What could spectroscopy tell us about a planet's atmosphere?

[Junghalli]

Here's something I'm wondering about. Exactly how much could we learn about the atmosphere of a possible habitable exoplanet from spectroscopy instruments like the Darwin Mission or Terrestrial Planet Finder would carry?

I mean, would we be able to know in detail the composition of the atmosphere, or would we just be able to tell, for example, whether or not there's some oxygen in it?

According to the TPF site:

The oxygen and ozone absorption features in the visible and thermal infrared respectively could indicate the presence of photosynthetic biological activity on Earth anytime during the past 50% of the age of the solar system. In the Earth's atmosphere, the 9.6 µm ozone band is a poor quantitative indicator of the oxygen amount, but an excellent qualitative indicator for the existence of even traces of oxygen. The ozone 9.6 µm band is a very nonlinear indicator of oxygen for two reasons. First, for the present atmosphere, low resolution spectra of this band show little change with the ozone abundance because it is strongly saturated. Second, the apparent depth of this band remains nearly constant as oxygen increases from 0.01 times the present atmosphere level of oxygen (PAL) to 1 PAL.

Does this represent about the limit of presently plausible technology or could we get more detailed information on things like the amount of oxygen in an exoplanet's atmosphere?

[Admiral Valdemar]

Short of sending a probe with an MS on it to analyse gas samples, you're somewhat limited. The likes of Beagle II could only manage to get such an instrumentn onboard with such size and low power usage because it was basically a very specific device with limited applications. You'd need a Lunar lander to send any of the LC-MS/MS units in the labs at my place, and then some given that doesn't account for the energy usage either. Oh, and maintenance.

So really, remotely, you're going to have to rely on getting the general composition via these fancy telescopes, instead of using mass spectrometry to find the more precise composition.

[Gil Hamilton]

Spectroscopy can tell you tons about an atmosphere, but its very hard to be quantitative unless you are close to the object of study (of course, you want that "Thar be nitrogen and oxygen signals, cap'n!" measurements before you invest a probe). We've studied all the gas giants in the solar system and the sun with microwave spectroscopy, for example. There are good things you can look for, for example, ammonia is the classic, because it has a very distinctive signal corresponding to its molecular inversion splitting that is in a band that is convenient to study. Studying that signal can tell you alot; it's a good temperature and pressure probe for one thing. It also tells you if the atmosphere is reducing or oxidizing. With microwave spectroscopy, you can study quite alot.

Of course, the BEST thing to do is send a probe with a fight capable GC-MS and a few other sampling instruments, ladel up some of the stew and see what's in it.