I watched a program over the weekend speculating on the source of all the water we now have on earth and I went awawy more confused than before. Long held theory was the source was comets, but after inspecting water from only one comet (Haley) they have ruled out that source. Something about the ratio of heavy hydrogen atoms in the water molecules not matching. Then they proposed the water was here all along, condensing on the rubble that eventually formed the earth, but again inspection of water contained in metorites suggested that the hydrogen ratio didn't match. So then they went on to propose the collision with the object that created the moon might be the source. My question is why can't it be a mix of all the theories? And why are they spending so much money to find out anyway?