How did the Earth get its oceans? The primordial Earth was a seething ball of magma, so the water that it began with would have evaporated into space. As a result, planetary scientists have long debated which of two types of objects, comets or asteroids, were more responsible for delivering Earth’s water.

A new study, published today in Science, says that asteroids were the source. The authors, led by Conel Alexander of the Carnegie Institution of Washington, in Washington DC, analysed the isotopic abundances of nitrogen and hydrogen in 86 primitive meteorites, and found that they coordinate with Earth’s.

Asteroids had already been the favored source. Studies of solar system dynamics suggest that there was a period of time around 3.9 billion years ago, called the Late Heavy Bombardment, during which the Earth would have been barraged, mostly by asteroids.

Even though comets are ideal sources, with their high percentage content of water, rich with amino acids, there are a few strikes against them. Six studies of comets from the Oort Cloud found that their isotopic ratios of heavy hydrogen (or deuterium) were much higher than Earth’s. When a 2011 Nature paper found isotopic levels of heavy hydrogen in the comet Hartley 2 to be similar to Earth’s, it revived interest in water-from-comets idea. But Alexander and his colleagues suggest that the overall levels of heavy hydrogen in Hartley 2 (and not just the levels in the comet’s ice) would be much higher.

Image Credit: NASA/Don Davis