In the Enuma Elish, a Babylonian epic that recounts the creation of the world, the heavens and the Earth emerge from a primordial abyss of brackish water. According to the biblical book of Genesis, water existed before land, life, and even light itself. Our ancient ancestors realized, just as we do today, that water is fundamental to life. But even though they could conceive creation stories for the Earth, moon, and stars, many cultures at the dawn of recorded history seemed baffled by the origin of our planet’s water.
Thousands of years later, the genesis of Earth’s oceans remains one of the key missing pieces in our modern creation stories. We know that water's precursors, hydrogen and oxygen, were abundant in the giant, cold molecular cloud that collapsed to form the sun and its planets just over 4.5 billion years ago. When the cloud collapsed, it became a twirling, incandescent disk of gas and dust with the glowing, nascent sun at its center. Small pieces of whirling debris collided and stuck together in the disk, gradually growing to form full-fledged worlds. Water ice was abundant in the cold outer disk, but near the sun, where all the rocky planets formed, the disk was far too hot to sustain more than a wisp of moisture. The Earth and its rocky siblings probably formed relatively dry. So where do Earth’s oceans come from?
Objects falling in from the water-rich outer solar system are the most likely way Earth got its water, and scientists have long argued back and forth trying to decide whether those deliveries came via asteroids, or rather comets. Today, scientists from the European Space Agency’s Rosetta mission to the comet 67P/Churyumov-Gerasimenko announced that a cometary origin for Earth’s water is an idea that’s all wet. Asteroids, it seems, are a far better bet. Their findings are published in the journal Science.
“Today asteroids have very limited water; that’s clear,” says study leader Kathrin Altwegg of the University of Bern. “But that was probably not always the case.” A wealth of geological evidence suggests that a barrage of water-rich impactors struck the Earth and other rocky planets nearly four billion years ago, a time when asteroids were probably wetter and far more numerous than they are today.
Using Rosetta’s ROSINA spectrometer, the team measured 67P’s ratio of ordinary hydrogen to deuterium, a heavier hydrogen isotope that contains an extra neutron. They then compared this “D/H” ratio to that found in Earth’s oceans, and discovered that the comet’s D/H ratio is three times higher than water on Earth. If comets were what filled Earth’s seas, then we should expect those D/H ratios to be the same.
For decades, asteroids had been the preferred explanation for Earth’s water. That’s partially because it’s relatively easy for them to get here from the asteroid belt, in the vicinity of Mars and Jupiter. Comets have a tougher trip, because they come either from a ring of debris called the Kuiper belt beyond the orbit of Pluto or from a spherical swarm called the Oort cloud, which stretches halfway to the nearest star.
This also helps explain why there are so many more samples of asteroids than comets to be studied on Earth – we call them meteorites. Out of every 10,000 water molecules on Earth, only three have heavy deuterium rather than normal hydrogen, and D/H measurements of water-rich meteorites are broadly consistent with this ratio. Scientists have also measured the D/H ratio for a dozen comets from the Oort Cloud, finding it to be twice as high as that for terrestrial water. The Oort Cloud, it seems, was not Earth’s oceanic reservoir.
However, the comet hypothesis for Earth’s water made a comeback in 2011, when the Herschel infrared space telescope measured the D/H ratio of water vapor venting from Hartley 2, a Kuiper Belt comet trapped in orbital resonance with Jupiter. Hartley 2’s D/H ratio mirrored the water in Earth’s seas, making some scientists suspect the comet was a frozen chunk of undelivered ocean.
Today’s Rosetta results instead suggest that Hartley 2 is instead just a strange quirk. The D/H ratio for 67P/Churyumov-Gerasimenko, which is also from the Kuiper belt, “is so high that you need only a small fraction of these comets to spoil the Hartley results if you mix them up to make terrestrial water,” Altwegg says. “You would have to assume that Churyumov-Gerasimenko is the exception to the Kuiper belt if you would believe that Kuiper belt comets are the origin of terrestrial water.”
There are still ways for the cometary hypothesis to once again surge ahead, though they face increasingly long odds. 67P/Churyumov-Gerasimenko, like all comets, emits multiple distinct jets of vapor. It is possible, though unlikely, that Rosetta’s measurements inadvertently sampled material from a particularly deuterium-rich jet. Even so, mission scientist Matt Taylor says that next summer the Rosetta team plans to fly the spacecraft directly through at least one individual jet to pin down potential differences in composition.
Researchers have only managed to measure the D/H ratios for a few Kuiper belt comets, so perhaps a wider survey could reveal Rosetta’s comet to be the outlier rather than Hartley 2. Similarly, new and upcoming efforts to retrieve fresh samples from asteroids, such as Japan's recently launched Hayabusa 2 mission and NASA’s OSIRIS-REx mission slated to launch in 2016, could provide surprising new data that once again turns the tables back in favor of comets.
Or, perhaps most probable of all, as more data are collected from asteroids and comets alike, the creation stories we now tell ourselves of one or the other source dominating water delivery will prove as naïve as the myths of our forebears. In the case of Earth’s oceans, it may not be either comets or asteroids that delivered them, but rather a significant mixture of both.