Regarding the ability of the atmosphere to block X-rays;
I seem to remember from some classes I took a while back that it
was the earth's electromagnetic field that blocked the gamma rays
(so-called 'hard' X-rays) that come from the sun. I recall some-
thing about how the earth periodically 'flips' the poles on it's
magnetic field resulting in a temporary loss of protection. A num-
ber of scientists have circulated some interesting speculations on
the effects of such an event upon the biosphere. I also remember
the planetary scientists were disappointed when it turned out that
Mars had no significant magnetic field. They seemed to feel that
w/o such a field the development of life would be greatly hindered,
if not impossible. Of course, I may not be recollecting accurately,
you know how undergrad courses are! ;*)
PHIL
[email removed]