On the recent news announcement that finds a mismatch in the heavy-to-regular water ratio on 67P found by the Rosetta mission and Earth's oceans, I wonder if the passage of time has been considered? Could the several billion years that comets have been exposed to radiation have increased their heavy water ratios by means of cosmic or solar neutron radiation absorption that Earth and its oceans have been protected from thanks to our atmosphere and magnetic field? I realize the chances of a free neutron striking just right at an atom to become absorbed are very minute, but surely over a long enough timespan (such as the billions of years the earth is theorized to have had its water content), this could have produced a measurably significant difference in heavy water ratios even if earth's oceans were seeded by comets? And/or could it possibly be a factor that with Earth's much more significant gravitational field (compared with comets), our water is much more stratified, such that heavy water stays lower in our oceans, and even sinking beneath our oceans as our ocean floors continually sink away from and slide under the oceans in subduction zones due to plate tectonics? Wouldn't it logically follow the water 'lost' from the oceans in this fashion would have a higher concentration of heavy water than the surface oceans, and thus over time the ratio of heavy water in earth's oceans would gradually decrease? Has either notion been explored and considered? I realize it is very early on in analyzing the Rosetta findings, and I know true science requires years of careful analysis, verifying results and peer review, but wonder if my science fanboyish ponderings have even slight grains of possibility to them, or if the theory of any common ancestry between comets like 67P and earth's oceans are as dead as the general media seem to make them out to be.