By Steve Goddard
ESA’s Venus Express mission has been studying the planet and a basic atmospheric model is emerging.
Venus has long been the CO2 bogeyman of climate science. In my last piece about Venus I laid out arguments against the claim that it is a runaway greenhouse which makes Venus hot. This generated a lot of discussion. I’m not going to review that discussion, but instead will pose a few ideas which should make the concepts clear to almost everybody.
If there were no Sun (or other external energy source) atmospheric temperature would approach absolute zero. As a result there would be almost no atmospheric pressure on any planet -> PV = nRT.
Because we have a sun providing energy to the periphery of the atmospheric system, the atmosphere circulates vertically and horizontally to maintain equilibrium. Falling air moves to regions of higher pressure, compresses and warms. The greater the pressure, the greater the warming. Rising air moves to regions of lower pressure, expands, and cools. The amount of warming (or cooling) per unit distance is described as the “lapse rate.” On Earth the dry lapse rate is 9.760 K/km. On Venus, the dry lapse rate is similar at 10.468 K/km. This means that with each km of elevation you gain on either Earth or Venus, the temperature drops by about 10C.
It is very important to note that despite radically different compositions, both atmospheres have approximately the same dry lapse rate. This tells us that the primary factor affecting the temperature is the thickness of the atmosphere, not the composition. Because Venus has a much thicker atmosphere than Earth, the temperature is much higher.
dT = -10 * dh where T is temperature and h is height.
With a constant lapse rate, an atmosphere twice as thick would be twice as warm. Three times as thick would be three times as warm. etc. Now let’s do some experiments using this information.
Experiment # 1 – Atmospheric pressure on Venus’ surface is 92 times larger than earth, because the atmosphere is much thicker and thus weighs more. Now suppose that we could instantly change the molecular composition of Venus atmosphere to match that of Earth. Because the lapse rate of Earth’s atmosphere is very similar to that of Venus, we would see little change in Venus temperature.
Experiment #2 – Now, lets keep the atmospheric composition of Venus constant, but instead remove almost 91/92 of it – to make the mass and thickness of Venus atmosphere similar to earth. Because lapse rates are similar between the two planets, temperatures would become similar to those on earth.
Experiment #3 – Let’s take Earth’s atmosphere and replace the composition with that of Venus. Because the lapse rates are similar, the temperature on Earth would not change very much.
Experiment #4 – Let’s keep the composition of Earth’s atmosphere fixed, but increase the amount of gas in the atmosphere by 92X. Because the lapse rates are similar, the temperature on Earth would become very hot, like Venus.
Now let’s look at measured data :
http://www.astro.wisc.edu/~townsend/resource/teaching/diploma/venus-t.gif
http://www.astro.wisc.edu/~townsend/resource/teaching/diploma/venus-p.gif
Note that at one Earth atmospheric pressure on Venus (altitude 50km) temperatures are only about 50 degrees warmer than earth temperatures. This is another indication that atmospheric composition is less important than thickness.
Conclusions : It isn’t the large amount of CO2 which makes Venus hot, rather it is the thick atmosphere being continuously heated by external sources. It isn’t the lack of CO2 on Earth which keeps Earth relatively cool, rather it is the thin atmosphere. Mars is even colder than earth despite having a 95% CO2 atmosphere, because it’s atmosphere is very thin. If greenhouse gases were responsible for the high temperatures on Venus (rather than atmospheric thickness) we would mathematically have to see a much higher lapse rate than on Earth – but we don’t.
WUWT commentor Julian Braggins provided a very useful link which adds a lot of important information.
“The much ballyhooed greenhouse effect of Venus’s carbon dioxide atmosphere can account for only part of the heating and evidence for other heating mechanisms is now in a turmoil,” confirmed Richard Kerr in Science magazine in 1980.
