When an astronomer looks at the atmosphere of a planet and sees carbon and oxygen, how do they know if any of the oxygen is O2? How do they know it's not all tied up as CO2?

The emission and absorption lines are different for molecules than for individual atoms. They would see CO2 lines and/or O2 lines.

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wow, ever since high school I've thought that atoms, even when part of a molecule, emitted a given wavelength.

thnaks.

A very broad overview....

It depends on the type of spectroscopy and at what energies. For most of what astronomers do, they use visible, IR, or UV. The energies of those emissions or absorptions often correspond to the energies of the bonds in compounds. For example, IR lines tend to be things like bending or stretching of bonds in compounds.

Now, in some atoms, metals particularly, visible or UV energies will correspond to changes in energy in atoms, particularly changes in electron energy states. That's why sodium vapor lights glow that orange color. But some of those transitions are in the upper UV or higher, and so we don't usually look at oxygen atoms or molecules that way.

Once you get into x-rays and gamma-rays, you are talking about energy states in atoms (usually core electrons), and so the spectroscopy is more element specific.

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That's a pretty sad commentary on the state of high school science education! Molecules have all kinds of ways to respond to light, especially low energy light, that atoms just don't have, like vibrations and rotations of the atoms within the molecule.

Ken G. True, molecules have all kinds of ways to respond to light....and they can respond in the same way to neutrinos via the neutral current. While an opaque wall will stop them from responding to visible light....nothing will shield them from the neutrino sea. Pete

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A second rate theory explains after the fact.
A first rate theory predicts.
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That's true, but note the ability to shield from light is also why light is so important and why neutrinos are not-- because the shield is made of molecules too, interacting with light. Thus there's a kind of devil's bargain here-- for neutrinos to be so unshieldable, they also have to be pretty unimportant when they arrive. Not to knock neutrinos, of course!

All one needs is little ole telescope for neutrinos [or, here ], and an appropriate continent.

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and a sad commentary on present day cosmology. McKellar used the rotations of molecules to discover the CMB

Quote:

The first observations of the CMB were made by McKellar using interstellar molecules in 1940. The image at right shows a spectrum of the star zeta Oph taken in 1940 which shows the weak R(1) line from rotationally excited CN. The significance of these data was not realized at the time, and there is even a line in the 1950 book Spectra of Diatomic Molecules by the Nobel-prize winning physicist Gerhard Herzberg, noting the 2.3 K rotational temperature of the cyanogen molecule (CN) in interstellar space but stating that it had "only a very restricted meaning." We now know that this molecule is primarily excited by the CMB implying a brightness temperature of To = 2.729 +/- 0.027 K at a wavelength

now why did he not get the Nobel prize instead of Penzias and Wilson?
Cheers,
Lyndon

I don't know for sure, but probably because he didn't realise the consequences. But Herzberg sure deserved his. Have you ever seen that book? It must have taken him a million years to do all those experiments....

Interesting that he overlooked the importance of the CN line, too, and even made a comment about it. I wonder what his reasoning for that was. That's not to say anything bad about Herzberg, who is pretty much my hero. But I wonder if it was something he kicked himself about later on, or if he just lacked a critical piece of information.

I think also that the very existence of interstellar molecules was somewhat under debate back in those days, though the book in which I read that might be exaggerating the situation somewhat.

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True,
But i find it fascinating how spectroscopy and and the OP together with molecules in the IGM have improved our understanding of the universe. However Penzias and Wilson didn't know what they had found either - so why them?
Cheers,
Lyndon

Hi Tofu,

The following is my effort to answer your question.

Attached is Jack Barrett’s review paper that describes how and why gas molecules and diatomic gases can or can not absorb and emit infrared radiation. (Note the symmetrical diatomic gases like oxygen and nitrogen can not absorb or emit infrared radiation. Compare them to a gaseous water or carbon dioxide molecule that has a dipole moment.)

Remember these gaseous atoms and molecules are not ionized. (i.e. The radiation emitted by an ionized gas when it gains a missing electron is a much higher frequency.)


Greenhouse molecules, their spectra and function in the atmosphere

http://www.warwickhughes.com/papers/barrett_ee05.pdf