Abstract: The provenance of water and organic compounds on the Earth and other terrestrial planets has been discussed for a long time without reaching a consensus. One of the best means to distinguish between different scenarios is by determining the D/H ratios in the reservoirs for comets and the Earth’s oceans. Here we report the direct in situ measurement of the D/H ratio in the Jupiter family comet 67P/Churyumov-Gerasimenko by the ROSINA mass spectrometer aboard ESA’s Rosetta spacecraft, which is found to be (5.3 ± 0.7) × 10−4, that is, ~3 times the terrestrial value. Previous cometary measurements and our new finding suggest a wide range of D/H ratios in the water within Jupiter family objects and preclude the idea that this reservoir is solely composed of Earth ocean-like water.
The pop views seem to be that the water supply came via comets and asteroids, or the Sun. The measurements done on the comets seem to challenge these ideas.
Personally, I hold to assumption that Earth and perhaps most planets and moons are old dark stars, older than the currently accepted age of the Universe. They evolve from an active fusing phase of development. Over time the star sheds atoms, stops fusing and changes. This is not a popular assumption to hold to and I've already been ridiculed by a friend for holding to this. However a few thinkers have come to this idea and although I don't necessarily agree with all of their ideas, I have a listed references and quotes here in the past.
The Earth's water supply could have been homegrown, that is chemically synthesized at a certain phase of Earth's stellar evolution. Or perhaps the water supply could have been supplemented to the Earth prior to the succession of locations it assumed close to the Sun; prior to when it got locked into a gravitational relationship with the Sun (in an inverse square regime). In other words the Earth could have marched around the galaxy as a rogue star/planet, and perhaps taken in water from interstellar clouds. Or perhaps Earth could have crossed close to other stars such as TW Hydrae (a young star erupting H2O) prior to coming close to the Sun. Prior to coming close to the Sun, I think the Earth was froze over; a hodgepodge of chemicals such as water, methane, ammonia, CO2 (dry ice), carbon monoxide and other volatiles. Underneath the ice was probably a liquid water supply with more chemicals and rock. Earth would have appeared as an astonishing desert. An ice world, like Hoth from Star Wars. And the moon? Well Earth and Moon could be a binary star system. The Moon looks older than the Earth to me. Heck some Moon rocks were radiometrically dated around 12 billion years. They took a mean date. Jupiter and her moons were once a star system that crossed paths with the Sun and so on.
Now this view is not the popular or accepted view. People accustomed to nebular hypothesis and Big-Bang Universe would heavily criticize these ideas. But you know I would just shrug my shoulders. If you invoke Big-Bang I would laugh because its assumptions are ridiculous. If you invoke radiometric dating, I would say time is subjective, observer dependent. Atoms do not remember when to decay in accord with your predefined contexts. Besides how would one date lava? And decay seems random. Who is to say that decay rates would not vary pending an astronomical object's location or an atom's location? We all agree the Sun has some pretty profound effects on the Earth and her atoms. And make manifest to me the decay process. Is it billiard balls moving in and out of the atom? Are you going to invoke the mystical binding energy, again? Radiometric dating is sophisticated guesswork. If you invoke supernova theory, I will say if every star goes supernova why are there not more observed? And nebular hypothesis well I have a collection of criticisms of that bloated assumption filled with ad hoc.
And as far as current astrophysical evidence suggests we have just discovered thousands of so called exoplanets. Some of these planets are in star systems that defy nebular hypothesis. In addition there are rogue planets and moons not in inverse square gravitational relationships with younger stars. So I hold to the idea that a star is a young planet, and a planet is an old star. I think this is the future. And it is fascinating because there could be so many variables. But think of it. We could be living on something that was once a star.