Planetary atmospheres

[Moontanman]

Does the size vs the density of a planet have an effect on the extensiveness of the atmosphere? More specifically would an earth sized planet with 2x earths gravity have a more or less extensive atmosphere than a planet 2x the size of the earth with 2x the gravity of earth or a planet 2.5 times the size of the earth with 2x the gravity? 

[Janus]

Too many other variables to consider. For example, Venus is slightly less massive than the Earth, but has an atmospheric pressure 95 times that of Earth.

[Moontanman]

1 hour ago, Janus said:

Too many other variables to consider. For example, Venus is slightly less massive than the Earth, but has an atmospheric pressure 95 times that of Earth.

I think I'd have to say "with all other things being equal" if that helps. 

[studiot]

On 8/10/2022 at 6:12 PM, Moontanman said:

Does the size vs the density of a planet have an effect on the extensiveness of the atmosphere? More specifically would an earth sized planet with 2x earths gravity have a more or less extensive atmosphere than a planet 2x the size of the earth with 2x the gravity of earth or a planet 2.5 times the size of the earth with 2x the gravity? 

Hi, Moon.

I'm not sure why you homed in on the 'extensiveness' of the atmosphere  ?

As Janus points out there are many variables involved and it should be borne in mind that the importance of a planet's atmosphere does not depend upon its extensiveness.

A fine example of that is the discoveries made by the New Horizons team in respect of Pluto, whose atmosphere was regarded as 'thin to non existent' before the spacecraft flew by.
Now we know the profound effect this thin to non existent atmosphere has on that planet's geohistory.

 

 

[Moontanman]

2 hours ago, studiot said:

Hi, Moon.

I'm not sure why you homed in on the 'extensiveness' of the atmosphere  ?

As Janus points out there are many variables involved and it should be borne in mind that the importance of a planet's atmosphere does not depend upon its extensiveness.

A fine example of that is the discoveries made by the New Horizons team in respect of Pluto, whose atmosphere was regarded as 'thin to non existent' before the spacecraft flew by.
Now we know the profound effect this thin to non existent atmosphere has on that planet's geohistory.

 

 

The idea i am referring to is that a small dense object's gravity falls off faster than a large less dense object with the same surface gravity. 

[mistermack]

5 hours ago, Moontanman said:

The idea i am referring to is that a small dense object's gravity falls off faster than a large less dense object with the same surface gravity. 

I'm speculating here, but I would imagine that that's because on a small dense object, if you raise up by one metre, that's a greater percentage of the distance from the centre of gravity than a metre on a large less dense object. Because on the less dense object, you are starting from a 'higher' surface.

[exchemist]

9 hours ago, Moontanman said:

The idea i am referring to is that a small dense object's gravity falls off faster than a large less dense object with the same surface gravity. 

Does it? Surely it depends only on the the planet’s mass, per Newton’s law?

[mistermack]

6 hours ago, exchemist said:

Does it? Surely it depends only on the the planet’s mass, per Newton’s law?

You would have to do the maths. For a less dense planet to have the same surface gravity, it would have to be more massive, and would have a bigger radius, so the maths is quite different for the two cases.

[studiot]

16 hours ago, Moontanman said:

The idea i am referring to is that a small dense object's gravity falls off faster than a large less dense object with the same surface gravity. 

On first reading I thought only mass was involved, but on more carefull reading I see that you have incorporated density difference.

I have now had the time to sketch out some maths.

 

[swansont]

16 hours ago, Moontanman said:

The idea i am referring to is that a small dense object's gravity falls off faster than a large less dense object with the same surface gravity. 

Do you mean the density of the two is equal?

[Moontanman]

45 minutes ago, swansont said:

Do you mean the density of the two is equal?

No, I am referring to the gravity at the surface, density would vary as the surface gravity differs on each object. The larger object would have less density, I used to have a site I could go to that would calculate these things for me after I plugged in the parameters. If i remember correctly the larger object would have 15.625 times the volume of the earth, the mid sized object would have 8 times the volume of the earth and the smaller one would have 1 times the volume of earth. which would mean the smaller object would have to have 2x the density of earth, the medium sized object would have the same density as earth and the largest object would have less density than earth. I can't remember the mass of the larger object but it seems as though it was someplace close to 12 times the mass of the earth.  

1 hour ago, studiot said:

On first reading I thought only mass was involved, but on more carefull reading I see that you have incorporated density difference.

I have now had the time to sketch out some maths.

 

Thanks for the math but I am afriad it is beyond my limited ability. 

19 hours ago, studiot said:

Hi, Moon.

I'm not sure why you homed in on the 'extensiveness' of the atmosphere  ?

As Janus points out there are many variables involved and it should be borne in mind that the importance of a planet's atmosphere does not depend upon its extensiveness.

