I don't understand -- it says 1.6 out of 5000 are deuterium. Why are we looking for meteorites and comets with precisely that ratio? Why couldn't it have been t

I don't understand -- it says 1.6 out of 5000 are deuterium. Why are we looking for meteorites and comets with precisely that ratio? Why couldn't it have been that the distribution of all comets to impact the earth yielded an average deuterium proportion of 1.6/5000?