun has heated the inner planets enough that most of their gas has escaped. Some gases are more prone to being lost due to the sun's heating than others, gases with the lightest molecules tend to be most susceptible. This is why there is little hydrogen and helium, the two lightest elements, in Earth's atmosphere but the gas giants have more hydrogen and helium than anything else. The smaller the planet or other body, the lower its escape velocity, and the less energy gas molecules need to reach escape velocity. This is why smaller bodies, like astronauts, asteroids, and all but the largest moons do not retain any atmosphere.
The moon has Sodium/Potassium in its atmosphere. Mostly Argon, but also Helium, CO2, CO, N2 and CH4, Hydrogen and some radon/polonium. About 10^-14th of the density of earth's atmosphere. In space in comparison you will find between 10^-16 and 10^-28 times lower pressure than earths atmosphere.

To the gas giant planets now.. What you claim is an educated guess, something thought to be true a few years ago, but this is no longer the case. We have observed gas giants in other starsystems, which are close to their sun, and rocky planets further away.

6. When it comes to the second lightest gas, Helium, our Earth will lose the last of it in the 2040 to 1050 time frame. The second paragraph in the linked webpage says that helium is a non-renewable resource. I wonder if fusion energy will be a reality by then and the helium by-product will prove enough to supply the users of helium sufficiently.

It seems to help having a spinning, molten iron core (as Earth has) that produces a magnetic field, that creates a magnetosphere, that shields the atmosphere to some extent, thus preventing the solar wind from stripping away the Earth's atmosphere.

I would also think the amount of gas that a mass attracts is related to the amount of gas nearby celestial objects attract, as well as its distance from the mass, etc because it seems like a tug-o-war situation to me.

7. The weight of a gas has little to no effect on how high in the atmosphere it is, nor on how quickly it is removed
This is only true if the atmosphere is thick enough that turbulent mixing dominates over molecular diffusion. See my answers in this thread for more details : will hydrogen gas be on top in atmosphere?

8. Originally Posted by Zwolver
A celestial body has atmosphere because it wasn't removed by solar wind, vacuum, angular momentum etc. The moon has an atmosphere, the sun has atmosphere, even large asteroids have atmosphere, only how much it has, varies.

The weight of a gas has little to no effect on how high in the atmosphere it is, nor on how quickly it is removed. Same rules apply to a feather and a lead weight, they drop the same speed. And even though helium is lighter than argon, it also disperses at different rates, which prevents us from having to breath too much xenon and argon.

And no, if you are too small, the vacuum will keep the atmosphere too low to detect around you.

And yes, if a spaceship leaves the atmosphere it creates turbulence which throws some air into space.
Thank you for your explanation, It clears things up however I believe you misinterpreted the Gas density point but none the less I am grateful.
To clear up the reason I stated Gas density: It could be the case that the gas is not light enough/too heavy to be pulled in by a particular body and so It cannot form an atmosphere with a particular gas. For Instance, If it were Sulfur Hexafluoride floating by It would be far too heavy to be gravitationally pulled towards the body (assuming it is a relatively light object) and so It would not form an atmosphere hence the reason I considered it a factor.
Feel free to correct me If I am wrong.

9. Originally Posted by jrmonroe
It seems to help having a spinning, molten iron core (as Earth has) that produces a magnetic field, that creates a magnetosphere, that shields the atmosphere to some extent, thus preventing the solar wind from stripping away the Earth's atmosphere.
Are you referring to the Helium? If so, Where did you read this? Seems Interesting.

10. Originally Posted by Ascendance
Originally Posted by jrmonroe
It seems to help having a spinning, molten iron core (as Earth has) that produces a magnetic field, that creates a magnetosphere, that shields the atmosphere to some extent, thus preventing the solar wind from stripping away the Earth's atmosphere.
Are you referring to the Helium? If so, Where did you read this? Seems Interesting.
No, he is referring to all of the gasses in the atmosphere.

