THREE years ago, the comet 17P/Holmes exploded with a blast comparable to a small nuclear bomb. Would you believe that an exotic form of ice was responsible?
Comet 17P/Holmes became a million times brighter when it erupted in 2007. A freak collision with an asteroid could have explained that blast, had it been a one-off. But the same comet also exploded in 1892, suggesting something else might be triggering the outbursts. Now William Reach of the California Institute of Technology in Pasadena and his colleagues think the culprit may be an exotic and unstable form of water ice at the comet's heart.
http://www.newscientist.com/article/mg20527483.800-suns-warmth-blows-comets-icy-heart-apart.html