Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: JMLCarter on 06/04/2011 22:02:54
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Surface gravity is about the same.
Is it the composition of the atmosphere (being mostly CO2 not N2),
its density factor is aprox 2, (but that doesn't impact pressure??)
Surface Temperature is 737 K - which could give us a pressure factor of 737/232 = about 3.
Scale height of atmosphere is 15 to 20km compared with the earth's 8.9km, another factor of up to 3.
Still missing a factor of about 10 - help.
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The Earth's surface contains a lot of carbon and oxygen in the form of carbonate rocks and organic carbon (presumably venus was at one time much the same) so if a significant portion of this was converted into carbon dioxide by burning etc it would not take all that much to produce the same sort of atmosphere on the earth.
The original atmosphere of the earth was assumed to be much higher density carbon dioxide which has been converted into organics by life this helped to keep the earth warmer when the sun was considrably fainter billions of years ago
In fact the carbon dioxide runaway will happen on the earth eventually as the sun gradually gets warmer when earth will turn into a planet like venus before it is fried completely.
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OK, some data;
Density of Nitrogen STP 1.251 g/ml
Density of Carbon Dioxide STP 1.562 g/mL
After density, temperature and height, there is still a factor of 10 to be accounted for.
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I think it's just because there is a lot more of it. According to Wiki -
"The atmospheric mass is 93 times that of Earth's atmosphere while the pressure at the planet's surface is about 92 times that at Earth's surface"
Presumably, if you pumped a huge amount of additional nitrogen or CO2 into Earth's atmosphere, the density and pressure would increase dramatically too.
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Carbon dioxide (95% on Venus) isn't that much denser than nitrogen (70% Earth) (see figures above)
The relative density of these gasses at STP cannot account for a 93xmass difference in the atmosphere (or a 92xpressure difference)
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Could it be related to the relative spins of the planets? If memory serves me correctly, a Venetian day is longer than its year. Perhaps the Earth's centrifugal forces counters a large part of it's atmospheric pressure (weight).
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Carbon dioxide (95% on Venus) isn't that much denser than nitrogen (70% Earth) (see figures above)
The relative density of these gasses at STP cannot account for a 93xmass difference in the atmosphere (or a 92xpressure difference)
Yes, it's true that CO2 is not much denser than nitrogen at STP. However, on Venus, the CO2 is not anywhere near STP.
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Please read all this summary post before answering [;)]
Please read the original post before answering, or now read this more detailed summary. (Various factors have been accounted for, e.g. I know PV/T is constant for an ideal gas.)
1) We can account for pressure since p is_proportional_to depth and the atmospheric "scale heights" (real term, look it up) are
Earth 8.9km Venus 15 to 20km expected pressure ratio say about 3, worst case
2) Surface gravity on venus is 0.904g (accounts for rotation as on earth variation in g from pole to equator is only 0.3%), so about the same; expected pressure ratio = 1.
3) Surface Temperature is 735 K - which could give us a pressure factor of 735/232 = about 3.
4) Density due to composition I estimate by considering a pure N2 earth atmosphere and a pure CO2 venusian atmosphere. Well it's just a rough estimate (N2 is lighter than O2 anyway).
Density of Nitrogen STP 1.251 g/ml
Density of Carbon Dioxide STP 1.562 g/mL
This accounts for a factor of about 1.25
TOTAL of all factors so far identified = 3 * 1 * 3 * 1.25 = 10.5
OK, so what are we missing, at least a factor of 92/10.5 or aprox 9 in both atmospheric mass and pressure.
What did I miss or do wrong???
Please read all this summary post before answering [;)]
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I think what you are missing is that the total mass of Venus atmosphere (that's the amount of all the gas in the atmosphere) is 93 times greater than the total mass of the Earth's atmosphere, so it should not be a big surprise that the atmospheric pressure on the surface of Venus is 92 time greater than it is on Earth.
That's why I posted the quote from Wiki [:D]
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Please read the original post before answering, or now read this more detailed summary. (Various factors have been accounted for, e.g. I know PV/T is constant for an ideal gas.)
I think what Geezer's getting at is that PV/T=kN. That N is number of particles. The number of particles, and hence the total atmospheric mass is different on the earth than on Venus, so you can't simply compare them. If the Wiki's numbers are right, and Venus has an atmosphere 93 times more massive than earth's, I think that pretty much accounts for the pressure at the surface. You simply have 93 times more weight above you pushing down on Venus than on the earth. All the concerns about temperature, etc. are minor tweaks to the fact that there's simply a lot more atmosphere.