The greenhouse theory does not explain the even surface temperatures from the equator to the poles: “atmospheric temperature and pressure in most of the atmosphere (99 percent of it) are almost identical everywhere on Venus – at the equator, at high latitudes, and in both the planet’s day and night hemispheres. This, in turn, means the Venus weather machine is very efficient in distributing heat evenly,” suggested NASA News in April 1979. Firsoff pointed out the fallacy of the last statement: “To say that the vigorous circulation (of the atmosphere) smooths out the temperature differences will not do, for, firstly, if these differences were smoothed out the flow would stop and, secondly, an effect cannot be its own cause. We are thus left with an unresolved contradiction.”
======================================================
An update for those interested in what Venus looks like at the surface.
On March 1, 1982, the Soviet Venera 13 lander survived for 127 minutes (the planned design life was 32 minutes) in an environment with a temperature of 457 °C (855 °F) and a pressure of 89 Earth atmospheres (9.0 MPa). The photo composite above shows the soil and rocks near the lander.
Here’s another Venera image that shows a hint of yellow atmosphere. – Anthony
Dr. Bill,
As I have been teaching physics too, I hope that you don’t teach that:
Temperature is defined in terms of the average Translational Kinetic Energy of a molecule, and the expression is (1/2)mv² = (3/2)kT, because the former is wrong and the latter a particular case.
The corect definition is that under conditions for the energy equipartition theorem to hold true, the average of kinetic energy per degree of freedom = (1/2) k.T.
For a particular case of a monoatomic molecule (f.ex He) that has only 3 translationnal degrees of freedom we have average (1/2m.v²) = 3.(1/2 k.T).
However for polyatomic molecules, rotational and vibrational degrees of freedom add to the translational and average (KE) = (N/2).k.T where N > 3 that is the general case.
This shows readily in the fact that when one adds the same amount of energy to He and to H2, the translational kinetic energy of H2 will increase much less than the one of He because the additional non-translational degrees of freedom of the H2 molecule will use up a portion of the supplied energy, what He can’t do.
Because details indeed matter, one has to add that even that is no longer true when the energy equipartition theorem doesn’t apply like f.ex in the stratosphere.
On Venus, the perfect gas law is only applicable near the top of the atmosphere. In most of the lower atmosphere, temperatures and pressures exceed the critical points for CO2 and SO3 reaching a critical density. SO3 reacts with what little water vapor exists to form H2SO4 aerosol leaving none to react with or absorb CO2 as it does on earth. CO2 on Venus and Mars is not being recycled into oceans.
RE: Mike McMillan says: (May 11, 2010 at 3:14 am) “Ralph is correct.”
My comments did not apply to short-term conditions on earth. If convection *never* happens at any level in a given planetary atmosphere then adiabatic heat transfer cannot happen. Without convection, I believe that this hypothetical and probably unrealistic planet would most likely have something equivalent to the stratosphere going all the way down to the surface. Heat exchange by thermal conduction is, by definition, non-adiabatic.
Fred H. Haynie
The ideal gas law doesn’t work perfectly mathematically in any range of temperatures and pressures, because molecules have a finite size. It is always an approximation.
Nevertheless, the sign of the relationships is accurate. When CO2 freezes, the volume can’t decrease any more, so the pressure drops with falling temperature.
P = cT/V
DesertYote has it backwards . In a static situation the temperature in a gas , even with a gradient in density maintained by gravity , will come to a uniform temperature ; there will be less energy per unit volume where there is less mass per unit volume .
I just posted this on Lubos Motl’s blog :
I continue to fail to see how the interior of an externally radiantly heated sphere can come to a higher mean temperature than that calculated by StefanBoltzmann and Kirchhoff for its externally observed spectrum given that all heat flow equations are from hot to cold .
If someone knows how to accomplish this trick , please show me the equations . We should be able to duplicate the phenomenon here on earth and solve all our energy problems forever .
The surface of Venus is over twice the ~327k , as calculated at my website , – on both its 244earthDay long day and night sides – calculated for a gray ball in its orbit .
I see no alternative than that Venus’s extreme surface temperature is due to its known vulcanism , perhaps due to solar tidal forces , held in by a very insulative atmosphere .
