The journal GCA isn’t monthly, like a lot of journals these days. So it’s time to catch up; here’s Geochimica et Cosmochimica Acta vol. 381, for 15 Sep:
Noguchi, T. Nakashima, D. Ushikubo, T. et al. Chondrule-like objects and a Ca-Al-rich inclusion fr… p. 131-155 .07.038
Yes, comets have been altered; they are not perfectly frozen relics, untouched since the start of our Solar System. We’ve seen this for almost twenty years now, since the Stardust results from Wild 2. At minimum, particles from the inner Solar System (therefore, heated and altered materials) got blown outward, and incorporated into the incipient comets. More recently, scientists are postulating the heating of early comets by radioactive isotopes, still present from the Solar System’s formation. The evidence, I would say, doesn’t cross the line of conclusiveness on that one just yet; we’ll see if other groups reach a consensus on it.
In sum, our notion of comets- never conclusive to start with- is a work in progress, and still progressing. Comets have elliptical, often inclined orbits, making them very difficult to study by space probes. The coma of a comet, which is basically always there when we can study it well (in the inner Solar System, and warmed by sunlight) gets in the way of telescopic studies. (JWST should help here- a large area helps us see it further out, and infrared wavelengths are less bothered by the coma.) Noguchi et al. are studying comets secondhand, via dust particles recovered from Antarctic ice. Due to the composition- vaguely close yet still unlike stony meteorites- we strongly suspect this dust is a comet sample. That comet sample is telling us, like the Stardust mission, that we need to sharpen our pencils on just what went on with these small Solar System bodies (SSSBs).