In addition to taking normal images of the impact in the visible spectrum, the HRI provides an infrared "fingerprint" of the material from inside the comet's nucleus.

BBC: Q&A: Deep Impact comet mission

It's never happened before that we've fired a very large bullet that's made largely of copper and aluminum, the size of a washing machine, at a comet that's about--the comet's nucleus is about four or five miles across--to make a big crater.

NPR: NASA 'Deep Impact' Probe Targets Comet

The first is that the asteroid is really a ball of ice, like the nucleus of a comet, and that the minerals which the spectroscope detected are only a coating.

ECONOMIST: Asteroids

Or so says David Eichler, lead author of a forthcoming Astrophysical Journal Letters paper positing that a sun-grazing comet roughly the size of Hale-Bopp (with a nucleus some 30 kms in diameter), could trigger cosmic ray-generating shockwaves large enough to initiate a global electromagnetic Armageddon.

FORBES: Sun-Grazing Comets As Triggers For Electromagnetic Armageddon