The heating of Venus
The Physics
What got me started
On my local Nextdoor website during August 2022 there has been a discussion of the effects of atmospheric CO2. Someone said there was no greenhouse effect. Another contributor posted a presentation by Carl Sagan testifying before Congress in 1985 on climate change.
https://www.youtube.com/watch?v=Wp-WiNXH6hI
The response to that from the person denying a greenhouse effect was to post a link to “Hyperventilating on Venus”, by Steve Goddard.
https://wattsupwiththat.com/2010/05/06/hyperventilating-on-venus/
I had not listened to the Carl Sagan presentation before I wrote my response to Goddard’s points. In fact Sagan only mentions Venus and other planets in a sentence or two. I found Sagan gave a very balanced, cautious presentation without alarmism. It is certainly worth listening to.
A good way to understand an issue is to start with an opposing view and work out in detail why you disagree. The devil is almost always in the detail though not always. Whatever view of an issue you have there will be many who share your view. If you only connect with those people you may never find the weakness of your viewpoint.
There is some basic physics to be known before you can have a discussion of the planetary greenhouse effect. Because my response became long I decided to post it online and post the link to the discussion on Nextdoor.
First, how I understand the greenhouse effect for a planet.
Here is a reminder of the physics you need to have at least touched on to understand what is happening. I have tried to avoid specialist jargon but that means some loss of precision.
Temperature is a measure of the average energy of motion of a large volume of atoms and molecules.
Although we use mostly the Celsius temperature scale, where ice freezes at 0 C and water boils at 100 C (for nit-pickers I should add at standard pressure), there is another temperature scale with an absolute meaning, the Kelvin scale. Zero Kelvin means zero energy of motion, though we can never get to that.
0 K is the same as -273.15 C,
273.15 K is the same as 0 C and so on.
Interstellar space has a temperature of 2.7 Kelvin. Anything in space with a temperature greater than that will have net outward radiation of its energy. The higher its temperature the faster it radiates. If it did not get any heating it would eventually also get to a temperature of 2.7 K.
The greenhouse effect on a planet is the extra heating caused by molecules in the atmosphere slowing the rate at which the planet (including its atmosphere) radiates its energy back to space. Sunlight does most the heating of the planet and atmosphere, though planets can get internal heat from radioactive decay.
It is like filling a leaking bath where the water level will rise until the higher water level pressure forces as much water through the leak as is being poured in. Incoming sunlight warms the planet and its atmosphere. As the temperature rises more energy will radiate out until there is a balance between in and out. The net effect is the planet and atmosphere becoming hotter.
How does the atmosphere have this effect?
By light I do not just refer to that which makes things visible to us but the whole range of what became known in the 19th century as electromagnetic waves. However, when light interacts with matter it behaves as a particle (called a photon for the past 100 years). This was worked out by Einstein; for this understanding he got the Nobel Prize in 1921.
The energy of the photon is inversely proportional to the wavelength of the light; infrared with long wavelength has lower energy than X-rays which have short wavelengths.
Infrared wavelengths that we feel as warmth range from 0.000 000 7 to 0.001 metre. [I space the digits to the right of the decimal point for clarity]
Light by which we see has a wavelength range from 400 nanometres (violet) to 700 nanometres (red), that’s 0.000 000 4 to 0.000 000 7 metre.
X-rays such as those used in hospitals for diagnosis have a wavelength in the range 10 to 100 picometres, 0.000 000 000 01 to 0.000 000 000 1 metre.
An X-ray could have 0.001/0.000 000 000 1 = 10,000,000 times more energy than that of infrared. X-rays can damage our cells.
When a photon is absorbed by an atom or molecule it does not exist trapped inside somehow, it is destroyed and the atom or molecule takes the energy the photon carried, to in some way speed up its internal parts. For example molecules vibrate by the distance between the atoms in it stretching and shrinking in a regular beat.
Another complication is that an atom or a molecule cannot absorb any photon it comes across, it can absorb only photons within a very specific set of energies. Different atoms and molecules will differ in the set of energies. One pattern is that atoms will mostly absorb in the range of visible light, molecules in the range of infrared. X-rays change the inner part of an atom, visible light the outer part of an atom.
Just as a final twist in the complication, as the gas pressure increases the molecules bash each other more frequently and violently. This causes the set of absorption energies to broaden into bands of absorption (same for emission, called pressure broadening).
That extra energy in the atom or molecule can be released: a photon of the same energy as that absorbed can be created and emitted; two lower energy photons could be emitted; in a collision with another atom or molecule that extra energy can become extra kinetic energy of motion of the things colliding, speeding them up. That extra kinetic energy is how things warm up.
As you can see warming is a complex process.
Next, the article by Goddard.
Goddard: “The first problem is that the surface of Venus receives no direct sunshine.”
Not relevant. Solar radiation penetrates the atmosphere. The issue is — does the resulting radiation from the warmed atmosphere get slowed down in getting out.