Those plus maybe a significant amount of radioactive material generating heat. I found this 1982 paper by Sean Solomon and James Head quite interesting.
As I understand the formation of our solar system, the heavier the element – the closer it tended to end up being WRT the Sun. (We got a lot of iron; maybe Venus got a lot of actinium?)
Steven Goddard 3:19
“What I am saying is that because of the high pressure on the surface of Venus, adiabatic heating causes the very high temperatures. Those kinds of temperatures aren’t seen on earth because of the lower pressure.”
The surface temperature is a function of the input power, the average height of emission to space, and the adiabatic lapse rate between this height and the surface. It depends on all three. The difference between Earth and Venus is the thickness of the atmosphere, but the mechanism isn’t solely due to pressure. I’m sure you know this, but statements like that above confuse.
8:44/8:51
Regarding Carl Sagan, I already gave a link to a paper by Sagan in which he sets out the adiabatic lapse rate theory of Venusian surface temperature. Evidently he wasn’t always consistent, but it’s only fair to point out that he did know about this.
Ralph, 4:42
“On a fine stable 1040mb day with zero convection and no downward flows there will still be an adiabatic lapse rate in the atmosphere (a dry one in this case). The lapse rate is always present, courtesy of gravity.”
At night, or during a polar winter, there is no solar heating at the ground, the warmer air rises and is not replaced, and you get an inversion layer, in which the adiabatic lapse rate is not followed. Likewise, in the stratosphere the temperature increases with altitude (i.e. the opposite of the adiabatic situation), and there is little to no convection. The atmosphere is stratified – hence the name.
Dr Bill,
5:22
“In the atmosphere, however, there is no external agent doing work on the gases. It is only the dQ part that matters, and that heating must occur at the bottom.”
“Adiabatic” means dQ = 0. Work is done on the gas by its surroundings through the compression as it descends in altitude. It’s just PdV.
“For what it’s worth, I am not trying to be a nuisance to Steve or anyone else, and I am sorry if my comments have created that effect.”
I shouldn’t worry about it. Nobody else does.
Argue for as long as you don’t understand or don’t agree (and feel like it). It’s how blog discussion works.
11:29
“This drag slows the molecules down, and transfers much of the energy they picked up from gravity to the other molecules through which they are moving.”
Are you arguing that gas loses internal heat energy by friction?! Think about it.
Jbar, 3:55
“Sure, I could have suggested it myself. Do you have a source? I’d love to read it. From a scientific paper or book? Not from the blogosphere.”
I assumed you’d just Google for the term. It’s standard scientific terminology.
wayne: May 11, 2010 at 1:07 am
Hi Wayne,
Sorry to make you wait so long, but my job thing keeps interfering with my recreation. 🙂
You appear to be a bit miffed by what you interpret as my “tone”. Please keep in mind that I have no idea how old anyone is (not that it matters), nor how much knowledge they possess (which does sometimes matter), so I try to start from a place that seems accessible to everyone, including other readers who might be interested. I wasn’t being condescending.
Anyway, your original question (way back) was about two identical samples of gas where all molecules had identical speeds. What you presented in your table, however, doesn’t seem to be the same thing. You are now (as far as I can tell) allowing things to change, and I can’t see where you have specified a temperature, so I’ll use some sample numbers, and you can have a look at them. Here they are:
T=288K, P=90atm, V=1 liter, M=44g/mol. This gives n=3.74mol, and v=404m/s.
If you now want to keep the volume at 1 liter, the moles at 3.74, and the temperature at 288K, you can’t allow the pressure to change either, because P=nRT/V, and everything on the right-hand side is fixed. The only way to accomplish this is to make your container an insulated rigid box, and in that case, it doesn’t matter whether the new outside pressure is higher or lower, and the molecular speed is still 404m/s. It’s pretty much like being inside of a perfectly designed submarine.
