Detecting life on Earth (and other planets)

Neat. [Link]

This spectrum covers part of the ultraviolet and visible range of light. The big dips in the line are due to absorption of light from certain molecules; these dips are like fingerprints for specific atoms and molecules. You can see that LCROSS clearly detected ozone (O₃) and water, which you might see on any old planet. But it also saw a feature that is from free oxygen (O₂) — it’s subtle but it’s definitely there — and that’s something you don’t see just anywhere. Why not?
O₂ is pretty unstable; plop some of it in a random planet’s atmosphere and in a few weeks it’ll be gone, combusted, combined with other molecules to make carbon dioxide or rust or something like that. The only reason we have a lot of it in our air (more than 20% of the Earth’s atmosphere is O₂) is because we have life in the form of plants. They make it as fast as animals and chemical reactions use it up, keeping it in equilibrium.
In fact, looking for free oxygen is one way scientists may eventually look for life on planets orbiting other stars. It’s an incredibly difficult observation, but not impossible, and in the future better technology may allow us to search for the elusive dip in a spectrum that tells us that aliens are breathing (or whatever aliens do) there.
Incidentally, the forest of little dips in the spectrum may be due to vegetation itself, which would be a direct detection of life. I’ll take that, too.