One of the more fascinating aspects of this subject—world views, paradigms—is that we apparently know much more about the cosmos than we know about our own backyard. Studies of the big bang knowledgably discuss events said to have transpired one or two millionth of a second after the big bang began, but our very best theories of the solar system’s origins are very speculative at best. [From this blog here.]
I confess that until the Rosetta space craft landed on Comet 67P/Churyumov-Gerasimenko I did not know that the earth had obtained all of its vast masses of water from a bombardment by comets beginning 4.1 billion years ago. That bombardment evidently ended 3.8 billion years ago and is known as “the late bombardment.”
The theory here is that the earth was way too hot back 4.6 billion years ago to hold water—but now we have a lot of it. How did it get here? Well, theory has it that every solar system has a hot and a cold zone; the line that marks it, measured from the sun, is known as the Frost Line. It is 5 astronomical units out, thus about 465 million miles away, and roughly just this side of Jupiter’s orbit. Out there grains of water formed in the infinite past exist as granules that the nudge of gravity can aggregate into comets. Comets are mostly ice and dust. Asteroids, by contrast, are mostly rock. Back 4 billion years ago or so, vastly more comets swept the skies; something disturbed their path; the Late Bombardment began; and now we have surf on our coasts.
The four most abundant elements in nature (or at least in the solar system) are Hydrogen, Helium, Oxygen, and Carbon; of these Helium is not reactive. You would think that the distribution of water—formed of the two most abundant reactive elements—would be more or less uniform and that the earth would always have had a lot of it. So why do we assume that the earth had to acquire water rather than having it all along? The theory is that with great heat a water vapor can more easily escape the atmosphere (atomic weight of 18) compared with carbon dioxide (atomic weight of 44).
Sorting out how the loss of water could have happened depends on four factors: the planet’s temperature, its mass, its magnetosphere, and solar wind. Great heat will vaporize all water; if the planet is not protected by its magnetosphere (which is itself generated by its interior mass and outer tectonic plates) the lighter water vapor will be swept away by the solar wind. Venus and Mars generate minimal magnetospheres and therefore have little water. We owe ours now—or so we thought before Rosetta made its measurements—to comets and the protection of that splendid aurora borealis.
But the news is, well, puzzling. Rosetta has discovered that the water on 67P/C-G is predominantly heavy water, deuterium oxide, itself due to the presence of heavy Hydrogen, an isotope. It acts pretty much like water—but life as we know it has adapted to ordinary rather than heavy water. If all we had was D2O, our cells would refuse to divide when it came time. Well, we’ll be back for another launch to see if 67P/C-G is an anomaly or common. If comets are all or mostly D2Os, we’ll have to revise our theories of everything—not least our theories of how the solar systems little objects formed. Which illustrates how little we really know.
To round this out, I would propose that we name 67P/C-G “The Little Horse” (“Parvulus Equus?”). That’s what it looks like. And finally, those names. The comet was discovered by two Ukrainian astronomers in 1969. Klim Churyumov saw it while examining a set of photographs taken by Svetlana Gerasimenko. That was back in the good old days of the Soviet Union—before the tiny bombardment, of parts of the Ukraine, began in 2014.