The Heat of Venus

[max8166]

I know that the general consensus is that Venus has a run away greenhouse effect contributing to it's high surface temparature. I have a new theory that I would like to discuss. As we know the rocky planets were all formed about 4 billion years ago, at the time, all the planets were molten but have cooled over the following years. What if the atmosphere of Venus somehow acted as a blanket reflecting the heat back so the molten planet was unable to cool? On Earth our thin atmosphere makes our planet radiate heat, just go out on a cloudless night, even in the middle of summer the temparature falls away, rapidly, however when the night is cloudy the temparature stays steady, even in the winter. From what I have been able to gleen from the Internet Venus actually reflects alot of the radiation from the sun, more than the Earth which is why it is so bright in the night sky.

[mantiss]

It all gets messier when you account for the atmospheric composition. Water Vapor reflects some % of light away and allows some % of IR radiation through. The cloud composition of Venus differs (it's Mainly CO2) a well known Greenhouse gas. So Reflection is high but the CO2 traps whatever heat reaches to the ground and doesn't let it out, causing the heat buildup observed today.

On a side note, check these reprocessed Venus images from the Venera legacy, there are some fantastic cleanup and sharpening of the original data made by Don Mitchell http://www.mentallandscape.com/V_DigitalImages.htm

[aurora]

An atmosphere is a lot less efficient insulator than a thick layer of basalt.

Even Venus' atmosphere.

[Kaptain K]

The atmosphere of Venus is mostly CO2, but the clouds are mostly SO2.

[max8166]

It all gets messier when you account for the atmospheric composition. Water Vapor reflects some % of light away and allows some % of IR radiation through. The cloud composition of Venus differs (it's Mainly CO2) a well known Greenhouse gas. So Reflection is high but the CO2 traps whatever heat reaches to the ground and doesn't let it out, causing the heat buildup observed today.

So are you saying that Venus was once cooler than today? Where is the evidence for that? Does anyone know why CO2 traps IR radiation when O2 and N2 don't(as much)?

[max8166]

An atmosphere is a lot less efficient insulator than a thick layer of basalt. Even Venus' atmosphere.

How do you get to that? A vacuum is perfect insulator, we insulate our houses with cavity walls, a gas between two solids. A thick layer of basalt would convect and radiate thats why mercury is solid so close to the sun, no atmosphere to insulate it, all the latent heat radiates into space which means that mercury is cold on the non sun facing side.
Does anyone know how much radiation comes from the dark side of Venus? (the side not facing the sun)
Besides which our planet has a molten element below a cool surface. I am trying to say that maybe the heat on Venus is just latent heat and not caused by a run away greenhouse effect.

[Tim Thompson]

Does anyone know why CO2 traps IR radiation when O2 and N2 don't (as much)?

Yes, at least in the same operational sense that the "why question" is always answered in the empirical sciences. The CO₂ molecule does not "trap" IR, it absorbs it much more efficiently than either O₂ or N₂. In a tenuous environment, a CO₂ molecule would absorb an IR photon, wait around a while (a very short while), and then radiate the enrgy as microwave photons in some random direction, so as to get rid of the extra energy it had absorbed. However, in a dense environment, the CO₂ molecule will collide with some other molecule, any molecule will do, in an even shorter while, and transfer that energy by collision (kinetic energy) instead of radiation. That's how radiative energy (photons) gets turned into kinetic energy, which shows up as the kinetic temperature of the gas. So any atmosphere with CO₂ in it will tend to be warmer than one without it, even if the one without it is even more massive (obviously this is not an unlimited affair).

On Earth, CO₂ in the troposphere will tend to do the same thing, convert absorbed photons into kinetic energy. However, in the stratosphere it works the other way around. A CO₂ molecule will absorb energy in a collision, and then wait so long for another collision, that it radiates away a microwave photon or few. So in the stratosphere, CO₂ is a refrigerant, not a greenhouse gas.

A vacuum is perfect insulator, ...

Not really. It works only to prevent heat loss by conduction or convection, since a vacuum has nothing in it that can conduct or convect. But planets (and stars and galaxies & etc.) lose heat to the vacuum of space by radiation, which is easiest to do into a vacuum.

A thick layer of basalt would convect and radiate ...

It would convect only if it were fluid, like Earth's mantle. Otherwise it will only conduct. Radiative energy travels through an atmosphere much quicker than it does through basalt. Likewise, convection & conduction (especially convection) are much more efficient in an atmosphere. I don't think there can ever be a case of an atmosphere insulating the planetary interior. Rather, the basalt in the interior will restrict the flow of energy, such that any energy that gets to the planet surface, will get out through the atmosphere on a much shorter timescale than it traveled through the planet (this certainly is true for terrestrial planets, but I won't swear to it for gas giants).

