Are Comets the Origin of Earth's Oceans?

A new study complicates the long-held idea that comets created Earth’s oceans.


Water, water everywhere, but where did it come from? One common explanation is that the water in Earth’s oceans was brought by comets, which bombarded the planet during its earliest period. It’s a simple, logical, and testable story.

But that doesn’t mean it’s right. A new study published last week in Science revealed that the water on Comet 67P/Churyumov-Gerasimenko doesn’t match that found on Earth. Specifically, instruments aboard the Rosetta probe measured the relative amount of deuterium in the comet’s water and found it was roughly three times higher than the amount in Earth’s oceans. Comets are chemically pristine, mostly unchanged over the Solar System’s 4.5 billion year history, so a mismatch in the deuterium content complicates the story of Earth’s water.

However, it’s wrong to say the new Rosetta results rule out comets as the origin of the oceans. That’s partly because we’ve only gotten this kind of measurement for a small number of comets, and only one of those—Comet 103P/Hartley 2—had the same relative deuterium content as Earth. Both 103P and 67P are Jupiter-family comets, which are widely thought to have formed in the outer Solar System and plunged into smaller orbits thanks to Jupiter’s gravity. The difference in their chemistry makes the story even messier, so it’s likely we’ll be talking about the origin of Earth’s oceans for many years to come.

That’s an important question: while Earth has a lot of water on its surface today, that hasn’t always been the case. After it formed, our planet had a surface of molten rock when any liquid water would boil away; that period of time is known as the Hadean Eon for its hellish character. While Earth’s deep interior has a lot of water in it, that’s mostly trapped in the crystal structure of minerals rather than in giant underground reservoirs, so it’s unlikely the oceans were entirely fed by water bubbling up from below.

That leaves another possibility: maybe Earth’s water came from space. After the Hadean Eon, Earth was bombarded by meteoroids (small asteroids) and comets, which might have brought enough water with them to make the oceans. That’s why comets especially look promising: they contain a lot of water ice, while it would take a lot more meteoroids to do the same trick.

The key to testing the hypothesis is in chemistry: if comets or meteoroids have the wrong abundance of atoms, they’re not the responsible party. One chemical test involves measuring the relative amount of deuterium in water. Deuterium is an isotope of hydrogen containing a proton and neutron in its nucleus, while normal hydrogen has only a proton. While deuterium—denoted “D” to distinguish it from the “H” of hydrogen—is rare, a tiny number of water molecules are made of HDO instead of the usual H2O. (“Heavy water”, or D2O, is even less common in nature, though nuclear engineers make and use it in some reactors.)

The Rosetta probe has recently been overshadowed in the news by the Philae lander it carried, but the orbiter is itself a sophisticated robotic laboratory. One of its instruments, the Rosetta Orbiter Sensor for Ion and Neutral Analysis (ROSINA), is a device known as a mass spectrometer, capable of identifying different types of atoms and molecules. Rosetta researchers used ROSINA data to determine how much HDO is mixed in with the normal H2O. From that, they extracted the ratio of the number of deuterium atoms to the number of hydrogen atoms.

They found that there are roughly 1,900 hydrogen atoms for each deuterium atom in the water on Comet 67P. However, even that ginormous difference still shows about three times the amount of deuterium than we see on our planet. If 67P is typical, then comets alone couldn’t be responsible for Earth’s water: some other source would have to dilute the deuterium-rich water to the levels we measure. For example, a common type of meteorite has similar mineral content to Earth, but a lot less deuterium.

And that’s why the story is complicated. According to theory, objects closer to the Sun got less deuterium than those that formed farther out, due to the way chemical elements got distributed in the chaotic early years of the Solar System. Most deuterium measurements we have come from comets originating in the Oort Cloud, an icy region far beyond the planets; these match the chemistry of Saturn’s moon Enceladus. (Halley’s Comet is likely an Oort Cloud comet.)

By contrast, 67P is a Jupiter-family comet, thought to have been born in the Kuiper Belt out past Neptune’s orbit. Comet 67P’s deuterium content is even higher than that of Oort Cloud comets, and definitely inconsistent with the measurement from fellow Jupiter-family Comet 103P. That could mean Jupiter-family comets might originate from a diverse range of birthplaces instead of all from one part of the Solar System … or something else weird might be going on.

To solve the problem, we need to study a lot more comets and meteorites. Rosetta-style missions aren’t going to be practical for all of them, but we have deuterium data from only a dozen comets so far. Every new comet we can study will bring us closer to understanding the story of the origin of Earth’s oceans.