How much does air pressure contribute to surface temperature?

[David H]

Venus and Mercury have very different atmospheric pressures and composition, but how much of the surface temperature of a planet is related to that pressure compared to the composition? If you had an identical composition and pressure was double, or identical pressure but totally different composition which one would overcome, by how much and why. It's only one question but broken down to its parts.

[beso]

The temperature increase from pressure is all about change. For a given quantity of gas, double the pressure and the temperature doubles if no heat is lost from the system.

However the pressure itself does not cause a higher temperature.

[New Judge]

More relevant than the atmospheric pressure by itself is the effect that the atmosphere has in trapping the heat energy applied to the surface of the planet. The moon has no atmoshphere but is roughly the same distance from the Sun as the Earth is. However,the moon is searingly hot on the side facing the Sun and perishingly cold on its dark side. The Earth has a more equal temperature becuase the heat energy provided by the Sun is trapped within the air whilst that same air also insulates the Earth's surface preventing it suffering the full impact of the Sun's energy.

[Teddio]

Temperature and Heat are two different things.
A body, such as the surface of the Earth, radiates electromagnetic radiation. As the temperature increases, the wavelength of the radiation decreases.
The Sun is a very hot body and gives off short wave electromagnetic radiation.
This penetrates the Earth's atmosphere, with little absorption, and heats up the ground. The warm ground gives off longer wavelength raiation (infrared) which is easily absorbed by atmospheric molecules such as carbon dioxide, methane and water.
An increase in pressure of a gas will increase the number of radiation absorbing molecules and hence increase the degree of absorption.

[beso]

Great answer Teddio.

I was thinking more of the gas equation in my answer.

[David H]

Being an investigator but with an interest in science I do need to know these basics, I think the first answer (is that Boyle's law from my 1976 O level by any chance?) was what I was mainly after, as is a vital piece in the planetary temperature equation, but I will copy the rest and learn it as it is also important consequential effects from the effect of having an atmosphere.

[Zacs-Master]

In Mercury and Venus' case, isn't proximity to the sun a factor. I believe Mercury has a geostationary orbit with obvious effects.

[beso]

It has just been announced that the Messenger probe has found water on Mercury. Surprising on a planet where lead will melt in some places.