Doppler water hack

2017-Jun-01, Thursday 20:29
mindstalk: (science)
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.

Martian water

2017-May-31, Wednesday 22:02
mindstalk: (Default)
So I've been watching videos in this Mike Brown/Caltech Coursera on the solar system. I think it's free for anyone to audit. Lots of cool stuff, even for this PlSc major; I wasn't that deep in it, and haven't kept up.

Like, apparently the top meter of Mars, above 60 degrees latitude, is like 30% water. How do we know? Cosmic rays knock neutrons out of nuclei, which hit more nuclei, either causing gamma rays or escaping on their own. Gamma ray frequencies tell us what nuclei they're from. Thermal as opposed to fast neutrons indicate the presence of something able to slow neutrons down, and that something is pretty much hydrogen. (No one expects Mars to be covered in ammonia...) So between those signals... we see lots of water signs.

More visually, the Phoenix lander dug. Ice! And we've observed some fresh meteor craters, which start out with a shiny white center, that goes way due to either sublimation or dust covering the ice, I'm not sure which.

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