If you want to reduce the pressure to 1atm, but keep the moles and volume fixed, then you have to actively cool the gas to reduce its temperature, and the pressure will decrease to match it. For the numbers I used above, the new temperature would be 3.2K (and we’ll ignore liquefaction), and the new molecular speed would be 42.6m/s. It is certainly true that 404/42.6 = sqrt(90), as you concluded, but in order to get that, you have to allow the temperature to change, and I think that during your substitutions, you didn’t account for that.
/dr.bill
Keith Minto: May 11, 2010 at 12:42 am
Hi Keith,
As I said (perhaps flippantly) in an earlier post, it could be the Borg doing something on the surface of Venus for all I know, but from Steve’s graph, it appears that the temperature decreases linearly with altitude in the troposphere of Venus. This signifies convection, and thus I conclude that the energy for this process comes from some kind of heating at ground level, just as it does on Earth. The thing that gets cooled is the Earth’s surface, and the energy has to go someplace, so it ends up in the atmosphere. Other amounts of energy are “dumped” to space by direct radiation from the ground and from the atmosphere, but that’s not the process we’ve been discussing here. In the end, radiation is the only way to get the energy “off-planet”. The convective processes don’t do that. They just redistribute things around the Earth while that process is taking place. It also, helpfully, makes life at the surface tolerable.
/dr.bill
TomVonk: May 11, 2010 at 5:41 am
Everything you objected to is included – correctly – in my earlier post to which you refer.
The flaw in your reasoning is in the following phrase:
what He can’t do
In actual fact, Hecan use up just as much energy as H2. The only difference is that the He will convert all of it into translational motion, and thus increased temperture, while H2 will “store” some of it in the rotational modes, and will not increase its temperature as rapidly. Heat capacity – remember?
/dr.bill
There were several other recent posts oriented in my direction that I don’t know how to respond to, or which would require more time to think about than I currently have available. My apologies for that, but it can’t be helped.
There was one other note which (as they say) “wasn’t even wrong”. I’m sure you can sort them out.
/dr.bill
Here’s a paper on the Hadean atmosphere. “The inception of the oceans and CO2-atmosphere in the early history of the Earth”
Earth’s core is circulating molten iron that generates our magnetic field. Venus has no magnetic field. That means either that its core is not nearly as electrically conductive as ours OR … the planet has been cooling, and continues to cool, a lot more quickly than earth because of a much higher thermal conductivity between the core and the surface thus explaining its high temperature atmosphere.
Steven, I have a few questions. I would appreciate it if you would consider each of them deeply and thoroughly before answering.
1. Earlier, you wrote: At temperatures where the atmosphere is a gas, the pressure is fixed by the weight of the column of air. The temperature controls the volume (i.e height of the atmosphere.) That is why the cold poles have a thinner atmosphere than the tropics.
P = T *c/V
This is the most fundamental physics. When the temperature decreases, the volume has to decrease. The volume is area * height. Area is fixed, so the only thing which can change at the cold poles is the height of the atmosphere.
Is it your contention that the poles being cold is what CAUSES the height of the atmosphere to be lower at the poles… because the lower temperature causes that column of air above the surface to become more dense, and thus shorter for the same mass of particles above it?
In short: What do you think is the driving force behind the total height of the atmosphere at the poles being what it is?
What I see as a critical question is this: Why do you think the poles are cold?
2. Earlier, you wrote: With a constant lapse rate, an atmosphere twice as thick would be twice as warm. Three times as thick would be three times as warm. etc.
Do you believe that it would be possible to have two different planets in the same orbit, where the planet with the much thicker atmosphere has a much colder surface temperature? What do you believe would tend to govern the temperatures at the surfaces of each planet?
3. Earlier, you wrote: If you changed the composition of Venus atmosphere to 100% nitrogen, the planet would be much colder due to the lack of absorption of IR. The atmosphere would also necessarily be much thinner (lower volume.) P = nRT/V
However, if you changed the composition of Venus atmosphere to 95% nitrogen and 5% CO2, the temperature and height of the atmosphere would not be hugely different than it is at present. Why? Because the greenhouse effect is logarithmic. After the first few percent, additional CO2 makes much less difference to the temperature.
