How Earth got its water is an ongoing question in solar system evolution. A new study suggests comets are more likely to be the answer than previously thought. But could the answer simply be too hard for us to figure out; should we just not worry, and can we simply say that a loving God did it?
When the solar system formed, there was a basic temperature gradient — it was hot in the center, where the sun was forming, and it got cooler as you went farther away from the nascent star. The location in the solar nebula that was about 100°C (212°F), is called the “Frost Line” where the water molecule would no longer be a volatile gas, but it would be a liquid and could be accreted to a forming object in an appreciable quantity. The frost line is about where the asteroid belt is.
Hence the problem: If liquid water could not form where Earth was, then how did Earth become the relatively water-rich world it is today?
For the last few decades, the favored solution has been delivery by comets. Comets are mostly water-ice, we know they impact objects, and we know that the impact rate was much higher in the very early solar system than it is today (in fact, I’m attending a workshop on the early solar system bombardment history in February where the focus will be on this).
A problem with this has to do with what’s called the deuterium/hydrogen ratio. Basically, water comes in two forms, “normal” water which is the familiar H2O (two hydrogen atoms and an oxygen atom), and HDO (one hydrogen, one deuterium, and one oxygen). The latter is known as “heavy water” and you may have heard about it in relation to nuclear fusion.
Deuterium is a heavy form of hydrogen. A normal hydrogen atom has one proton in the nucleus. Deuterium has one proton plus a neutron, making its mass about twice that of a normal hydrogen atom … hence “heavy water” when it’s incorporated into the water molecule. It’s still considered hydrogen because the number of protons is what determines what atom it is. (And for those who like the extra credit information, tritium would be one proton and two neutrons.)
Getting back to the problem, the deuterium/hydrogen ratio (abbr. as D/H) is the normal ratio of heavy water molecules to normal water molecules found on an object. Earth’s oceans have a D/H of about 1.56 x 10-4, or basically a bit more than 1 out of every 10,000 water molecules is heavy water. Comets, though, have been measured to be about (2.96±0.25)x10-4, or around 70% too high. Asteroids are too low at (1.4±0.1)x10-4.
So where did the water come from?
New Proof that Comets Watered the Earth
So proclaimed the title of an October 11, 2011 Time article. That’s right, “Proof.”
My problem with this statement is that we never have absolute “proof” in science. We have evidence that adds to the “conclusivity” (yes, I just made up a word) of a hypothesis. Proofs are in mathematics. Proofs never apply to real life. If you’re interested in this subject, I’ve written probably two relevant posts on it (post 1, post 2).
The article in question (Hartogh et al. 2011) is about a recent Nature Letter (a very short paper) that measured the D/H value in a comet named 103P/Hartley 2. The D/H measured in that comet came out to be (1.61±0.24)x10-4 … which overlaps with Earth. This particular comet was from a different part of the solar system than previous comets with a D/H measurement, which is part of why this is a new result and why it was hyped up a bit.
The effect of this work is to revitalize the comets delivering water hypothesis, clearing up one of the biggest problems with it: We now do have a potential source for water that matches a significant constraint.
If you’re interested in reading more about it, other than the title, I do suggest the Time article.
But I Thought Goddidit
This brings us to the Answers in Genesis’ “News to Note” from October 15, 2011, specifically the second item. They don’t necessarily dispute the basic science of the article, rather the “view:”
“Nevertheless, in an effort to avoid a biblical explanation for the origin of all things—in other words, God as Creator—many cling to this explanation despite its aberrant physics. While the isotope ratios in the comets and asteroids are of scientific interest, they tell us nothing about the origin of the solar system. …
“The Bible explains the origin of the water on Earth and the origin of the entire universe. And the time of this Creation, about six thousand years ago, does not exceed maximum comet lifespans or demand a hypothetical birthplace to replenish them. … [God] made the Earth with its generous supply of water, not as a hot molten world that would boil away its water. After providing the Earth with an atmosphere, dry land, and plant life, He created the solar system and the other stars. He specifies that He made the sun, moon, and stars on the fourth day of Creation week. There is no way to blend the Genesis account of Creation with secular ideas of cosmology such as the big bang and the nebular hypothesis without calling God a liar.”
I really don’t think at this point that I need to go into detail about this and my position on it. It really is interesting to see, though, how these people are so willing to stick their heads in the sand and would be perfectly content in the Dark Ages of Europe a thousand years ago.
This was an interesting piece of science news, one that I knew some creationist somewhere was going to have an issue with, and one that I hoped the news media would not spin too broadly. I was right on the first, wrong on the second. With the latter point, these things are subtle, but using words like “proof” or “prove,” “hypothesis” versus “theory,” and “believe” versus “think” are words that shape significantly the public perception of science, how it works, and how “definitive” it is.
After over half a decade of fighting, the science-/evidence-based medicine crowd has succeeded in making that the term people use for what had been generally referred to “western” medicine. It’s a long battle, but maybe some day we’ll be able to get people to use some of these basic science terms correctly. At least when referring to science.