A fine example of that is the discoveries made by the New Horizons team in respect of Pluto, whose atmosphere was regarded as 'thin to non existent' before the spacecraft flew by.
Now we know the profound effect this thin to non existent atmosphere has on that planet's geohistory.

 

 

By vastness I am alluding to the volume and mass of the atmosphere, like how Saturn's moon Titan has a very large atmosphere, deeper than even earth's. 

[exchemist]

1 hour ago, mistermack said:

You would have to do the maths. For a less dense planet to have the same surface gravity, it would have to be more massive, and would have a bigger radius, so the maths is quite different for the two cases.

Oh I see what you mean. The gravity actually at the surface will have lateral components, due to the fact the mass is not all lined up beneath one's feet, whereas as one climbs higher above the surface the force increasingly becomes all orientated downwards.

So yes, I can now see that a more diffuse mass will have weaker surface gravity than a more concentrated body of the same mass. So, given that, at a large distance above the surface the gravity from both would tend toward the same value, that would imply the gravity falls off more slowly with the diffuse body than the more compact one.   

[studiot]

49 minutes ago, Moontanman said:

By vastness I am alluding to the volume and mass of the atmosphere, like how Saturn's moon Titan has a very large atmosphere, deeper than even earth's. 

Yes I understand that, but I am asking why is that important ?

Did you catch my comment/question about the importance of Pluto's very tenuous atmosphere ?

51 minutes ago, Moontanman said:

Thanks for the math but I am afriad it is beyond my limited ability. 

It is really only very simple arithmetic to derive the end formula which is vry simple and all that matters.

Skip to the end and let swansont or joigus or janus dissect it, as I'm sure they will.

I will try to plot a suitable graph of the result, that may help visualisation.

I should also point out that some planets are spinning faster than others and the rotation makes a measurable difference to the apparent gravity.
My calculations assumed a non rotating planet.

[mistermack]

46 minutes ago, Moontanman said:

By vastness I am alluding to the volume and mass of the atmosphere, like how Saturn's moon Titan has a very large atmosphere, deeper than even earth's. 

There are other variables at play too. Mars once had a substantial atmosphere, with resulting greenhouse effect and liquid water. But it lost nearly all of it. The difference between Mars and the Earth was I believe the magnetic field, which on Earth protects the atmosphere from the worst effects of the solar wind. Also the greater mass of the Earth increases the escape velocity. And Mars has little current volcanic activity, so replacement gases are slow in coming.

So over millions of years the solar wind stripped the atmosphere off Mars, which caused the current freezing conditions.

Compare Venus to Mars, it's much more massive, so even though the solar wind is stronger, it's kept it's atmosphere. It's also likely that it's still volcanically active. 

[Moontanman]

On 8/12/2022 at 9:59 AM, studiot said:

Yes I understand that, but I am asking why is that important ?

I am setting the stage for a series of short stories set on a large planet with a 10 bar atmosphere and a group of human researchers who live on the planet. I am trying to make the initial set up as accurate as possible. I am also trying to get a reasonable way to include helium in the atmosphere. 

On 8/12/2022 at 9:59 AM, studiot said:

Did you catch my comment/question about the importance of Pluto's very tenuous atmosphere ?

Yess I did. 

On 8/12/2022 at 9:59 AM, studiot said:

It is really only very simple arithmetic to derive the end formula which is vry simple and all that matters.

Skip to the end and let swansont or joigus or janus dissect it, as I'm sure they will.

I will try to plot a suitable graph of the result, that may help visualisation.

I should also point out that some planets are spinning faster than others and the rotation makes a measurable difference to the apparent gravity.
My calculations assumed a non rotating planet.

My planet will rotate in about 36 hours. 

On 8/12/2022 at 8:07 AM, swansont said:

 

Edited August 13, 2022 by Moontanman

[mistermack]

1 minute ago, Moontanman said:

I am setting the stage for a series of short stories set on a large planet with a 10 bar atmosphere and a group of human researchers who live on the planet. I am trying to make the initial set up as accurate as possible. I am also trying to get a reasonable way to include helium in the atmosphere. 

That's difficult to picture. Helium rises here on Earth, so it's likely to float upwards and away in a 10 bar atmosphere. What about gravity, if the planet is bigger and more massive than the Earth? How would the humans cope with it?

[Moontanman]

2 minutes ago, mistermack said:

That's difficult to picture. Helium rises here on Earth, so it's likely to float upwards and away in a 10 bar atmosphere. What about gravity, if the planet is bigger and more massive than the Earth? How would the humans cope with it?

The gravity on this planet should be enough to hold onto helium, at least i hope so, if not I'll have to go with a oxygen nitrogen atmosphere like the earth. 

Humans will be limited to short stays on the surface, weeks, but will have mechanical exoskeletons to help them cope. The atmospheric pressure is equal to 330 feet below the ocean's surface on Earth which is doable but will have to have something to prevent nitrogen narcosis. . 

Edited August 13, 2022 by Moontanman