Does anyone know how much radiation comes from the dark side of Venus? (the side not facing the sun)

Yes, exactly the same amount that comes out the top of the dayside atmosphere. This discovery was made by Nicholson & Pettit, at Mt Wilson Observatory, in 1924, and constitutes the first astronomical use of other than eyeball light, so far as I know (excepting of course Herschel's original discovery of infrared, which was not otherwise used to make any observational discoveries until Nicholson & Pettit). See Notes from Pacific Coast Observatories: Radiation from the Dark Hemisphere of Venus, Publications of the Astronomical Society of the Pacific, Vol. 36, p. 227-228, August 1924; Temperatures on the Bright and Dark Sides of Venus, Publications of the Astronomical Society of the Pacific, Vol. 67, No. 398, p.293, October, 1955. Their discovery has since been confirmed many times over.

Besides which our planet has a molten element below a cool surface.

Indeed it does. But the heat energy that arrives at the surface of Earth, from beneath, is on average about 57 mw/m² through the continental crust, and about 78 mw/m² through the thinner oceanic crust. Compare that to about 1.37 kw/m², about 20,000 times greater, from the sun. The heat flux at the surface of Earth is clearly dominated by the sun, not by the flow of heat from the interior.

I am trying to say that maybe the heat on Venus is just latent heat and not caused by a run away greenhouse effect.

Understood. But I think we know enough about Venus, and about planets in general, to rule that out. The mass of venus is only 81.5% of Earth's mass, and the volume of Venus is only 85.7% of Earth's volume. Since Earth has already cooled to the point that geothermal flux is insignificant compared to insolation, Venus should have cooled likewise, and a lot quicker than did Earth. The atmosphere could not have insulated Venus so well, and the atmosphere we see now is only a temporary affair anyway, as are all planetary atmospheres. They evolve much faster than the solid bodies do, and there is little doubt that a few million years ago, the atmosphere of Venus was rather different than it is now. That variability, coupled with the poor insulating abilities of atmospheres in general, makes it a pretty strong conclusion that the current surface heat is a result of greenhouse heating, and not heat internal to the atmosphere.

I remember a nice paper by Arkan-Hamed that might be useful to look at: On the thermal evolution of Venus, J. Arkani-Hamed, Journal of Geophysical Research, vol. 99, no. E1, p. 2019-2033, January 1994. Unfortunately, JGR does not make its papers available free, even old ones through ADS, but I'm sure that you can find it, and the papers which cite it, in an appropriate library. Also try the gargantuan book Venus II - Geology, Geophysics, Atmosphere, and Solar Wind Environment, Bougher, Hunten & Phillips (editors), 1300 pages of more than you ever wanted to know about Venus (University of Arizona Press, 1997). The last chapter in the book is on mantle convection and thermal evolution of venus. My talk.origins webpage might be of interest too, though I guess it could use an update or two: Is the Planet Venus Young?

[SpockJim]

I know that the general consensus is that Venus has a run away greenhouse effect contributing to it's high surface temparature. I have a new theory that I would like to discuss. As we know the rocky planets were all formed about 4 billion years ago, at the time, all the planets were molten but have cooled over the following years. What if the atmosphere of Venus somehow acted as a blanket reflecting the heat back so the molten planet was unable to cool? On Earth our thin atmosphere makes our planet radiate heat, just go out on a cloudless night, even in the middle of summer the temparature falls away, rapidly, however when the night is cloudy the temparature stays steady, even in the winter. From what I have been able to gleen from the Internet Venus actually reflects alot of the radiation from the sun, more than the Earth which is why it is so bright in the night sky.

I want to add a correction here...

Even when It's a clear 95 degree sunny day here in MN it can dip down to about 82 on some nights and thats it. I really love the gulf moist air!!!
Our atmosphere is not thin, It's approximatley 52 miles thick.

Although I do agree about the winter cold and clouds deal. but it's not really that big of a difference. usually 5-7 degrees.

[Ken G]

Aurora's point was simply that no amount of atmospheric insulation can have any significant impact on the (huge) time it takes for energy released in the interior to make its way to the surface. Point taken.

A CO₂ molecule will absorb energy in a collision, and then wait so long for another collision, that it radiates away a microwave photon or few. So in the stratosphere, CO₂ is a refrigerant, not a greenhouse gas.

