New long period comets are believed to be pristine matter which has never been processed by photochemical interactions with the sun. The older long-period comets which have been around the sun many times, like Halley's Comet, have been profoundly affected by deep outgassing through numerous surface vents, and intense radiation fields in the inner solar system. This is probably the major reason why these comets change in brightness according to different albedo or 'reflectivity' laws. The details of whether the 'law' will have an index of 2 or 6 or some number in between will depend on the detailed chemical properties of their surfaces, and how these change in time as the comet falls nearer the sun.