Water’s origins on Earth have long been a subject of debate in the scientific community. One theory is that it hitched a ride on asteroids from deep in our solar system. Another idea is that water molecules were created in the cosmic dust that formed the planet, though some scientists believe the radiation from our young Sun would have burned too fiercely for any to remain. However it got here, geological evidence shows that water’s been present on Earth from the very beginning. 

This week, New Scientist rounded up the evidence for the origin of water on Earth. The biggest recent discovery came from volcanic basalt rocks, which have remained largely unchanged in the Earth's mantle since its formation. A few years ago, a team led by planetary scientist Lydia Hallis sampled basalt rocks that had been spewed from the Earth’s interior by volcanic eruptions on Canada’s Baffin Island. What they found in the rock was hydrogen that could be traced back 4.5 billion years to the birth of our planet. 

The hydrogen molecules also held clues as to where in the solar system they had originated from. By looking at the ratios of heavy hydrogen (deuterium) to normal hydrogen in water molecules, geologists can determine whether the water came from Earth or somewhere else in space. The rocks Hallis’ team studied contained relatively small amounts of heavy hydrogen: a ratio about 22 percent less than that of present-day seawater. This finding suggests that Earth’s water didn’t arrive via a bombardment of meteorites; the hydrogen isotope measured on meteorites is normally higher. 

The data point to water being formed by the same dust cloud that formed the Sun and solar system, despite earlier theories that the Sun’s strong radiation, combined with the lack of atmosphere, would have made this impossible. Theoretical simulations from Nora de Leeuw and colleagues at University College London illustrate how this might have been achieved. It’s feasible that the dust particles had such a strong grip on the water molecules that they were able to hold on even in face of the sweltering conditions. 

However, not all scientists are convinced. Horst Marschall, a geoscientist at Woods Hole Oceanographic Institution in Massachusetts, says the mixing in the discovered hydrogen inclusions still leaves room for doubt. If it is true that water formed with our planet, then that opens up the possibility for the same to be true for other planets in our solar system—and potentially throughout the galaxy as well. 

[h/t: New Scientist]