November 16, 2015 | David F. Coppedge

New Earth Ocean Theory Is All Wet

Time to rewrite the textbooks again. Earth started out wet, scientists now claim, overturning decades of dogma.

“Earth may have kept its own water rather than getting it from asteroids,” reads a story title in Science Magazine , a summary of a paper in Science. The authors concluded, from divination of lavas on Baffin Island collected in 1985 (Astrobiology Magazine), that the mantle must have gained its water directly from the protoplanetary nebula.

Astronomers had been telling the public for many years that Earth started out dry and got its water from comets. When the deuterium ratio of comet ice turned out to be too high, they had a problem.

Now, however, instead of apologizing for their mistake, they are bragging that the discovery is “exciting” and suggests that habitable planets may be much more common throughout the universe. For Science Magazine, Julia Rosen writes,

In the prevailing model of an initially dry Earth, hydrating the planet seemed like “more of a one-off event,” Hallis says. However, if the planet managed to keep water from the solar nebula before it evaporated away, there’s no reason other planets couldn’t do the same thing. Hallis says that her results could mean that water-rich planets like Earth are not so rare after all.

Live Science joins this chorus, claiming “the study could also have far-reaching implications for deciphering where water came from — and how it was lost over time — on other planets in the solar system, and even on planets orbiting distant stars.”

The new theory messes up old beliefs, Rosen indicates, that bits of meteoric rock called carbonaceous chondrites had brought water to the Earth. It also violates tradition that the dust disk was too hot at Earth’s radius to hold any water. To get around that objection, they had to get sneaky:

Traditionally, the main objection to this idea has been that the inner portion of the protosolar nebula, where Earth formed, would have been too hot for water to hang around. But Hallis’s team suggests that water floating around in the nebula snuck into our nascent planet by adsorbing to dust particles. They cite previous modeling work suggesting that this mechanism could allow a significant amount of water to survive the brutal temperatures and violent processes by which dust particles coalesced to form planets.

The paper admits that temperatures in the habitable zone would have been 440 to 1340 degrees Kelvin. Those water molecules would have needed a tight grip to hang on to the dust. Maybe they “snuck” onto the back side of the grains to avoid the solar wind.

The researchers merely assumed that these dust grains would clump into planetesimals and then planets. They spoke of Earth’s accretion six times in the short paper without explaining how tiny dust grains accrete, which they usually don’t. And there’s another problem keeping the traditionalists clinging to their traditions:

However, some scientists aren’t ready to abandon the asteroid hypothesis just yet. That’s because, on top of bringing water, they are also believed to have delivered much of Earth’s so-called volatile elements, namely, carbon, nitrogen, and noble gases, says Conel Alexander, a cosmochemist at the Carnegie Institution for Science in Washington, D.C. To explain the abundance of these elements, there would have had to have been enough impacts to also deliver Earth’s water, he says. “That still seems to me the simplest and most attractive explanation.”

Ciesla says that the new results will force scientists to re-evaluate the process of Earth’s formation. Perhaps the team’s adsorption model is correct, or perhaps water came to Earth aboard a kind of asteroid that hasn’t yet been found, or that no longer exists because it all went into making the Earth. “What we have to do is try to understand what fits and what doesn’t,” he says.

The perhapsimaybecouldness index of all these articles is non-trivial. Of special note is the appeal to what “would have had to have been,” that uses the verb have three times in six words. It also seems a bit unscientific to appeal to entities that have never been found or no longer exist.

They’re at least getting warmer. The Earth started out with all of its water, according to a textbook that never needs revising.


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