The abundance of water on Earth is one of the most important parameters that make our planet finely tuned for life. But how did Earth come to be so rich in this vital resource? That is a question that has vexed scientists for some time. Smithsonian Magazine explains the problem:
Water molecules were surely part of the dusty swirl that coalesced into the Sun and its planets beginning about nine billion years after the Big Bang. But Earth’s early history, including epochs with high ambient temperatures and no enveloping atmosphere, implies that surface water would have evaporated and drifted back into space. The water we encounter today, it seems, must have been delivered long after Earth formed.
As a result, many have proposed that water was delivered to Earth by asteroids or comets. Comets, being giant balls of ice (and rock), are the preferred candidate. However, the Rosetta Probe that landed on Comet 67P last month found that the isotopic signature of the water on 67P is very different from the signature found in water on Earth. See, for example, the article from the BBC, “Rosetta results: Comets ‘did not bring water to Earth.’”
Water, as everyone knows, is composed of two hydrogen molecules and one oxygen molecule. Hydrogen atoms typically have a single proton and no neutron. But hydrogen also has an isotopic variant known as deuterium which has one proton and one neutron. In Earth’s oceans, about one in every 6420 hydrogen atoms is deuterium. According to Professor Kathrin Altwegg of the University of Bern in Switzerland, quoted in the BBC article, the prevalence of deuterium on Comet 67P is “more than three times higher than on the Earth, which means that this kind of comet could not have brought water to the Earth.”
Other comets studied also have different deuterium concentrations than in water on Earth. However, we’ve only studied a few comets so the jury is still out. BBC reports:
Professor Altwegg believes that Kuiper Belt comets did not bring water to Earth.
She said: “The conclusion here is that in the reservoir of the Kuiper Belt, we have very diverse comets that probably came from different regions of the early Solar System.
“We have light water in some comets and very heavy water in other comets. We have to assume the mixture of all these comets is something that is heavier than what we have on Earth, so this probably rules out Kuiper Belt comets as the source of terrestrial water.”
Altwegg thinks asteroids are the answer to how water came to Earth. Can asteroids — which are less water rich than comets — explain how Earth accumulated so much water? This is the first time we’ve landed a spacecraft on a comet, so who knows what future studies might show. For now, it doesn’t look like comets are the best explanation for how water arrived on Earth.