Venus's Atmosphere

Factual Questions
1

The pressure at the surface of Venus’s atmosphere is 90 atm. My question is, how much of the quantity of gas in the atmosphere is due to its high temperature? If the mean surface temperature of Venus were reduced to be comperable to the Earth’s. how much of the gas in the atmosphere would be in liquid form, and what would the resulting pressure be?

Initially, I thought of just looking at the composition of its atmosphere, and then comparing the boiling points of its constituents to say, 27 degrees C. The only problem is, I don’t think that’ll work. There’s water in our atmosphere, and solid water too, despite our mean temperature being in the liquid range of water.

Anyone have any ideas on how to approach this problem?

2

Can’t give you numbers, but the tremendous atmospheric pressure stems from the massive cloud layers and extreme solar load. Venus is a massive pressure cooker.

3

From one website:
"In the runaway-greenhouse explanation, Venus was said to be so hot that its water existed only as vapor and had no chance to condense to liquid on the surface. Water vapor rose into the atmosphere, where radiation from the Sun cracked it into separate oxygen and hydrogen atoms. The hydrogen escaped into space and water couldn’t form.

But the Ames researchers, looking at different climates on Venus, Earth and Mars, didn’t like the runaway-greenhouse explanation. That old theory forgot that the Sun was 25 to 30 percent cooler 4.5 billion years ago. It also did not account for the water loss."
http://72.14.203.104/search?q=cache:3XECJoprSAMJ:www.spacetoday.org/SolSys/Venus/VenusGreenhouse.html+venus+greenhouse&hl=en

4

Thank you, that’s very informative, but I was wondering how much Venus’s atmospheric pressure would drop if the eman surface temperature were reduced to 27 degrees C.

5

One of the reasons Venus’ surface is so hot (hotter than Mercury) is because of the thickness of the atmosphere. As pressure increases, the temperature of a gas also increases. (PV=kT)

If you could magically cool the surface of Venus to 27 C, it would do nothing to change the heat of the atmosphere itself and the surface would eventually reheat to 450C.

Why is Venus’ atmosphere so thick? That’s the interesting question…what seems to make sense to me is that because Venus doesn’t have plate techtonics like Earth, interior pressure builds to a point where Venus has regular planet-wide volcanic events, thus throwing gobs of gases into the atmosphere. Repeat over 4 billion years and eventually you get an atmosphere that 90 times thicker than Earth’s even though they may have had a similar “original” atmosphere.

6

Isn’t the reverse true?
The temperature is the result of heat absorbed from solar radiation less that lost to space on the side opposite the sun!
As a result the presure and volume are the pressure and volume of the existing atmosphere whatever and however much there is of it.

7

I don’t think this is correct. The volume of Venus’s atmosphere is not constant. It seems to me that the pressure is really a function of how much gas there is and the mass of Venus. It works that way on earth. The atmospheric pressure does not go up proportionately to rise in temperature during the day.

8

The formula PV=kT relates the product of Pressure and Volume to the product of the proportionality constant k, of the gas/gas mix under consideration, to the Temperature. The pressure and volume are a function of TEMPERATURE. The temperature is an independant variable, Pressure and Volume are dependent variables. (Thermodynamics 101.)
The composition of the Venusian atmosphere is not too different from Earth’s!
FYI ** Atmosphere of Venus **

9

My reading of that data shows a huge difference in atmospheric composition, or at least in chemical concentrations.

10

If you decrease the temperature of Venus’ atmosphere, the pressure at the surface will actually remain the same!

Pressure is force per unit area, and to understand atmospheric pressure, what you want to do is imagine an area of the surface, A, say a circle of diameter 1 m, and then find the weight of all the gas above that circle. Imagine a cylinder rising straight up above your circle, all the way from the surface up into space. If you take the weight of all the air in that cylinder, that’s the force of the air pressing down on the circle. Divide by the area, and you have the atmospheric pressure.

Now, weight just equals the mass of the atmosphere times the acceleration of gravity. The acceleration of gravity is smaller by a few percent at the top of the atmosphere, but for now let’s just take it to be constant.

If you decrease the temperature of the atmosphere, that reduces the average speed of the molecules in the atmosphere. As a result, the cylinder will decrease in volume by getting shorter, and be more compressed by gravity. However, the mass in our imaginary column will remain the same, so the weight of the column of air, and the pressure will remain the same!

Now, if you want to get fancy about it, if the column gets shorter, i.e. closer to the surface, were g is greater, the weight of the column will actually increase slightly!

The amount of weight contained in Venus’ clouds is very small contained to the mass of the carbon dioxide atmosphere, so freezing out the clouds would actually have very little effect on the pressure. So until you reach the point where carbon dioxide begins to freeze, so that carbon dioxide is actually removed from the atmosphere, the atmospheric pressure will not decrease.

11

Right on!

PV=kT is the expression relating the Pressure and Volume to Temperature. It is known as the ideal gas law and does not apply to other ‘chemical vapors/gasses in the atmosphere,’ only to ‘ideal’ gases, in a closed container.

12

Yeah, the tricky thing about atmospheres is they aren’t contained by a rigid container. They’re just stuck to the surface by gravity and they get thinner and thinner as they go up.

The ideal gas law is still a good way to characterize atmospheres in most circumstances, actually, but you can’t just assume that V is a constant, and P is provided by the weight of the layers of atmosphere above.

13

That’s a very good explaination of the use of the ideal ga law in this situation, but how much of the gasseous material in venus’s atmosphere would be in a solid or liquid state if the mean global temperature were comperable to Earth’s? Earth’s mean global temperature is comfortably within the liquid range of water, so only a very small proportion of the water on earth is a gassesous state. If Venus’s temperature were reduced, how much of the atmosphere would no long be a gas? The sulphruic acid should me mostly in a liquid state for example, right?

14

Is there any technology existing that would allow one to “syphon” away some of the CO2 into space. Some sort of tube in cythero-synchronous orbit (if that’s the word) that dips into the higher end of the atmosphere and a pump just sucks some of it up into the vacuum of space…?

15

It’s true that you’re not going to get CO[sub]2[/sub] (the primary component of Venus’ atmosphere) to precipitate out until you get awfully cold. But it’s my understanding that that’s not the only, or even primary, means of sequestering carbon. Isn’t much of the carbon on the Earth in the form of carbonate rocks? And if you cooled off Venus, wouldn’t the equilibrium shift to much more carbon being locked up in rocks? In that case, one might be able to significantly reduce the mass (and therefore pressure) of Venus’ atmosphere, even just at Earth-like temperatures.