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OK, some data;
Density of Nitrogen STP 1.251 g/ml
Density of Carbon Dioxide STP 1.562 g/mL
After density, temperature and height, there is still a factor of 10 to be accounted for.
Don't know what I'm missing here..... water has a density of 1.0 g/ml, according to the above numbers, both nitrogen and CO2 are denser than water!?
Seems like your numbers should be more like g/L...
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If you took two columns of liquid, mercury and water let's say, you could calculate the differences in pressure soley by considering the density ratio and using that as scaling factor directly, because there's no compression factor.
But you can't with gasses. ALthough PV/T is constant for a given gas, in a column, the pressure gradient is logrithmic, and since you ask, THAT's where the equation has fallen apart.
In my humble opinion.
Thx
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thanks daveman. The density figures are wrong.
STP figures
are 1.98 g/L for CO2
and 1.251 g/L for N2
Ok, this gives a factor of about 1.6 not the 1.25 from before.
Also, looks like you are right about the compression factor. I have found
http://en.wikipedia.org/wiki/Barometric_formula
so P0=Pheh/scale_height
I can't just punch a number into this, it has to be done layer by atmospheric layer... [xx(]
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Of course, you might also consider dividing the total mass of the atmosphere by the surface area of the planet to calculate the force exerted by the atmosphere per square meter of surface, but I suppose that would be rather too easy.
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There seems to be rather a circular argument going on here, I take it the mass of the atmosphere has been calculated by measuring the surface pressure and working back.
I recall there was a Russian lander back in the sixties that survived long enough to send back both pressure readings and a ground picture
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There seems to be rather a circular argument going on here, I take it the mass of the atmosphere has been calculated by measuring the surface pressure and working back.
I recall there was a Russian lander back in the sixties that survived long enough to send back both pressure readings and a ground picture
I was wondering about that. I don't think there is any other way to determine the pressure (and hence the mass) other than by measuring it.
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I don't suppose anyone ever actually weighed all of Earth's atmosphere either, so presumably its mass is simply derived from air pressure at the surface and the surface area.
Rather than asking why atmospheric pressure on Venus is 92 times that on Earth, perhaps the question should really be why shouldn't it be 92 times that on Earth [:D]
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There are rather indirect ways of measuring the thickness of planets atmosphere by noting how a remote radio signal varies as it is occulted but nothing beats landing a barometer.
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JMl carter your fundamental error is the way atmospheric pressure increases with the depth of an atmosphere it is not a linear function of the depth of the atmosphere but an EXPONENTIAL one. This arises because gases are compressible as opposed to liquids which are much less compressible.
I am not sure how you have defined the depth of the atmospheres of the earth and venus. because they all fade out asymptotically. let us assume they are at equal minimum pressures.
you defined the earth's atmosphere as about 9km on the earth the pressure difference is about 4.25 of that at ground level so an atmosphere three times this depth would be a factor of 4.25 greater for every depth increase equal to the depth of the earth's atmosphere that is two additional factors of 4.25 over the earth's atmosphere i.e. about 4.25 x4.25 = 18 times that of the earth assuming other factors were the same. you have accounted for a factor of 3x1.25 (taking out your original linear factor of 3 ) this gives a factor of 68 times the earth's atmosphere which is not all that far off considering the crudity of the information we are working with
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I'm pretty pleased with the explanation from Daveman, and Soul Surfers estimate of it by squaring of depth.
Thanks for solving this for me. You may have guessed I have been doing some thinking about green house effects of different atmospheres; now some sort of basis for comparison is established between earth and venus.
As an aside along the way I discovered this weird truth:
http://www.thenakedscientists.com/forum/index.php?topic=38597.new#new (http://www.thenakedscientists.com/forum/index.php?topic=38597.new#new)
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At Venus' surface, the air pressure is 92 times the Earth's surface atmospheric pressure. Venus' surface atmospheric pressure is the same as what you would feel if you were 1 kilometer below the ocean surface on the Earth. The deepest free-divers can get down to around 160 meters and divers breathing special mixtures of gases can get down to 730 meters. If you want to send someone to Venus, that person would need to be in something like a diving bell.Venus explorer would also need a very powerful cooling system: the surface temperature is 737 K (= 477° C) This is hot enough to melt lead and is over twice as hot as it would be if Venus did not have an atmosphere.