Much of what Tim Thompson is explaining is very clear and informative. I learned a lot. But I'm not buying the collisional transfer to kinetic energy part. A greenhouse gas is defined by virtue of its ability to warm the ground, not itself. Any process that absorbs or scatter infrared light on the way out, more than it does the visible light on the way in, will warm the ground. After all, half the re-emitted light goes back downward. It makes no difference if there is time to thermalize the radiation via collisions.

[Ken G]

Does anyone know why CO2 traps IR radiation when O2 and N2 don't(as much)?

I believe that the asymmetry of the molecule is crucial for supporting a permanent dipole moment. O2 and N2 are symmetric, so must have their dipole moment induced, so is a second-order coupling to the radiation. However, O2 and N2 do yield Rayleigh scattering, I'm not yet clear on the reason behind that difference, perhaps it's that Rayleigh scattering is nonresonant.

[GOURDHEAD]

So Reflection is high but the CO2 traps whatever heat reaches to the ground and doesn't let it out, causing the heat buildup observed today.

"Whatever heat" may be too strong a phrase. Each molecule in the atmosphere only traps that part of the spectrum to which it is opaque. Since it is opaque to both incoming and outgoing frequencies, it will protect from the same incoming radiation that it traps; however due to the inverse square law it traps more than it excludes.

[Tim Thompson]

Does anyone know why CO2 traps IR radiation when O2 and N2 don't(as much)?

I believe that the asymmetry of the molecule is crucial for supporting a permanent dipole moment

Indeed. CO₂ is linear & symmetric, and so has no permanent dipole moment. However, it can develop a temporary dipole moment during many vibrational modes, so that rotational & vibrational modes can be simultaneous. In the case of CO₂ and IR absorption, that's because CO₂ has a significant family of vibrational states that absorb at wavelengths around 15 microns. Earth is bright in the (thermal) 10-15 micron range, so the CO₂ 15 micron absorption is well placed to be significant.

But I'm not buying the collisional transfer to kinetic energy part. A greenhouse gas is defined by virtue of its ability to warm the ground, not itself. Any process that absorbs or scatter infrared light on the way out, more than it does the visible light on the way in, will warm the ground.

But this is incorrect.

When a molecule absorbs a photon, the energy that used to be the photon will go into mechanical energy of the molecule, or into exciting the electron cloud in one of the atoms. In this case, the photon is going into vibrational energy of the molecule. If the molecule collides with another molecule, while it is still vibrating, then that energy (or part of it, as allowed by quantum mechanics) will transfer as mechanical energy to the other molecule, either as a vibrational, rotational, or kinetic state. This is how radiative energy is converted into thermal energy, and is a basic & crucial process in atmospheric physics.

The idea that the greenhouse effect involves "trapped" radiation has made its way into the common language, but is not correct. The greenhouse effect actually has to do with the ability of the atmosphere to maintain a temperature that is higher than that of cold space. The atmosphere warms the surface by emitting IR photons down onto it. So the surface of the planet reaches thermal equlibrium with the warm atmosphere, rather than simply radiating all of its photons into space. Therefore, the ability of the atmosphere to warm itself is not only crucial to the greenhouse effect, it is the greehouse effect.

In the case of Earth & Venus, scattering of thermal infrared photons is essentially non existent, in the gaseous atmosphere. I used single scattering in the radiative transfer algorithms for the ASTER project, because we needed to concern ourselves with the effect of aerosols. But if it's just gases, the thermal IR photons are either absorbed or ignored (not so for visible light, where scattering is more important than absorption).

The important things to remember here are that the transfer of energy from radiative to mechanical is a fundamental process, and that the greenhouse effect is a function of the temperature of the atmosphere. There are a lot of places to read about this. I'm a fan of John Houghton's book The Physics of Atmospheres, Cambridge University Press, 2002 (3rd edition). He also has a book entitled Global Warming that has a far more general discussion. But perhaps the best discussion of the details is in Richard Goody's book, Principles of Atmospheric Physics and Chemistry, Oxford University Press, 1995; see chapters 3 (Absorption and Scattering) and 4 (Radiative Transfer). But none of them describe the process of transfer from radiative to mechanical energy, on the grounds that the reader is expected to already know that (except for Houghton's global warming book, where that's too technical for the book anyway).

[Ken G]

The greenhouse effect actually has to do with the ability of the atmosphere to maintain a temperature that is higher than that of cold space.

