The Way to Dusty Death

It's our planet's greatest murder mystery: what causes the occasional mass species extinctions that plague us? Space.com offers some new speculations, so far based on model possibilities, not hard evidence: dense clouds of cosmic debris that our solar system might have passed through in its twirlings through endless space, taking up to a half million years for us to escape. Such clouds could either freeze out life or, under certain conditions, destroy the ozone layer, saith Alex Pavlov, author of a couple of papers on the topic appearing in Geophysical Research Reports.