Originally Posted by ATKINS

Your reasoning is based on the long-held but mistaken belief that cometary comas and tails are necessarily composed of water produced by sublimating ice. The belief originated in the visual aspect of comets as seen from the Earth. It was a perfectly respectable belief half a century ago when Fred Whipple coined the term "dirty snowball" because observation possibilities were limited to ground-based telescopes. The most recent observations and findings, obtained thanks to infinitely more sophisticated means and methods, have now falsified the theory. The corresponding evidence can be found here, here, here, here, here and here.