How pristine are cometary nuclei?

The claim that comets are the best obtainable source of original solar nebula material is assessed. To fully interpret the cosmochemical record contained in the comet nuclei, modifications since their formation in the primordial solar nebula, 4.5 Gyr ago must be considered. Possible processes include: comminution and collisions during formation; heating by short-lived radionuclides; accretion of interstellar gases; erosion by interstellar dust; polymerization by galactic cosmic rays; and heating and thermal evolution when the comets return to the planetary region. Consideration of these processes has important implications for where and how a sample is obtained on the cometary nucleus, and how it is preserved during the return to Earth. The possible dynamical history of short-period comets is discussed.