Last week Science Express published two papers ( here and here ) about the Deep Impact mission to comet Tempel. This spacecraft basically slammed into the comet to dig out the pristine material deep under the surface. When that stuff jetted out, both spacecraft and ground telescopes were able to identify the components by their spectra.
Based on the behavior of the comet flume, astronomers think that the comet is a very weak and dusty collection of dust-sized particles, with almost no solid surface. This may be bad news for the Rosetta mission, which is trying to send a lander down to the surface of a comet. It may just sink straight down once it lands.
The main chemical result from Deep Impact is that the inside of this comet is a whole soup of organics , including methyl cyanide, a reactive small organic which can easily polymerize into the precursors of DNA among other things. The earth was absolutely pummeled by comets and their kin during its early history; thus these objects may have delivered the precursors of life to the earth's surface.
A second finding of interest was the presence of clays and carbonates , which require liquid water. Thus the comet might be an agglomeration of material which formed in very different parts of the solar system. I think this mixing, maybe under the gravitational influence of the early Jupiter, may be an important part of making a terrrestrial planet. The rocky core has got to form close in, but the water and organics are best made further away. However this works out, the comets are a big part of the Earth's history.
See also the writeups at ..of Cabbages and Kings and plvs vltra .