The Earth is wet. Not only does it have substantial surface oceans, the outer part of the planet - the lithosphere - also contains a healthy amount of water and other volatile compounds. Exactly where all this water came from during the planet's assembly some 4 billion years ago has long been debated. Although objects like comets - prone to smack into the Earth every so often - can carry a lot of water, it's not quite the right flavor to match the stuff sloshing around in your bathtub. Comets carry more deuterium (heavy hydrogen) in their water molecules than the terrestrial version. One solution is that another family of celestial objects; asteroids or meteorites, are the culprits. Formed at warmer, less distant places in the young solar system, some of these rocky bodies carry just about the right deuterium mix.There's another issue though. Did the water get 'painted' into the surface of the young Earth in a veneer, or was it implanted earlier on, as a more integral part of the planet's composition? A new paper in Science this past week seems to shed some light on the question. By studying isotopic ratios of silver, as well as some other elements, the authors were able to demonstrate similarities between the composition of rocks from the Earth's mantle and some types of so-called primitive meteorites. But they hit a snag - one set of measurements suggests that the Earth's inner core formed very quickly, in a mere 10 million years, the other suggests a much more leisurely process, taking up to 100 million years. The neat solution that they offer up is that as the Earth assembled from lumps of material agglomerating due to gravity, the composition of the lumps changed over time. The starter mix was much drier than the later stuff.
They also point out that the addition of water-rich material could have pretty much all happened when the Earth was hit by a Mars-sized proto-planet, long hypothesized as responsible for the formation of the Moon, 4.53 billion years ago. Here's the bit that I find most interesting. Based on their model this proto-planet would have actually had a very similar composition to that of the young Mars. Picture our baby solar system, with another substantial, terrestrial-like, planet forming in a similar orbit to the Earth. Although gravitational dynamics probably sealed its fate early on, I cannot help but wonder if a few small nudges could have set things off along a very different track, to a dry Earth and an additional, significant, wet world...
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I don't get this. If water can form on comets, asteroids, and other proto-planets, why not on Earth as it formed in the same way? Painted on? Implanted? Why does this qustion even come up?
It's partly a question of energetics. A big planet like the Earth is formed from the agglomeration of a *lot* of stuff (it would take about 1 trillion kilometer sized objects to make the Earth). These pieces collide/fragment/stick over ten million years or so. The forming planet melts through due to both radioactive decay (more prevalent 4 billion years ago) and the energy from all this colliding material. Hence retaining volatiles (water) can be difficult. Also, water only freezes out into solid form quite a distance away from the proto-Sun during planet formation. So while comets and some asteroids have a large solid water component, a planet like the Earth, forming much closer to the proto-Sun need *not* have much water - unless material from further out in the system is involved in its formation...and hence the discussion of when and from where the Earth got its water.
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