11. Originally Posted by Zwolver
Originally Posted by Ascendance
Originally Posted by jrmonroe
It seems to help having a spinning, molten iron core (as Earth has) that produces a magnetic field, that creates a magnetosphere, that shields the atmosphere to some extent, thus preventing the solar wind from stripping away the Earth's atmosphere.
Are you referring to the Helium? If so, Where did you read this? Seems Interesting.
No, he is referring to all of the gasses in the atmosphere.
Ah, right Sorry. I misunderstood.

12. Originally Posted by jrmonroe
When it comes to the second lightest gas, Helium, our Earth will lose the last of it in the 2040 to 1050(sic) time frame.
This is very badly phrased, to the point of being wrong as written. The availability of readily accessible helium using present methods will likely occur in the timeframe you note. There will still be very large quantities of it in above the planet.

13. Originally Posted by jrmonroe
When it comes to the second lightest gas, Helium, our Earth will lose the last of it in the 2040 to 1050 time frame. The second paragraph in the linked webpage says that helium is a non-renewable resource. I wonder if fusion energy will be a reality by then and the helium by-product will prove enough to supply the users of helium sufficiently.

It seems to help having a spinning, molten iron core (as Earth has) that produces a magnetic field, that creates a magnetosphere, that shields the atmosphere to some extent, thus preventing the solar wind from stripping away the Earth's atmosphere.

I would also think the amount of gas that a mass attracts is related to the amount of gas nearby celestial objects attract, as well as its distance from the mass, etc because it seems like a tug-o-war situation to me.
Well how do you explain Venus, which has no magnetic field and a very dense atmosphere? The type of atmosphere a rocky world might have is very dependent on how active the planet is (vulcanism), then gravity and magnetic fields play their part in the equilibrium that establishes itself on a planet. Personally I would not call what the moon has, an atmosphere. But the gas molecules are close to the moon and can be referred to as an atmosphere even though we have a problem making a vacuum as good as that on the moon here on Earth.

14. Originally Posted by Ascendance
Earth has its atmosphere because of Its mass right? So that begs the question how much mass does an object have to have in order for it to gain an atmosphere or does every object have a miniature atmosphere of their own? I suppose it would also depend on the following: The composition of the atmosphere in question e.g The gas density and whether or not other heavier objects with greater gravitational pulls are cancelling out the object in question.

On a side note: by atmosphere I don't necessarily mean it has to be thick one like the one earth has, I'm Just wondering is there a starting point at which a object begins to gravitationally attract gas or would it only depend on the two factors I stated above?

So for example, If i were floating around in the middle of space with no other bodies with a greater mass in sight to cancel out my gravitational pull and a cluster of gas floated by. Wouldn't that air be gravitationally attracted to me and become my atmosphere considering I would have a much higher mass then the gas? if so why or why not? I was also wondering when a spaceship escapes the earths atmosphere does it then take a bit of the atmosphere with it because It is heavier then the gases? Or does the gravitational pull from the earth outweigh the pull from the spaceship? If not, What else stops the spaceship from not achieving a small atmosphere of air?
If my question does not make sense, I will kindly elaborate.
I think I'm with danhanegan on this. I'm fairly sure I was taught it is all to do with whether or not the velocity of the molecules at temperatures prevailing in the atmosphere is greater or less than the escape velocity. Of course the molecular speed is distributed over a bell curve and there will always in theory be some "tail" of energetic molecules able to escape. But the bigger this "tail" fraction becomes the more rapidly the atmosphere will be lost.

I seem to recall that one can do a calculation that shows that on Earth the rms molecular velocity of hydrogen exceeds escape velocity, hence why there is almost no free hydrogen in the atmosphere, but I could be misremembering this.

15. Someone mentioned escape velocity but didn't also mention that escape velocity decreases with distance from the Earths surface. In the exosphere layer of the atmosphere the escape velocity is very small. Please read the comments clipped from Wikipedia below.

