The two comments submitted previously as I type this explain why the article title on HN is as it is. I was puzzled by what the title MEANT, so I looked into the article, and what the article says farther down is "Previous measurements of the deuterium/hydrogen (D/H) ratio in other comets have shown a wide range of values. Of the 11 comets for which measurements have been made, it is only the Jupiter-family Comet 103P/Hartley 2 that was found to match the composition of Earth’s water, in observations made by ESA’s Herschel mission in 2011." When I first read the headline, I wasn't sure if the claim was that the water vapor had mixtures of other chemical molecules in it, or what.
Okay, a different isotope ratio in water from a comet as contrasted with water generally found on earth would indeed be a clue to how water might have traveled from one orbiting body to another early in the development of the solar system. This kind of isotype checking (for isotopes of other elements) is one of the things done to confirm that rocks found on earth are presumptively from other parts of the solar system.
Okay, a different isotope ratio in water from a comet as contrasted with water generally found on earth would indeed be a clue to how water might have traveled from one orbiting body to another early in the development of the solar system. This kind of isotype checking (for isotopes of other elements) is one of the things done to confirm that rocks found on earth are presumptively from other parts of the solar system.