Temperature And Atmospheric Pressure

[David H]

Is it possible to calculate the relative temperatures of planets from simply measuring the law of greater atmospheric pressure raising the temperature directly, which ought to explain the reason why Venus is warmer than Mercury? It should also explain why Earth is warmer on average than its nearby moon with hardly any, although it still reaches 40C at the equator.

[jake-the-peg]

Only to a certain extent

You have to take into account a lot of other complex factors not least of which is the composition of the atmosphere.

Venus is so hot because of the runaway greenhouse effect - its atmosphere is almost entirely carbon dioxide.

Factors like its inclination to the sun and gelogical activity will also play a part.

Look for example at Io which is constantly stretched under Jupiters gravitational field and is full of volcanic activity

[Hypognosis]

The atmospheres of the gas giant planets are certainly deep enough (several earth diameters) for pressure effects to be the dominant source of heating, as opposed to solar radiation. See the para featuring refs 29 to 32 in:-

http://en.wikipedia.org/wiki/Jupiter

Also, the inverse square law tells us that a planet ~five times further away from the sun than us, as Jupiter is, gets 1/25 as much solar energy per unit area. I can't get much agreement from the 'net on what the figure for earth is because it is now snarled up by solar panel promotional bunf. If you look at

http://en.wikipedia.org/wiki/Insolation#Projection_effect

you'll see what sun angle (on account of latitude) does to affect how much energy the ground absorbs.

There's even a bit about Milankovich cycles, as the cyclical axial tilt changes further alter the projection angle effect.

//It should also explain why Earth is warmer on average than its nearby moon with hardly any//

My Collins guide (Ridpath/Tirion) says "Each spot on the moon is subjected to two weeks of daylight, during which temperatures reach the boiling point of water (100'C), followed by a two-week night when temperatures plummet to -170'C."

I make that an average of -35'C for the moon. I don't know what average you prefer to ascribe to earth (please let us know) but is a mere 10 miles* of atmosphere enough to account for the discrepancy?

(*Actually, if I remember rightly, about 90% of our atmosphere is found in the bottom 10,000 feet)

Greenhouse effect is what stops the planet from being an ice cube and that's with under half a percent CO2 in the mix.

[David H]

I am getting the impression there is a lot of material suggesting various aspects of the greenhouse effect are assumptions. The major reason is you cannot get a greenhouse effect without a solid barrier like glass. Gas moves, so trying to measure temperature as a function of endless layers of gas changing from one level to another in temperature and density, as if they are simply able to be compared with static glass barriers is no more realistic than trying to apply the ideal gas law to the same complex atmosphere as the greenhouse effect. I have read since that both only work simply in lab conditions, and while no one is put off applying the greenhouse effect as if it can be calculated just as well in situ I have never seen the same effort put into calculating the huge atmospheric pressure on Venus on surface temperature.

Had I been qualified I would not have been asking these questions here, so am not able to assess such propositions, but the holes those are have punched in the greenhouse effect makes me suspicious why atmospheric pressure is not factored into average surface temperatures when if a control planet were able to be compared (the moon being our best opportunity) one with a massive pressure and one with very little, and a double bind with two more with the same pressure and totally different atmospheres then we wouldn't really need to wonder as it would be directly observable and any theories could be gradually inferred.

[Hypognosis]

If you read the paragraph in the Jupiter article, you will note a comment to the effect that the planet achieves its net output of heat due to steady contraction of its diameter. 2cm/year doesn't sound much but the surface area is huge.

I'm using that as the exception to prove the rule though. In the case of an inner-solar-system ball of rock with no atmosphere, the act of dumping gases onto it will cause the gases at the surface to experience pressure increase as more gas piles on top. The act of becoming compressed will, as you say, cause temperature increase.

But then what? The atmosphere stabilises at a certain depth. No further prersure-rise-related temperature rise. The initial heat begins radiating away into the ground or into space. Everything after that is all down to solar radiation, topping up the nightime heat losses.

If pressure is being disregarded for the purposes of calculations, then this is (imho) the reason why. The assumption (you rightly pointed out assumptions were being made) is that the planet in question has a well established atmosphere which is neither undergoing active gravitational collapse nor losing bulk quantities to ablation by solar wind (current theory has it that this is how Mars lost most of its atmospheric depth).

Incidentally, the greenhouse effect that climate scientists go on about involves air molecules absorbing photons of one wavelength and, due to energy losses, re-radiating them at a longer wavelength. Furthermore, it is about incoming sunshine, all wavelengths, hitting the ground, being re-radiated at longer wavelengths before it encounters the greenhouse gas for the, abovementioned, second round. Direction of the heat emission is random so 50% is re-radiated into space and 50% back down to the ground.

(I've seen an infographic which explains this visually and with more nuances to it but I can't remember where, at the moment. If I find it again, I'll try to post a link to this thread).

The more familiar glass greenhouse does all that and more - the physical barrier stops convection currents escaping and taking heat away from the interior. It's the archetypal 'closed system', at least until someone opens the door or a vent. ;-)

In the case of a planet, it is gravity which prevents the atmosphere from escaping into space such that convection cannot take heat out of the system. Radiating the heat away is its only means of escape and that is the nub of the issue with greenhouse gases partially blocking that, as described above.