OK, so no nuclear stripping, but infra red heat to "distill" as it were, and then perhaps stripping the comet of one isotope over another as the comet proceeds through the intense magnetic influence of the sun
I only ponder if going in and out of the sun's electromagnetic and radiation fields could have an effect on the detected ratios of H2/H1 on comets?
I do realize planets would tend to be stable in their detected ratios, because of their stable orbit. But comets are not so stable.
Is it possible, just possible, that over sufficient passes close to the sun (relatively speaking) the ratios could change?
And if it is possible, then the comparison for purposes of stating where water came or did not come from becomes less clear.
Related, just what is the tail composed of?
The ratio in comets and the oceans works out to about 1:6410, the ratio in Jupiter and other large extraterrestrial sources is about 1:6420; since the deuterium or heavy water is more prevalent in comets and in Earth’s oceans, stripping the neutrons would give the opposite result, similarly to differential heating.
There are two types of comet tails: dust and gas ion. A dust tail contains small, solid particles that are about the same size found in cigarette smoke. This tail forms because sunlight pushes on these small particles, gently shoving them away from the comet’s nucleus. Because the pressure from sunlight is relatively weak, the dust particles end up forming a diffuse, curved tail. A gas ion tail forms when ultraviolet sunlight rips one or more electrons from gas atoms in the coma, making them into ions (a process called ionization). The solar wind then carries these ions straight outward away from the Sun. The resulting tail is straighter and narrower. Both types of tails may extend millions of kilometers into space. As a comet heads away from the Sun, its tail dissipates, its coma disappears, and the matter contained in its nucleus freezes into a rock-like material.
http://hubblesite.org/reference_desk/faq/answer.php.id=19&cat=solarsystem