Astronomers have been arguing for some years about comets brought Earth its water. Then in 2011, an international team of astronomers using the Herschel Space Observatory to study Comet Hartley 2 (103P/Hartley) published their results on the first comet confirmed to contain ocean-like water.
For decades, the accepted wisdom has been that comets brought a large proportion of water to the primordial Earth. In spite of the seemingly logical connection between comets and oceans, there has been one serious problem with that theory: the composition of water thus far detected in comets has differed fundamentally from that of the Earth’s oceans, so they couldn’t possible be a primary source. This problem was serious enough to threaten the comet source model altogether. Or at least it was until now.
The Rosetta spacecraft has detected some strange water vapor streaming from Comet 67P — water that's significantly different to what we have here on Earth. The startling discovery challenges the popular assumption that much of our water was delivered here by comets.
By 'weird' water we're talking about major differences in the deuterium-to-hydrogen ratio (D/H). Comet 67P has water that's not only different from what we have here on Earth, it's also markedly different from water observed in other Solar System bodies.
For comets to be a source of the Earth’s ocean water, they too must contain just the right ratio of heavy to regular water. But until Comet Hartley 2, no comet had been found to meet this vital criterion.
In fact, the specific chemistry of comets was unknown until the 1980s, when the first direct measurements of comet ice were made on Halley’s Comet and — years later — Comet Hyakutake. Unfortunately, these two comets contained twice as much heavy water than is found in water on Earth. That meant they, and comets like them, couldn’t possibly be a source of ocean water. The comet model was sinking, fast.