Doppler water hack

Say you want to look at Mars and know if it has water.

Basic spectroscopy review: Perfect blackbodies radiate in all wavelengths. Real blackbodies are made of atoms and molecules whose electrons absorb some wavelengths of light. So, it's simple: look at Mars, and if the wavelengths absorbed by water are missing, it has water!

Problem: *our* atmosphere has lots of water, so we'd expect to not see those wavelengths no matter what we looked at, because they'd be absorbed by our atmosphere's water.

You could get around that by putting a telescope In Spaaaaace, which makes everything cooler. But that wasn't much of an option in 1963.

Earth and Mars usually have up to relative motion, up to 30 km/s (said Mike Brown in a popular lecture.) That means Doppler shift of the light from Mars, changing its wavelength (and frequency). Not by much, one part in 10,000, but that's apparently enough to shift narrow absorbtion bands into transparent regions of our own atmosphere.

So, new plan! You look at Mars at quadrature or something, hoping not to see certain wavelengths, which are water-like but shifted: if they're there, our air lets them in, so if they're not, Mars never emitted them in the first place.

And if you do see them, then Mars doesn't have much water.

(Spoiler: we mostly see them, and Mars doesn't have much water. Well, on the atmosphere, or emitting light from the surface. Shit ton underneath it, but those are different observations.)

I thought I knew basic spectroscopy. But using Doppler shift as an information hack against our own atmosphere? That's new to me.