The oceans are a mystery in more ways than one, but you might not expect the answers to come from a pack of electronics and a comet. But that's what the European Space Agency (ESA) says about the unmanned Rosetta probe orbiting comet 67P/Churyumov–Gerasimenko. Though 67P is making its first visit to the inner Solar System and won't come closer to the Earth than hundreds of millions of miles, it is throwing new light on one of the fundamental questions in Earth's history: Where did the oceans come from?
According to current theories, the Earth was formed as the disk of dust and gas that surrounded the primordial Sun coalesced into planets. The water in the oceans could have come from that, but the formation of the newborn Earth 4.6 billion years ago left it an incandescent molten mass that would have boiled away any water into space before the crust even had a chance to form. And yet, today three quarters of the Earth is covered by water. So, what was the source?
One idea is that the water came from comets. It's a logical source. Comets are "dirty snowballs" made up largely of ice, so a steady rain of them during the early history of the Earth when the planet was being bombarded by objects from space could have been the origin of the oceans. However, proof needs more than plausibility; it needs evidence.
To see if comets could have been the source of Earth's water, scientist's turned to isotopic analysis. Water is made up of an oxygen atom and two hydrogen atoms, and the hydrogen atoms are made of a proton and an electron. However, if a hydrogen atom also has a neutron in its nucleus, it becomes a hydrogen isotope called deuterium.
What's clever about this is that the ratio between normal hydrogen atoms and deuterium isn't fixed. It depends on the origin of the atoms, and by matching the ratios of one source against another, scientists can deduce where the water in any particular part of the Solar System probably came from because the ratio changes the farther away from the Sun you get. This means that water that comes from the Asteroid Belt between Mars and Jupiter will be different from that coming from the Kuiper Belt beyond Pluto, which will in turn differ from that of the Oort cloud on the fringes of the system.
This is where Rosetta comes in. Among the suite of instruments carried by the orbiter is the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA), which is a pair of mass spectrometers and a pressure sensor used to determine the chemical composition of comet 67P's coma, the temperature and speed of the gas molecules, and last, but not least, the ratio of various isotopes – including hydrogen.
According to astronomers, comet 67P is a fossil of the earliest days of the Solar System because it came from the Oort cloud, which has remained unchanged for billions of years. These icy fragments float in space until a passing star or other massive object disturbs them and sends them hurtling toward the Sun. As they do so, some are captured by the gravitational pull of Jupiter and swung into new orbits. In the case of 67P, it now swings around the Sun in an elliptical orbit with a period of 6.5 years, coming no closer to the Sun than a point between Earth and Mars before heading back out beyond Jupiter.
What all this means is that 67P is made up of very ancient water and if comets are the source of Earth's oceans, then the isotope ratios should match to a reasonable degree. However, they don't. ESA says the 67P confirms measurements taken of 11 other comets that have been studied – all of which, except one, showed very different deuterium/hydrogen ratios from that of Earth. And the odd-comet out, called comet Hartley 2, is a Jupiter Family comet, which has a different origin from those in the Oort cloud.
In all, ESA says that the deuterium/hydrogen ratio of the Earth is 1.56 ×10–4 while 67P's ratio is 5.3 x 10–4. This makes 67P's ratio three times higher than that of Earth's oceans and is higher than that of any other Oort cloud comet measured so far.
On the other hand, meteorites that originated in the Asteroid Belt do show a close match with Earth, so ESA scientists believe that even though there isn't as much water in asteroids as in comets, the former is a more likely source of Earth's seas due to a massive bombardment over millions of years. It also indicates that Jupiter Family comets may have some surprises as well.
"This surprising finding could indicate a diverse origin for the Jupiter Family comets – perhaps they formed over a wider range of distances in the young Solar System than we previously thought," says Kathrin Altwegg, principal investigator for ROSINA. "Our finding also rules out the idea that Jupiter Family comets contain solely Earth ocean-like water, and adds weight to models that place more emphasis on asteroids as the main delivery mechanism for Earth’s oceans."
The Rosetta team's results were published in Science.