It is generally believed that a comet consists basically of a loose conglomeration of frozen gases with embedded material similar to that found in the carbonaceous chondritic meteorites, and consequently that comets may be nearly pristine samples of the original solar nebula1−5. We show here that thermal processing within comets could have played an important part in determining their present state; in particular, we find that liquid water might have been available in some comets over geologically and biologically significant spans of time. It follows that a cometary origin is not excluded for some thermally metamorphosed meteorites and asteroids, that comets may contain quite complex organic molecules, and that comets may have played a role in the origin and conceivably even in the subsequent evolution of terrestrial life.