Goddard: “The way a greenhouse effect works is by shortwave radiation warming the ground, and greenhouse gases impeding the return of long wave radiation to space.”
It may work that way on Earth but why should Venus be the same as Earth. The sunlight does not have to reach the ground for there to be a greenhouse effect. In a greenhouse, it’s the glass that impedes the return of long wave radiation to space and the Earth is not surrounded by glass windows.
Goddard: “The next problem is that the albedo of Venus is very high, due to the 100% cloud cover. At least 65% of the sunshine received by Venus is immediately reflected back into space.”
So what? There is about 35% that gets through, what happens to that? It is absorbed otherwise the albedo would be 100%.
Goddard: “The third problem is that Venus has almost no water vapor in the atmosphere. “
Not relevant. Water vapour may have the major greenhouse effect on Earth but the atmosphere of Venus has mostly CO2 and some high clouds of sulphuric acid aerosol that also impedes return of long wave radiation to space.
Here is a useful graphic of the structure of the atmosphere of Venus:
https://www.aeronomie.be/en/encyclopedia/venus-atmosphere-stable-cloud-layer-covers-planet
Goddard: “Each doubling of CO2 increases temperatures by 2–3C. So if earth went from .04% CO2 to 100% CO2, it would raise temperatures by less than 25–36C.”
So here on Earth doubling CO2 from 0.04% to 0.08% would increase the temperature by 2C according to Goddard. On this he could be right.
Goddard: “The excess CO2 does not begin to compensate for the lack of H2O.”
Why? This is just assertion without evidence. Venus has a different atmospheric composition and temperature from Earth. Above the CO2 atmosphere Venus has clouds of aerosols that are 75% sulphuric acid and 25% water where “sulfuric acid water solution droplets will absorb solar radiation in the near IR, about 2μ … … will scatter solar radiation.” The fact is that Venus is hot. The energy going in is sunlight.
https://journals.ametsoc.org/view/journals/atsc/30/1/1520-0469_1973_030_0095_rabdos_2_0_co_2.xml
Goddard: “So why is Venus hot? Because it has an extremely high atmospheric pressure. … Temperatures in Earth’s atmosphere warm over 80C going from 20 kPa (altitude 15km) to 100 kPa (sea level.)”
Just because temperature changes with altitude, as well as pressure, does not mean that pressure causes the temperature change. This is a total non-sequitur.
Goddard shows a graph from R Stull, “Meteorology for Scientists & Engineers”. This is merely a look-up chart for different thermodynamic states for the Earth’s atmosphere. In no way does it indicate that one parameter is the cause of another.
Goddard, “The atmospheric pressure on Venus is greater than 9,000 kPa. At those pressures, we would expect Venus to be very hot.”
But not because of the pressure.
In the Mariana Trench, 11,000 metres deep (pressure 110,000 kPa), the water temperature ranges from 1 to 4 C. The surface (pressure 100 kPa) temperature of the sea ranges from -2 to 30 C. No pressure effect there. Pressure does not determine temperature.
Goddard, “The high temperatures there can be almost completely explained by atmospheric pressure — not composition.”
How? What is the physics of this? It is more temperature that determines pressure just as in a pressure cooker. Pressure is the force of molecules banging into whatever, temperature is a measure of the kinetic energy — the faster the molecules move the higher that force, the greater the pressure.
If you rapidly compress a volume of gas the temperature goes up because you are putting energy into it. It has a higher pressure but unless insulated the temperature will return to the surrounding temperature and although the pressure will drop it will still be higher than before compression. Pressure does not determine temperature.
On Venus, molecules in the atmosphere absorb most of the entering sunlight before it reaches the surface. Because the atmosphere is dense the molecules are in continuing collision and some of the absorbed energy will become extra kinetic energy of those molecules, which means increasing temperature. Most of the radiation from the molecules will be infrared that is absorbed easily by other nearby molecules — it stays around a long time being gradually converted to kinetic energy making the atmosphere hotter and hotter. The high pressure on Venus and the broadening effect it has on molecular absorption will enhance this effect. Some of the radiation will eventually get to the top of the atmosphere and radiate into space; that top of atmosphere will be coldest because ‘space’ is cold. The surface of Venus is hot because it is in contact with the hot atmosphere. The heat will move from the hottest region at the surface up to the coldest region at top of the atmosphere.
There is volcanic activity on Venus that emitted the CO2 in the distant past. This will also contribute to heating. There is a study, an example of modelling Venus to understand how it got to its present state: “The hot atmosphere of Venus might cool its interior” https://phys.org/news/2010-09-hot-atmosphere-venus-cool-interior.html
Whether the heating comes from absorption of solar radiation by the atmosphere or from the planet’s interior you could not have a better example of the greenhouse effect due to atmospheric gases slowing the outgoing radiation.
Postscript: Fascinating detail on the structure of Venus:
[This is my first post on Medium and I have not explored formatting fully]