From Wikipedia

The atmosphere has a mass of about 5.15×1018 kg, three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. The Kármán line, at 100 km (62 mi), or 1.57% of Earth's radius, is often used as the border between the atmosphere and outer space. Atmospheric effects become noticeable during atmospheric reentry of spacecraft at an altitude of around 120 km (75 mi). Several layers can be distinguished in the atmosphere, based on characteristics such as temperature and composition.

Structure of the atmosphere
Principal layers

Exosphere: >700 km (>440 miles)
Thermosphere: 80 to 700 km (50 to 440 miles)
Mesosphere: 50 to 80 km (31 to 50 miles)
Stratosphere: 12 to 50 km (7 to 31 miles)
Troposphere: 0 to 12 km (0 to 7 miles)

Exosphere

The exosphere is the outermost layer of Earth's atmosphere (i.e. the upper limit of the atmosphere). It extends from the exobase, which is located at the top of the thermosphere at an altitude of about 700 km above sea level, to about 10,000 km (6,200 mi; 33,000,000 ft). The exosphere merges with the emptiness of outer space, where there is no atmosphere.

This layer is mainly composed of extremely low densities of hydrogen, helium and several heavier molecules including nitrogen, oxygen and carbon dioxide closer to the exobase. The atoms and molecules are so far apart that they can travel hundreds of kilometers without colliding with one another. Thus, the exosphere no longer behaves like a gas, and the particles constantly escape into space. These free-moving particles follow ballistic trajectories and may migrate in and out of the magnetosphere or the solar wind.

The exosphere is located too far above Earth for any meteorological phenomena to be possible. However, the aurora borealis and aurora australis sometimes occur in the lower part of the exosphere, where they overlap into the thermosphere. The exosphere contains most of the satellites orbiting Earth.

16. Originally Posted by Bad Robot
Someone mentioned escape velocity but didn't also mention that escape velocity decreases with distance from the Earths surface. In the exosphere layer of the atmosphere the escape velocity is very small. Please read the comments clipped from Wikipedia below.

From Wikipedia

The atmosphere has a mass of about 5.15×1018 kg, three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. The Kármán line, at 100 km (62 mi), or 1.57% of Earth's radius, is often used as the border between the atmosphere and outer space. Atmospheric effects become noticeable during atmospheric reentry of spacecraft at an altitude of around 120 km (75 mi). Several layers can be distinguished in the atmosphere, based on characteristics such as temperature and composition.

Structure of the atmosphere
Principal layers

Exosphere: >700 km (>440 miles)
Thermosphere: 80 to 700 km (50 to 440 miles)
Mesosphere: 50 to 80 km (31 to 50 miles)
Stratosphere: 12 to 50 km (7 to 31 miles)
Troposphere: 0 to 12 km (0 to 7 miles)

Exosphere

The exosphere is the outermost layer of Earth's atmosphere (i.e. the upper limit of the atmosphere). It extends from the exobase, which is located at the top of the thermosphere at an altitude of about 700 km above sea level, to about 10,000 km (6,200 mi; 33,000,000 ft). The exosphere merges with the emptiness of outer space, where there is no atmosphere.

This layer is mainly composed of extremely low densities of hydrogen, helium and several heavier molecules including nitrogen, oxygen and carbon dioxide closer to the exobase. The atoms and molecules are so far apart that they can travel hundreds of kilometers without colliding with one another. Thus, the exosphere no longer behaves like a gas, and the particles constantly escape into space. These free-moving particles follow ballistic trajectories and may migrate in and out of the magnetosphere or the solar wind.

The exosphere is located too far above Earth for any meteorological phenomena to be possible. However, the aurora borealis and aurora australis sometimes occur in the lower part of the exosphere, where they overlap into the thermosphere. The exosphere contains most of the satellites orbiting Earth.
Yes I like very much the point about molecules observing ballistic trajectories, with almost no intermolecular collisions.

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