Ever since the European Space Agency’s Rosetta spacecraft caught up with comet 67P/Churyumov-Gerasimenko in August this year, scientists have been eagerly anticipating one particular piece of data—the analysis of its water content. That’s because the origin of Earth’s oceans remains a hotly debated topic, but one of the leading hypotheses is that most of it came from comets.
Now, the results are in, and it seems that cosmic snowballs are no longer the prime suspect. The water on Earth doesn’t match up with the water surrounding 67P, leading scientists to point the finger at another suspect: asteroids. However, some have argued that this conclusion is a little hasty, and that more data is needed before comets can be eliminated.
When our planet was born some 4.6 billion years ago, it probably possessed water, but it would have been so hot that anything that was present would have likely boiled off. So what was responsible for filling Earth’s surfaces with vast oceans that make up more than 70% of the total? Scientists aren’t sure, but many believe collisions with remnant debris from the formation of our solar system, either comets or asteroids, could have seeded Earth with water and organic compounds. That’s why one of Rosetta’s main missions was to sniff 67P’s water vapor and compare its “flavor” with that of Earth’s water.
More specifically, using Rosetta’s ROSINA instrument, scientists wanted to look at the ratio of hydrogen to deuterium—a form of hydrogen with an extra neutron. On Earth, the majority of water molecules are composed of two hydrogen atoms and one oxygen atom, but sometimes a deuterium will replace a hydrogen. Although these water molecules have the same physical properties as H2O, they are slightly heavier. The proportions of heavy and light water are significant because they don’t easily change and thus stay the same for long periods of time.
The idea is to gather data on these ratios from various different space objects, including asteroids and different types of comets. Broadly, there are two kinds of comets: those that came from the Kuiper Belt outside Neptune and Pluto, the so called Jupiter-family comets, and those from the more distant Oort cloud that originally formed in the Uranus-Neptune region, or far comets.
So far, the deuterium/hydrogen (D/H) ratio has been measured on 11 different comets, covering both types, but only one has matched up with Earth: the Jupiter-family Comet 103P/Hartley 2. However, as reported in Science, Rosetta found that the D/H ratio was more than three times higher than the values found in Earth’s oceans and on Hartley 2.
This not only demonstrates that Jupiter-family comets are more diverse than originally believed, but also suggests that Kuiper Belt comets probably didn’t bring water to our planet. Instead, the researchers say, the findings support models that advocate asteroids as the origin for our oceans, since various meteorites hailing from asteroids have D/H ratios that match up with the water on our planet. However, others have argued it’s far too early to rule out comets given the fact that measurements have only been taken from water vapor surrounding 67P, and the D/H ratios change as the gas escapes the comet’s surface.