Finally, a comet has been found with a deuterium-to-hydrogen (D/H) ratio that is close to that found in Earth’s oceans.  That had not been true of many other comets.  Astrobiologists claim this ration in Comet Hartley 2 as “proof” that our water came special delivery from water-balloon comets.  But why do they believe that, what constitutes proof, and what new problems does the “proof” lead to?

Astrobiology Magazine, a government-sponsored NASA site, wrote this headline: “Proof that Comets Brought Oceans to Earth.”  The devil, naturally, hides in the details.  Even though scientists from Max Planck Institute, using the Herschel Space Telescope, detected a D/H ratio in Comet Hartley 2 that closely matches the ratio in the oceans, there are difficulties.  Any measurement involves the elimination of error from known sources of error.  What can trip scientists up is error from unknown sources.  Another stumbling block is inference: what does the measurement mean?

At the end of the article (actually a reprint of the press release from Max Planck Institute), just after celebrations that “a key peice [sic] in the puzzle of how Earth became habitable for life,” were underway that some of the devils emerged in the details:

However, the new results also raise new questions. Until now, scientists assumed that the distance of a body’s origin from the Sun correlated to the deuterium-to-hydrogen ratio in its water. The farther away this origin lies from the Sun, the larger this ratio should be. With a “birth place” within the Kuiper belt and thus well beyond the orbit of Neptune, Hartley 2, however, seems to violate this rule. “Either the comet originated in greater proximity to the Sun than we thought”, says Hartogh, “or the current assumptions on the distribution of deuterium have to be reconsidered.” And maybe Hartley 2 is a so-called Trojan that originated close to Jupiter and could never overcome its gravitational pull.

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