Do you believe that it is purely a function of the PERCENTAGE concentration of CO2 or other GHG which matters with respect to thermal transfers from the surface of a planet to space, or is it more a question of the ABSOLUTE amount of these gases between the surface and space?
Modifying one of your previous thought experiments, consider the following:
Experiment #5: Take Venus and modify its atmosphere in the following fashion: Incrementally remove CO2 from the atmosphere and substitute N2 such that the total atmospheric mass remains identical. Continue the modification process until the total mass of airborne CO2 on Venus is identical to the mass of current airborne CO2 on Earth. Post-modification, what do you believe would happen to the surface temperature of Venus?
Lets see if I have understood this!
Venus is a different planet to Earth, and on different planets, different things happen!
Nullius in Verba: May 11, 2010 at 11:15 am
Hi Nullius (if I may be so informal!),
Yes, I understand what adiabatic means, but what I was talking about was what happens in the rising gas, which is what’s doing work on the things it displaces as it rises, but without heating them or being heated by them. That’s what determines the lapse rate. The rising “package” does, though, have to get heated by the ground in the first place or it will not start rising to begin with. The adiabatic part is for everything that happens above the surface skin.
The return trip is another story. For one thing, the rising gas doesn’t come back down in the same place. There are lateral movements, turbulence, entrainments, and a lot of bouncing and mixing going on, which serve to homogenize things. This isn’t friction in the usual macroscopic sense, but it does siphon off energy from anything that gets going “too fast” and redistributes it to other molecules via collisions. In most cases, but not all, it eventually settles into a “sort of” Maxwell-Boltzmann distribution with a superimposed downward drift. If this were not the case, we would have high-velocity winds coming straight down at us all the time.
And thanks for the comment about “thickening my skin”. I’ll work on that, but I don’t like to be abrasive unless absolutely necessary. When I get to that point, though, I sometimes have “gasket issues”. 🙂
Barry Kearns, in experiment #5 I anticipate a surface temperature increase of about 100 to 150 kelvins because the lower molecular weight of N2 compared to CO2 means replacement on a mass basis results in 1.57 times as many molecules, which increases atmospheric volume (and thus height).
So even though the dry adiabatic lapse rate of N2 is slightly lower than CO2, the taller atmosphere would have about a 40% greater temperature difference between the surface and the cloud tops, giving a surface temperature of about 850 K.
It would still have the clouds of sulfuric acid and other bizarre chemicals, plus the dust, so I’m not sure how the IR picture would change.
dr. Bill
You said:
The return trip is another story. For one thing, the rising gas doesn’t come back down in the same place. There are lateral movements, turbulence, entrainments, and a lot of bouncing and mixing going on, which serve to homogenize things.
That’s why the Marines hate riding the dropship down to a terraforming planet for a bug hunt, man! It’s the express elevator to …
@ur momisugly Mike M
Interesting comments & link . I just mentioned the tidal forces as something which has brought Venus to slow retrograde motion . ( Wikipedia seems to give a rational description which it certainly fails to do for the basic calculation of a planet’s temperature based on StefanBoltzmann & Kirchhoff . )
Perhaps Venus’s lack of a magnetic field has more to do with its lack of rotation rather than a solid core .
Good , but no longer surprising , to see a number of people recognize that the only way the interior of a sphere can remain hotter than its surface is if it has an internal heat source . Adiabatic heating explains nothing because it’s transitory . The profile of atmospheric pressure caused by a planet’s gravity is essentially static .
I actually have been censored from “warmist” blogs because of my “obsession” with understanding the quantitative physics and proposing the “absurd” notion that Venus might have an internal heat source . Easier , I guess to just claim that heat can be made to go up hill and stay there .
Dr Bill,
The informality is fine.
If you look at the diagram for Hadley cells, you can see the air coming down over the desert latitudes. But yes, turbulence is more complicated. It siphons off the bulk motion of convection into heat. But the way you had written it, it looked like you was talking about the heat energy itself – the adiabatic temperature increase resulting from gravitational compression of descending air.