Again, the greenhouse effect has to do with the temperature of the ground, not the temperature of the atmosphere. Scattering mechanisms would work fine. It is true that typically infrared radiation is easily thermalized as you describe, but this is not at all an essential part of the effect.

The atmosphere warms the surface by emitting IR photons down onto it. So the surface of the planet reaches thermal equlibrium with the warm atmosphere, rather than simply radiating all of its photons into space. Therefore, the ability of the atmosphere to warm itself is not only crucial to the greenhouse effect, it is the greehouse effect.

This statement is only correct if you assume that the atmosphere must radiate according to its temperature. Processes that scatter infrared radiation, in principle, don't care what the atmospheric temperature is. What I was objecting to was the idea that it was crucial for the molecules to collide with something to share the infrared excitation energy. If they were to re-emit prior to colliding, you still have a perfectly nice greenhouse effect, whether this happens in practice or not.

In the case of Earth & Venus, scattering of thermal infrared photons is essentially non existent, in the gaseous atmosphere.

So now you are speaking to the issue of what actually happens. That's fine, I don't dispute that. I dispute the claim that you needed the collisions to get the greenhouse effect, when in fact, you have it as soon as you get the infrared photons to interact with the gas.

The important things to remember here are that the transfer of energy from radiative to mechanical is a fundamental process, and that the greenhouse effect is a function of the temperature of the atmosphere.

Again, what is not correct here is the implication that scattering cannot produce a greenhouse effect. Your statement is colored by working with gases that do thermalize the infrared radiation they absorb. But if they were instead to scatter it, more effectively than they scatter visible light, you would still have a very nice greenhouse effect in terms of warming the ground. You may feel that I am nitpicking because I'm speaking about hypothetical processes that happen to not be very important in practice, but I say it to avoid conceptual confusion in people who are trying to understand radiative transfer in principle.

[Andre]

Happened to post some info here about the heat of Venus:

http://www.bautforum.com/showthread.php?t=34742

[tracer]

It would convect only if it were fluid, like Earth's mantle.

Only the upper mantle of the Earth is a fluid, and even that is semi-solid (like pine pitch). The rest of the mantle is solid, due to the tremendous pressure caused by having the weight of the upper mantle on top of it.

Note that this was one of the many geophysics facts that the movie The Core got wrong.

[Ken G]

Did The Core get any geophysics right? Beyond the fact that there actually is a core?

[Doodler]

Would the incredibly long length of Venus's day add to the problem? On Earth, the day only gets so hot before rotation moves the surface into darkness and it cools, with direct overhead heating by the sun being at its peak intensity for a couple hours at a stretch. With Venus, you're baking in the sun for the better part of seven months from sunrise to sunset.

[Ken G]

I don't think that would matter, because Venus' atmosphere shares the heat very effectively around to the night side. There is almost no temperature variation, IIRC. I'm not sure of the mechanism that shares the heat though.

[tracer]

Did The Core get any geophysics right? Beyond the fact that there actually is a core?

Well, um, let's see ... they did get it right when they said that the Earth has an Outer Core made of molten metal, and an Inner Core made of solid metal. And a geomagnetic field. Does that count?

[Ken G]

Sure, as long as the geomagnetic field wasn't ridiculously strong, or some such thing. Of course, there's no rescuing the idea that the core could suddenly stop spinning. I guess the thought there was, out of sight, out of mind-- if you can't see it, it can do whatever it wants!

[max8166]

Could the heat of Venus be explained by a recent (in geological terms) collision say just less than a million years ago? Imagine two worlds colliding, the heat produced would be phenomenal. Only a few hundred thousand years of cooling to get to todays present climate.

[Tim Thompson]

Could the heat of Venus be explained by a recent (in geological terms) collision say just less than a million years ago?

It seems unlikely. A collision like that could harldy get away without leaving a clue or two behind. Magellan radar images of Venus do not show any surface features indicative of such an impact, and that's probably the strongest evidence against such an idea. There does not appear to be a hidden mascon left over by a large impactor (Runcorn, 1985). The obliquity of 177.36 degrees gives it a retrograde spin, but it would rotate prograde if it were budged by about 3 degrees. And its figure is round, the polar & equatorial axes are the same. It certainly does not look like something that was slammed into that recently.

[Ken G]

Worse, a collision could only permanently heat the core, not the atmosphere. The atmosphere would quickly (months?) return to its normal energy balance with solar heating. It would not be very easy for the heat coming from the core could to compete with that, at Venus' distance to the Sun.