The Evolving History of the Earth’s Oceans

I’ve been reading Sky and Telescope magazine for 45 years. In the “News Notes” section of the April issue, senior editor J. Kelly Beatty wrote a short piece titled, “Give-and-Take Origin for Earth’s Water?” In the article, he discussed several recently published studies and how they have affected the theories that planetary scientists have for the origin of the earth’s oceans. For many years the dominant view among planetary scientists was that the earth’s oceans were primordial. That is, they thought that the oceans’ water came from material that formed the earth 4.6 billion years ago and that the water condensed into oceans right away. This view dramatically changed three decades ago when most scientists embraced the theory that the moon resulted from a grazing impact by a Mars-sized body early in the earth’s history. That impact would have vaporized and removed what primordial oceans might have been present, leaving behind a parched earth.

Because comets have high water content, planetary scientists next turned to them to provide water for the earth’s oceans. Comets are small, so it would have required the collision of a few thousand large comets to provide enough water to equal the content of the oceans. Most astronomers think that comets were much more plentiful in the early solar system. Furthermore, they also think that the solar system underwent a period of very intense impacts, the late heavy bombardment, between 3.8 and 4.1 billion years ago, so one might expect that comet impacts were common then.

However, this idea has encountered a difficulty. Water contains hydrogen, and hydrogen has two stable isotopes. Most hydrogen atoms have a single proton in their nuclei, but a small percentage of hydrogen atoms have a proton and a neutron in their nuclei. We call this heavier isotope of hydrogen deuterium (normal hydrogen sometimes is called protium). In the earth’s oceans, a little more than 0.01% of hydrogen atoms are deuterium. In recent years, astronomers have measured the deuterium abundance in about ten comets. The first six comets so measured had deuterium about twice that of earth. Most astronomers were unconcerned about that, because they thought that these six comets came from the Oort cloud, while they thought it more likely that comets from the Kuiper belt delivered the earth’s water. This thinking supposedly was confirmed by the measurement of deuterium in Comet 103P/Hartley four years ago, which found a near-match for terrestrial deuterium abundance.

However, measurements late last year showed that Comet 67P/Churyumov-Gerasimenko had deuterium abundance 3 ½ times greater than that of earth. This comet too is supposed to be from the Kuiper belt, yet it has the highest deuterium abundance yet. It is a mystery why the deuterium abundance of comets varies so much, but it has become clear that the average deuterium abundance of comets, whether they come from the hypothetical Oort cloud or the Kuiper belt, is significantly higher than the earth’s. This effectively eliminates comets as the source of the earth’s oceans.

Mr. Beatty briefly discussed a proposal that primordial water on the earth may have been transported deep into the earth whereupon it was later reintroduced to the surface through volcanic activity. But the removal of surface water in this manner would have required that plate tectonics be operating from the very beginning; however, most planetary scientists think that plate tectonics came along a billion years after the earth formed. Since this is much too late to have saved primordial water before the hypothetical large impact that formed the moon, that suggestion probably is not correct. Beatty finally concluded that earth’s water likely came from asteroids. This discussion in Sky and Telescope reminded me of my blog a couple of months ago where I discussed much of this. I also have an upcoming article in the Answers Research Journal on the blurring distinction between comets and asteroids.

Biblical creationists know that the earth’s oceans are primordial. Water played an important role in creation. Genesis 1:2 mentions water twice. The first instance is when it says that “darkness was upon the face of the deep.” The Hebrew word translated “deep” refers to ocean depths, or deep water. The second mention is that “the Spirit of God moved upon the face of the waters.” The Day Two account (Genesis 1:6–8), when God divided the waters above and below, explicitly mentions water five times. On Day Three, God gathered the waters below so that dry land appeared for the first time. God called the gathered waters “seas.” Apparently, water covered the earth to considerable depth until Day Three. If this were not enough, 2 Peter 3:5 explicitly mentions the importance of water in the early creation. Hence, biblical creationists do not require asteroids, comets, or any other objects to deliver the ocean’s water.