Jump to content

SkySurveyBanner.jpg.21855908fce40597655603b6c9af720d.jpg

Does Venus really have a runaway greenhouse effect at all?


Recommended Posts

Morning all

I was thinking about good 'ol climate change / AGW etc. and reading some stuff on other sites.  One comment that comes up from time to time is that Venus has a mostly CO2 atmosphere and it's temperature is very high.   i.e.   lots of CO2 makes very high temperatures.

But then a penny dropped..   We're not comparing like with like.  There are more differences between Venus and Earth than just atmospheric make up.

Clearly Venus is somewhat closer to the sun than us..  but theoretically still within the goldilocks zone.   If Earth were where Venus is we would probably still have lots of water.

Venus also has a very thick atmosphere..  Now this is where I went "Hang On!"..  what's the pressure at the surface of Venus?..  it's lots and lots.. enough to crush the first spacecraft that ventured there, now I know from basic physics that Pressure Volume and Temperature are all related, compress a gas and it gets hotter.  I also know that all atmospheres have temperature gradients, walk up a mountain here on earth and things will get a whole lot colder.   So.. if we are going to compare Venus to Earth it would be better to compare like with like.. i.e. what is the temperature on Venus at an altitude where the atmosphere is at a pressure of 1Bar.

I had a little look around for the data and...   turns out the temperature is about 335k or a little under 40C...  so if Venus' atmosphere were the same thickness at Earth's we should perhaps expect see much much cooler temperatures despite the CO2?

hmmm..  what else can we compare..  well we could compare temperature at depth.

Venus' atmosphere is over 20km deep, at about 5km is roughly where we reach 1Bar..  so that leaves about 15km of depth...   what temperature are our rocks on earth at that sort of depth and how do they compare with Venus' surface temperature.

Well on earth 15km is going to be around 470C.. about 3.5C/100m gradient.

What's the temperature on Venus' surface?.. oh, it's about 500C.. a similar degree hotter to earth as at the 1 Bar hight

Now I'm certain there's a whole load of missing stuff and I've got the wrong end of the stick on others.. but the more I think about this the more I'm thinking, Venus doesn't have a runaway greenhouse effect caused by CO2..  what it has is a truly gargantuan atmosphere, one that if it were to exist on earth would likely render our planets surface somewhere around 400~500C...  regardless of what the atmosphere was made of.

Am I barking mad?

Link to comment
Share on other sites

Edit:

For some reason I got the depth readings wrong..   it's 50km, not 15km depth (reading multiple sources..  need to take more care)..   but the 1Bar Venus vs 1Bar Earth is as stated.

It means earth at 50km depth is substantially hotter than Venus..   (more reasonable result)..   but I think the main point still stands.

To compare like with like we should be measuring at comparable depths/pressures.

Link to comment
Share on other sites

I'm afraid you have a fundamental misunderstanding of the relationship between temperature and pressure.

Pressure and temperature are unrelated properties. However, increasing the pressure of a gas raises its temperature. This is because it contains the same amount of energy, but it is concentrated in a smaller volume. Think of the CO2 in a fire extinguisher, it reaches an equilibrium temperature with its surroundings despite being at higher than atmospheric pressure.

Venus is very hot because the CO2 in its atmosphere is largely transparent to the UV light emitted by the Sun, but opaque to the infrared light radiated by the planet itself. This is the greenhouse effect. If Venus had an atmosphere of nitrogen and oxygen its temperature would be far lower, as these gases absorb very little IR.

Link to comment
Share on other sites

Clearly Venus is somewhat closer to the sun than us..  but theoretically still within the goldilocks zone.   If Earth were where Venus is we would probably still have lots of water.

That is/was the problem, the water on Venus disappeared as the sun made it too hot and the radiation broke the water down.

So with no water there was no rain, with no rain the CO2 was not washed out of the atmosphere. So the CO2 built up and the greenhouse effect of CO2 took control

Owing to the closeness of the sun the lighter gasses also escaped, hotter.so more kinetic energy and they escape So you are left with the heavy gasses, CO2 and SO2, with those you get higher atmospheric pressure and with the greater they are hotter and so even more atmospheric pressure, Boyles law or Universal gas law will show why..

All comes back to the green house effect and oddly the lack of water to cause rain.

Also you cannot transport Earth to the orbit of Venus and expect the Earth to remain Earth like, it would almost certainly become as Venus is - water would escape, SO2 and CO2 would remain and Earth would have a runaway green house effect.

Link to comment
Share on other sites

exactly why I wanted a cross check.

Ok I'm wrong, which means maybe I'm learning something, but I still don't get exactly why I'm wrong..  please help me.

OK..  I do fully understand the relative transparency argument for far IR vs solar radiation.

Pressure/Temperature..   my understanding is that as a gas rises through the atmosphere the total atmosphere above it reduces, so the pressure on it reduces, so for the same amount of energy in that gas its temperature falls...   so if we compare Venus atmospheric temperature at 100km depth vs Earth at a much lesser depth we are not comparing like with like, we should be comparing Venus and Earth at comparable depths.

Link to comment
Share on other sites

That is/was the problem, the water on Venus disappeared as the sun made it too hot and the radiation broke the water down.

So with no water there was no rain, with no rain the CO2 was not washed out of the atmosphere. So the CO2 built up and the greenhouse effect of CO2 took control

Owing to the closeness of the sun the lighter gasses also escaped, hotter.so more kinetic energy and they escape So you are left with the heavy gasses, CO2 and SO2, with those you get higher atmospheric pressure and with the greater they are hotter and so even more atmospheric pressure, Boyles law or Universal gas law will show why..

All comes back to the green house effect and oddly the lack of water to cause rain.

Also you cannot transport Earth to the orbit of Venus and expect the Earth to remain Earth like, it would almost certainly become as Venus is - water would escape, SO2 and CO2 would remain and Earth would have a runaway green house effect.

thanks.

I thought earth hung onto it's gasses due to the huge magnetic field earth produces..   My understanding of venus is that it doesn't have a magnatic field anything like as strong as earths.

However, I agree..  there will be a point where if you cook earth enough it should lose it's water.

Link to comment
Share on other sites

Pressure/Temperature..   my understanding is that as a gas rises through the atmosphere the total atmosphere above it reduces, so the pressure on it reduces, so for the same amount of energy in that gas its temperature falls... 

That may be the source of your confusion. The energy in the gas doesn't fall, but it is dissipated over a larger volume.

Link to comment
Share on other sites

That may be the source of your confusion. The energy in the gas doesn't fall, but it is dissipated over a larger volume.

hmm   .  penny still not dropping..  got a link?

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.