It would be fair to say that it would be moving slower and in a more disorganised fashion, but the kinetic energy of the molecules could be just as high, and it wouldn’t need reheating. In fact, since it has just moved to where the ground is cooler, it would tend to transfer heat to the surface, before flowing over the surface back to the tropics. Heat is transferred from the hot tropics to the cooler temperate zones, where it is dumped; the temperature difference driving the heat engine.
In fact, it does cool overall on the cycle, but by radiation to space from higher altitudes, rather than by friction.
I was pretty sure it wasn’t what you meant, but most of this conversation appears to be two people in violent agreement, both misunderstanding what the other intends.
Like you, I like to get the details right, even when I agree in general.
I found a link to the 30-year old Dragon of Eden that Steve seeks to slay:
Per the request of the website owner, I’m not quoting,
but I urge the reader to consider each paragraph fully
http://users.tpg.com.au/users/mpaine/Cosmos_greenhouse.html
Also, Charlie Rose in his last interview with Sagan,
http://www.charlierose.com/view/interview/5787
I have a question on the spectral widening of CO2 absorption bands under the conditions found on Venus, which several people here have pointed to as explaining the potent CO2 GHG affect on that planet.
Doppler widening (due to temperature) at the surface of Venus should make the spectral lines only about 50% wider than CO2 at Earth’s sea level temperature, which means pressure widening of the spectrum must be the explaination.
CO2 has absorption peaks at 2.6 and 4 microns, yet the VIRTIS imaging spectrometer on Venus Express can see all the way to the surface at 2.3 microns, just 0.3 microns away from a CO2 absorption peak.
So to me it looks like there can’t be much widening of CO2’s spectrum.
Also, VIRTIS can see pretty well from 2.2 to 2.5 microns, just 0.1 microns from the CO2 peak.
Nullius in Verba: May 11, 2010 at 12:55 pm
Hi Nullius,
What you say perfectly valid, and I was, in fact, being a bit sloppy and simplistic in an effort to keep my original point from getting lost in tangential issues. Mea culpa ex animo.
I also liked the following:
I was pretty sure it wasn’t what you meant, but most of this conversation appears to be two people in violent agreement, both misunderstanding what the other intends.
The curse of our species sometimes! 🙂
/dr.bill
dr.bill Reur May 10, 2010 at 6:30 pm
Concerning S-B as it might apply to a body immersed in a fluid. (such as the Venus atmosphere);
Thankyou for responding to the first of several considerations I sequentially listed.
It would be good if you could also advise on step b} that is repeated below, in order to try to explain what might be happening on the surface of Venus.
b} If a dry body is immersed in say an Earthly thin gas, [it] having a lower temperature, there will not only be EMR emission resulting in net HEAT loss, but also HEAT loss via conduction and convection. Where does this “extra energy” come from without upsetting the balance of the EMR? Or, to put it another way, the surface molecules are busy losing energy as EMR, (photons), and as HEAT, (KE). Can they do that without affecting each other?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
BTW, you appear to be saying that wet water ice will emit twice as much EMR as dry water ice. (at its freezing point). I find that to be surprising because there is intimate contact between the water and ice such that I would have thought that the water would become the radiating surface of the body. Consider for instance painted concrete. Does the concrete still send photons through the paint laminate? From my limited reading, it would be phonons from the concrete, (which cannot radiate through space), not photons. I ponder too; dry, moist, (meaning water has penetrated), and wet concrete.
You’re right, it’s quite a headache for me.
Can you recommend any text books or studies that consider immersion of bodies in a fluid, WRT S-B?
And a follow up on the clear IR window from 2.2 to 2.5 microns.
VIRTIS for those interested.
I calculated that a 740K blackbody would emit 799 W/m^2 in the open window from 2.2 to 2.5 microns. That’s more than the solar energy Venus receives at the surface.
check please?