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Viewing Single Post From: The giant space ship example

Chris Ho-Stuart
Dec 4 2011, 11:52 AM
The easiest way to answer the original question is to simply use a Nitrogen/Oxygen mix in the starship atmosphere. Nitrogen and Oxygen are almost totally transparent to infrared radiation, which means that there is no greenhouse effect in the starship atmosphere. The thermal radiation emitted from the surface will pass straight out into space, pretty much the same as if there was no atmosphere at all.

Can we also assume that Nitrogen in addition to being almost totally transparent, also does not transfer energy [lose heat/energy] other than imparting energy through collision. That molecule collision in the atmosphere among exclusively nitrogen gas can impart kinetic energy, but doesn't cause nitrogen emit photons [radiate heat].

And that collisions of nitrogen gas with CO2 [or other gases] could cause these other gases to emit photons.

That we accept that this is true and applicable in an atmosphere:
"Electron impact excites vibrational motion of the nitrogen. Because nitrogen is a homonuclear molecule, it cannot lose this energy by photon emission, and its excited vibrational levels are therefore metastable and live for a long time.
Collisional energy transfer between the nitrogen and the carbon dioxide molecule causes vibrational excitation of the carbon dioxide, with sufficient efficiency to lead to the desired population inversion necessary for laser operation."

Or that only way a N2 molecule can transfer any of it's energy is by collision with something other than other N2 molecules.
And if one has a pure nitrogen atmosphere this means nitrogen can only exchange energy with the surface of the planet. And that nitrogen is unable to directly radiate energy into space.
[N2 molecules frantically "trades" kinetic energy with themselves, but the "community" only "exports" energy to the surface- it can not "export" energy to space. It needs a middleman to radiate energy into space.]

This would mean that with pure Nitrogen atmosphere if one control the heat loss of the surface, or controls the interaction of the atmosphere with the surface, you control amount energy needed
to warm the atmosphere. If you have near zero loss of heat from surface, one has near zero loss of energy from the planet.

With nitrogen atmosphere one could ignore the non-existent radiation of the atmosphere to space and focus on the transfer of energy of the atmosphere to the surface.
One might interested what percent of the atmosphere molecule collide with the surface.
How long does it take for an average nitrogen molecule to hit the surface.
Could have a thermal reflective surface?
Could thin layer of cooled air inches above the surface reduce loses- would this be the case "without even trying"- since there is no sunlight heating the ground, the cold ground should "naturally" cause a layer of cool atmosphere near the surface?

Note, other gases or most other gases can also function in similar manner to nitrogen- I am not mentioning them from a desire to simplify. Whereas greenhouse gases are considered, to act differently.
Namely, CO2 is considered as agent that may to some degree take the KE from N2 as well as the surface and emit this energy as radiation. It's possible that CO2 or H2O may in some manner reduce energy loss, but CO2 or H2O at high elevation should be a net loss of energy/heat- they may export energy towards the surface, but are the only exporter of energy to space in the upper atmosphere. A examination of pure CO2 or H2O atmosphere would also be interesting.
It would seem that in such examples, the surface temperature could be more or less irrelevant- "all the action" occurring mostly in the upper atmosphere.

[In the real world one is going to have mixtures/impurities- Oxygen is common though commonly chemically reactive- and so mostly in various compounds, CO2 is common as is water, nitrogen and with Hydrogen and Helium dominating this universe.
NASA is sending a spacecraft [Dawn]to Ceres [and is at Vesta at present time]. Ceres may have a largely H20 atmosphere:
"There are indications that Ceres may have a weak atmosphere and water frost on the surface. Surface water ice is unstable at distances less than 5 AU from the Sun, so it is expected to sublime if it is exposed directly to solar radiation."
So on Ceres' weak atmosphere H2O may be one of major gases. Anyhow it will be 2015 before Dawn reaches Ceres. And perhaps then see up close an atmosphere which could have H2O as main element as the atmosphere.]
Edited by gbaikie, Dec 15 2011, 12:53 AM.
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The giant space ship example · Physical theory for climate