Hmmmm, let's see here. Is it the early hour, lack of coffee or what that muddles my mind with a very puzzling question.
They say they want to learn more of the comet's primordial cores. If the comets are leftover stuff of which the solar system and planets are made of, would it not stand to reason that whatever is in the core of the comet should also be found in the planets like earth, for instance???
What am I missing here>
I'm not a scientist, nor did I play one on TV, but my understanding is that the inner planets are mostly iron, nickel, and dense rock. The gas giants are large enough to retain atmospheres of hydrogen and helium, along with lots of other light gasses. There is a formula that lets you calculate if a gas molecule will reach a planet's escape velocity at a given temperature - I forget it now but it was in my intro to Astronomy class in college. Anyway, we ended up with dense minerals and large enough to retain medium-sized molecules of gas. The gas giants keep almost everything, including light gasses. The other bodies are usually too small to retain any measurable atmosphere, and being outer bodies they have a sizeable portion of ice and dust, but not much metal - at least that is the theory my Astronomy professor explained.