Gravity versus atmospheric pressure

[Bill Phil]

3 hours ago, Mitchz95 said:

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.

Pluto, Triton, Mercury, Titan. And more have atmospheres. Yes, Mercury has one, too.

19 minutes ago, YNM said:

@ 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.

I'm just pointing out that high gravity isn't needed for atmospheres.

[YNM]

(Ignore, were having connection issues (and editor issues) on smartphone.)

Edited March 17, 2016 by YNM
If you want, please delete this post.

[kurja]

15 hours ago, Mitchz95 said:

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.

Thumb up Think how it feels like when you dive under water - there's heavy stuff above you, tons of water when you go down just a couple meters, but it's not pushing you down as pressure works equally in all directions. Quite the opposite actually because you get more buoyancy in the denser fluid.

[fredinno]

13 hours ago, Bill Phil said:

Pluto, Triton, Mercury, Titan. And more have atmospheres. Yes, Mercury has one, too.

I'm just pointing out that high gravity isn't needed for atmospheres.

Mercury does not have an "atmosphere" but an exosphere where none of the particles interact with each other.https://en.wikipedia.org/wiki/Atmosphere_of_Mercury

 

[Bill Phil]

31 minutes ago, fredinno said:

Mercury does not have an "atmosphere" but an exosphere where none of the particles interact with each other.https://en.wikipedia.org/wiki/Atmosphere_of_Mercury

 

It has an atmosphere composed of particles that are removed from the surface by solar wind. It's about 0.005 picobar.

[fredinno]

16 minutes ago, Bill Phil said:

It has an atmosphere composed of particles that are removed from the surface by solar wind. It's about 0.005 picobar.

It's thin enough not to be considered a true atmosphere. By your definition, the Moon would have an atmosphere.

[Bill Phil]

33 minutes ago, fredinno said:

It's thin enough not to be considered a true atmosphere. By your definition, the Moon would have an atmosphere.

It does.... ~10^-7 pascals... In the day.

There's still more stuff there than the surrounding vacuum.

Edited March 16, 2016 by Bill Phil

[sevenperforce]

Semantics. 

Most, I think, would require a thin collection of gas to have meaningfully measurable pressure and temperature in order to be considered an atmosphere. 

[fredinno]

7 minutes ago, sevenperforce said:

Semantics. 

Most, I think, would require a thin collection of gas to have meaningfully measurable pressure and temperature in order to be considered an atmosphere. 

Which is what exospheres lack. It's so thin, the particles do not act as a gas. https://en.wikipedia.org/wiki/Exosphere

[sevenperforce]

5 minutes ago, fredinno said:

Which is what exospheres lack. It's so thin, the particles do not act as a gas. https://en.wikipedia.org/wiki/Exosphere

Precisely my point. 

[Bill Phil]

18 minutes ago, sevenperforce said:

 

12 minutes ago, fredinno said:

Which is what exospheres lack. It's so thin, the particles do not act as a gas. https://en.wikipedia.org/wiki/Exosphere

But there's still gas particles there, and on Mercury there is pressure.

An atmosphere is just a layer of gases around a celestial object. There's gas around Mercury, it has an atmosphere. What the gas acts like is irrelevant.

[sevenperforce]

5 minutes ago, Bill Phil said:

But there's still gas particles there, and on Mercury there is pressure.

An atmosphere is just a layer of gases around a celestial object. There's gas around Mercury, it has an atmosphere. What the gas acts like is irrelevant.

We don't consider comets to have atmospheres. That's basically what we are talking about here. 

Again, it's semantics. If you want to define an atmosphere as any gas particles near the surface of a celestial body, fine, but the fact remains that there is a clear divide between bodies with persistent, gravitationally-bound gaseous atmospheric envelopes and bodies without anything like that. 

I'd also wager that the exosphere on Mercury is highly ionized and is thus more plasma than gas. 

[AngelLestat]

I think the only planets who possess an atmosphere are:

Venus, Earth, Mars, Jupiter, Saturn, Titan, Uranus, Neptune.

From all those Mars gases just barely qualify as an atmosphere, because its enough dense to have winds, climate, a considerable reentry effect and due its low gravity aerodynamics methods works.
But from a different perspective compared to earth, is almost the same than vacuum, lets said that vacuum habitats needs to stand for 12000kg, well mars habitats needs to stand for 11500kg or something like that, I dont remember the exact numbers.

All the other cases, we are talking about pressures that are only achievable with special laboratory pumps. Which in overall effect, is the same than nothing. 

[YNM]

Atmosphere... What is it ? Basically you're having gasses near any celestial body. It only matters (worth to be called atmosphere) when it's presence make massive changes to the face of the planet, such as how life blooms on Earth, how Venus have the hottest surface temperature, how Mars have clouds, how the gas giants have clouds, how Titan have lakes, how Pluto have weird coloring etc.

Anything more insubstantial isn't a proper atmosphere.

Edited March 17, 2016 by YNM

[kerbiloid]

Not a "weight of air column above you" makes the pressure, but "amount of air molecules per volume unit, at the given temperature".

Gravity does not create the atmospheric pressure, it just keeps air molecules in a volume around you, preventing these molecules from just flying away into space.
The greater is gravity, the more molecules are stretched together in the same volume.

So, atmospheric pressure is omnidirectional and it absolutely doesn't press your boots to the floor, pushing you down from your head. It presses you from all you body surface, from all directions.
You can be squished by gravity on an airless but large or dense planet, and you can happily breath in a barrel full with air flying in the deep space with zero-G.

[fredinno]

23 hours ago, AngelLestat said:

I think the only planets who possess an atmosphere are:

Venus, Earth, Mars, Jupiter, Saturn, Titan, Uranus, Neptune.

From all those Mars gases just barely qualify as an atmosphere, because its enough dense to have winds, climate, a considerable reentry effect and due its low gravity aerodynamics methods works.
But from a different perspective compared to earth, is almost the same than vacuum, lets said that vacuum habitats needs to stand for 12000kg, well mars habitats needs to stand for 11500kg or something like that, I dont remember the exact numbers.

All the other cases, we are talking about pressures that are only achievable with special laboratory pumps. Which in overall effect, is the same than nothing. 

TITAN IS NOT A PLANET

[Yourself]

On 3/17/2016 at 1:46 AM, kerbiloid said:

So, atmospheric pressure is omnidirectional and it absolutely doesn't press your boots to the floor, pushing you down from your head. It presses you from all you body surface, from all directions.

Technically atmospheric pressure does the opposite, it lifts you.  After all, that's just the buoyant force and it's directly proportional to the density of the atmosphere. For example, on earth at sea level the presence of the atmosphere makes you about 0.1% lighter than you would be in a vacuum.

All of this is because the air at your feet has an ever-so-slightly higher pressure than the air at your head, so the net effect over your body is an upward force.

Of course it's also possible to get the effect where the atmosphere holds you down.  That's how suction cups work.