November 3, 2009 | David F. Coppedge

Comets Didn’t Bring Earth’s Oceans

So much for the “water balloon” theory of how Earth got its oceans.  A new study by Belgian planetary scientists pretty much rules out comets as the source of our planet’s abundant water.  Their results are published in this month’s Icarus.1
    “The origin of water on Earth is still puzzling,” they began.  Our rocky planet, so close to the sun, should not have formed from ices.  Carbonaceous chondrites contain water sufficient to create a “veneer” of water after the Earth cooled, but ratios of osmium isotopes don’t match.  Comets have long been a popular source.  They’re cold, and made mostly of ice.  Could they have brought water by special delivery to a cooling earth?  Some TV documentaries have brought this story to life with colorful animations.  Early indications from deuterium-to-hydrogen ratios, though, cast doubt on the idea; comets have much more deuterium than ocean water.  Perhaps other classes of comets or asteroids have different ratios.  “Additional constraints are therefore mandatory,” the team said.
    Recently, the team found that nitrogen isotope ratios differed significantly between comets and Earth’s atmosphere.  Since any delivery of cometary nitrogen would have been accompanied by water, they used nitrogen isotope ratios to calculate how much water could have come from comets.  The result: very little, probably 6% at most.  Also, since the nitrogen isotope ratios is the same in both Oort Cloud and Kuiper Belt comets, the estimates apply to both classes of objects.  Bottom line: planetary scientists will have to find other sources.  The authors left that question unanswered.


1.  Hutsem�kers, Jean Manfroid, Emmanu�l Jehina, and Claude Arpigny, “New constraints on the delivery of cometary water and nitrogen to Earth from the 15N/14N isotopic ratio,” Icarus, Volume 204, Issue 1, November 2009, Pages 346-348.

The paper constrains, not disproves, naturalistic models, but it is clear that secular planetary scientists are running out of options (see 03/26/2002, 12/27/2007).  Cometary delivery was already a last resort for the bottom-up worldview.  The top-down worldview, that the Earth was created with its water and was designed with habitation in mind, has less arthritis and dandruff (i.e., less hand-wringing and head-scratching). 

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