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Earth's First Ocean

 

We’re finally to the point where we can start to talk about the very first waters that lapped the fiery shores of some ancient rocks. It may not have been the ocean of our dreams—it was probably hot and smelly and murky—but hey, it was the ocean, the only one in the Universe, as far as we know with certainty right now.

 

So let’s review where we’re at with our early Earth. We’ve separated it into internal layers and given it two types of crust, one of which is the perfect receptacle for liquid water. But when did liquid water exist on our planet and where exactly did it come from?

In Origin of the Earth and Moon (2000), Yutake Abe and others argue for a “wet” magmatic ocean prior to the formation of Earth’s crust. This hydrous mantle, derived from the accretion of water-bearing planetestimals during the early formation of Earth, gave rise to a thick atmosphere that blanketed Earth’s surface, trapping heat. The trapped heat melted (or maintained the molten state of) the Earth’s surface creating an “ocean” of magma in which significant quantities of water remained dissolved. Thus, a hydrous or wet magma ocean formed which, upon cooling of the atmosphere, released its water vapor and created a planet submerged in water, our ocean.

In an e-mail exchange with Michael Drake, a co-author of the Abe chapter and a prominent planetary scientist at Arizona State University, constraints on the time of formation of Earth’s first ocean may be found in the radioactive decay of 129I (Iodine) to 129Xe (Xenon). Using modern-day ratios of 129Xe in MORBs and the atmosphere, we can infer that an ocean was present within 5-7 half lives, or within 80-96 million years of the formation of the Earth.

As noted earlier, Mojzsis and others found circumstantial evidence for water in a 4.3 Ga zircon, indicating liquid water was at least present at this time. Thus, if we assume that at least some if not all parts of Earth’s surface were molten at the time of the lunar impact ~4.4 Ga, then the 100 million year timeline for the formation of Earth’s ocean appears reasonable.

While some progress has been made in constraining the timeline of formation of the ocean, questions concerning the source of Earth’s water have yielded fewer direct answers. Francois Robert in a 10 August 2001 Science article compares the modern-day deuterium (2H, also symbolized as D, the stable isotope of hydrogen) to hydrogen (H) ratios of water in carbonaceous meteorites, comets and Earth water. Such a comparison appears to indicate the Earth’s water came from carbonaceous meteorites. This analysis does appear to rule out comets as a source of water, an idea that was popular with a few scientists for a while. But similar D/H ratios do not provide a “smoking gun”: materials can arrive at similar isotopic ratios by different mechanisms.

Drake and Righter note two “schools of thought” for the origins of water on Earth in their 7 March 2002 paper in Nature. The first argues that Earth accreted dry: temperatures in the solar disk were too high for water to be present initially and Earth received its water from a “late veneer” of meteorites or comets that arrived subsequent to Earth’s formation under cooler conditions. The other school of thought argues that Earth accreted wet: Earth accreted from a mixture of hydrous and anhydrous planetesimals and, as such, derived its water internally through outgassing.

It should be clear to you now why arguments over how Earth accreted—through homogeneous or heterogeneous mechanisms—have a direct impact on arguments over the origin of Earth’s ocean. Homogeneous, dry accretion appears to favor the late-veneer proponents; heterogeneous wet accretion favors the “indigenous water” proponents.

Drake and Righter make good arguments against the late veneer, notably that the late-arriving carbonaceous meteorites could provide at best one volume of ocean under the most ideal circumstances (ignoring losses, assuming a high water content and allowing for special circumstances to explain observed ratios of elements and their isotopes in the modern-day mantle). They also argue against a significant role for comets in a dry or wet accretion. Their strongest arguments appear in favor of wet accretion from an as-yet-uncharacterized (or extinct) material within the solar nebula. As they put it, “Earth accreted at least in part from hydrous materials that are not present in our meteorite collections.”

Drake and Righter frame their arguments from a number of different perspectives and their paper is well worth a thorough reading. But despite their rather “untestable” conclusion (that Earth’s water comes from an unknown or unknowable source), all is not lost. New and more accurate techniques, more careful testing of assumptions and simply more research on the composition of solar materials will shed new light on their arguments. As with everything we’ve studied so far in this chapter, it’s a work in progress. That’s what makes science an exciting and ever-changing drama.

 

   
   
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