Why are X-rays absorbed in the atmosphere.

[Naty1]

X-ray observations of outer space are usually made from orbiting platforms. Yet radio and optical astronomy seems to work pretty well.

I got to thinking that it seems odd for the highest energy electromagnetic radiation to be so well blocked by Earth's atmosphere...I would have thought: more energy the more radiation gets thru...but apparently not.

Seems like the highest energy radiation (X-rays,gamma rays, and some UV) from space are blocked in the upper atmosphere protecting us from genetic damage; I would have thought it would take more dense atmosphere to block the most energetic radiation; again, apparently not:

There is a nice diagram here:

http://en.wikipedia.org/wiki/File:Atmospheric_electromagnetic_opacity.svg

What's the mechanism(s) that accounts for this absorption?? Is it the upper atmosphere that also blocks high energy radiation emitted from earth??

 

[JeffKoch]

Photoelectric absorption, compton, pair production - the total extinction cross section for high-energy photons is large enough that the atmosphere is optically thick even though it's relatively low-density, because it's so deep. Lucky us.