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Martian water

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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Damien Sullivan
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