Comety Show: Oceans from Space
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) some of the devils emerged in the details – just as they were celebrating that “a key peice [sic] in the puzzle of how Earth became habitable for life” had been found.
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.
How did the other news sources report the problems? PhysOrg’s reprint of a press release from University of Michigan was restrained, stating that “Herschel has shown that at least one comet does have ocean-like water.” Another PhysOrg article suggested that Comet Hartley 2 has undergone serious remodeling of its surface, and gained material from different parts of the solar system, calling into question whether its D/H ratio is uniform or representative. The BBC News was fair enough, simply headlining the fact that the comet’s water is “like that of Earth’s oceans.” It’s report also drew attention to new problems raised, quoting one planetary modeler who said “It opens up a new can of worms for us.” Universe Today was more assertive, calling this the “Best Evidence Yet That Comets Delivered Water for Earth’s Oceans.” But did they ask the Belgians? (11/03/2009). It seems the most realistic statement from the meager evidence is what the BBC News said based on a co-author of the report: the data “opened up the possibility that comets at least contributed to our water supply.” That leaves room for anything from complete delivery by big comets, to tiny comets that may have contributed a molecule or two.
Why do planetary scientists need special delivery of Earth’s water? They reject the Biblical notion that the Earth was formed out of water and by water (Genesis 1:2, II Peter 3:5). In their creation myth, the Earth condensed out of dust baked by the sun. For millions of years, hot lava pouring over the landscape made liquid water impossible. Needing water to keep the myth going on all the way to origin of life, they looked beyond Earth for water balloons, and found them in asteroids and comets. But alas, the D/H ratios of comets (which carry much more water than asteroids) did not match. Now you understand their excitement at the possibility that at least one comet has a match to the D/H ratio in ocean water.
Whenever data seems to confirm the secular creation myth, you can be sure somebody is not asking all the questions that should be asked. Here are a few sources of possible “unknown error”.
Do we know that the D/H ratio in the oceans doesn’t vary from location to location?
Do we know that the D/H ratio doesn’t vary with ocean depth?
Do we know that the D/H ratio on comet Hartley 2 doesn’t vary with location?
Could there be a fractionation process in the comet that sets up a deuterium gradient, such that the value measured remotely by Herschel does not reflect the bulk D/H of the entire comet?
How representative of comets likely to have impacted Earth is Comet Hartley 2?
Why is Hartley 2 being selected for proof when so many others falsify the notion that comets delivered Earth’s water? Is there selection bias?
How carefully tuned does the special-delivery hypothesis have to be to keep incoming big comets from vaporizing the water after the oceans form?
How carefully tuned does it have to be to prevent destroying Earth’s atmosphere?
Perhaps you can think of other questions. One comet with one data match does not confirm a theory, especially when the theory must be propped up with auxiliary hypotheses to keep it from being falsified. An unbiased observer might judge the celebrations premature, if not perfunctory: a right daft comety show, indeed.