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Earth has a lot of water. It’s almost like we’re hoarding the stuff for a not so rainy day. All of that water is more or less the defining feature of our planet when seen from a distance, but there’s been an ongoing mystery about where it all came from. With a few exceptions, vast amounts of water don’t appear to be particularly common in other parts of our stellar neighborhood.
It’s likely that much of our water was carried here by meteors and asteroids which impacted with the planet, delivering payloads of H2O. It’s also possible that water was present in the stuff which made up the Earth, and settled into place as the planet cooled. But in order to make a planet, asteroid, or meteorite with water in the first place, it has to come from somewhere.
A new study by Jerome Aléon from the Institute of Mineralogy, Physics of Materials and Cosmochemistry at Sorbonne University, and colleagues, traces the story of Earth’s water back to the very earliest moments of the solar system’s formation. Their findings were published in the journal Nature Astronomy.
“We looked at the composition of hydrogen in early solar system rocks. They have refractory inclusions, very small bits of rock, which we measured with a mass spectrometer,” Aléon told SYFY WIRE.
Mass spectrometers work by using an ion beam to dismantle a piece of the sample. The atoms are collected and measured to determine their mass, and that can tell us what they’re made of. Even isotopes of the same elements might have different masses depending on their composition. Hydrogen, for instance has two stable isotopes. There is the hydrogen you probably remember from high school chemistry class, with a single proton and mass one. Then you have hydrogen with an extra proton and mass two, which scientists call deuterium. The ratio of hydrogen to deuterium tells us when and how a particular sample was made, during the solar system’s formation.
“What we found were two different isotopic signatures distributed in the minerals,” Aléon said. “In some we found a deuterium to hydrogen ratio that corresponds to what we expect from the solar gas, and we found other minerals with a deuterium to hydrogen ratio similar to Earth.”
The measurements point to the existence of two gas reservoirs which were present at the very beginning of the solar system. One of these reservoirs was stripped almost entirely of deuterium, a consequence of deuterium burning which occurs in stellar fusion. The second was deuterium rich, a result of natural chemistry which occurs at very low temperatures during the formation of a protoplanetary disk.
“All of the molecules which are formed at these very low temperatures, including water and organic molecules, they are very rich in deuterium. By mixing these two gas reservoirs, you get something which is intermediate with a composition matching that of the Earth,” Aléon said.
We now know that the water which existed in the early days of our solar system had the same composition as what we see on Earth, something which wouldn’t necessarily be the case if that water formed later or ultimately came from elsewhere.
Moreover, it suggests that other planets and asteroids which formed in the same region at the same time should have similar compositions. That doesn’t mean they’ll have similar water reserves, there are a whole host of physical processes which conspire to strip objects of water when they don’t have the luxury of lush atmospheres like ours, but it does mean we might expect to see similar processes at work in other solar systems.
“It’s a very general physical process at work in the interstellar medium,” Aléon said. “The main H2 gas is everywhere and in all molecular clouds we measure we see very low temperatures producing deuterium-rich molecules.”
Stellar systems appear to form in the presence of these two interacting gas reservoirs which ultimately produce the sorts of water reserves we find on our own humble planet. The trick is figuring out how to soak it all up like Earth did.