Where does water on Earth come from?


In the last hours of the last day of February 2021, a 13-kilogram asteroid hurtled into the Earth's upper atmosphere at a speed of about 14 kilometers per second. As it passed through the stratosphere, the friction and heat of the atmosphere burned its exterior a deep black. The outer debris of the meteorite fell off in the firelight, and the huge fireball instantly lit up the night sky like a torch.

The largest fragment of the meteorite (about 320 grams) fell on a driveway in Winchcombe, England. Twelve hours later, scientists rushed to collect samples of the meteorite. This is the "freshest" meteorite material to date, "almost as timely as ideal." said Ashley King, a planetary scientist at the Natural History Museum in London, UK.

The Winchcomb meteorite belongs to a rare type of meteorite - a carbonaceous chondrite. Using its volatile components, researchers are trying to piece together one of Earth's biggest mysteries: Where does our planet's water come from? In the past, researchers believed that meteorites may have brought water to the Earth, but there is still fierce debate over how much water meteorites brought. Some people suggest that meteorites are like "a heavy rain" on the earth; others think that the contribution of meteorites is only "a drop in the bucket".

Before becoming a planet, Earth was once just a cloud of dust orbiting a young sun. This dust condenses into rock clumps through a process called accretion, and then collides together to form larger and larger rock aggregates, which eventually snowball into a complete planet.

The early Earth was not like the "pale blue dot" it is today. Its temperature was once as high as 2000°C, enough to evaporate any moisture on the surface. Scientists once thought this meant the Earth would have been very dry in its infancy, but a recent study published in the journal Nature suggests the planet may have been much wetter. Anat Shahar, a co-author of the study and a geochemist at the Carnegie Institution for Science, and colleagues noticed that many Earth-like exoplanets will Obtaining a hydrogen-rich atmosphere, they adjusted the model accordingly and re-simulated the formation process of the Earth. The results show that, contrary to previous assumptions, even if the virtual planet was once covered by a magma sea, there is still a large amount of water present on the planet's surface and wrapped into the mantle.

While this model suggests that the Earth may have retained large amounts of water since its formation, planetary geologists still believe that a large portion of the water comes from outside the atmosphere. "There is so much evidence," Shahar said, "that we can't refute it".King said the "smoking gun" is hidden in Earth's hydrogen. Hydrogen exists on Earth in two stable "flavors" - or hydrogen has two stable isotopes: hydrogen, whose nucleus has only one proton, and deuterium, whose nucleus is composed of one proton and one neutron. Water in the Earth's mantle contains 15% less deuterium than seawater, so the excess deuterium in seawater is likely to come from elsewhere.

Initially, astronomers speculated that deuterium-rich water came to Earth via comets. The place where the comet is born is very cold, and the comet core at the head is mainly composed of icy material, which may account for up to 80% of the mass. But in 2014, data from the European Space Agency's Rosetta comet probe revealed that the water of many comets has much higher levels of deuterium than isotopes in Earth's water. Scientists have proposed another hypothesis: It is the solar wind that pushes hydrogen and oxygen from space into the Earth's atmosphere. However, many scientists insist that there is too little deuterium in these molecules. "It's difficult to use these sources together to explain the hydrogen isotope ratio of Earth's water," says Megan E. Newcombe, a petrologist at the University of Maryland.

So where can we find the “just right” isotope ratio? Eventually, researchers found the answer in meteorites—specifically, a type of primitive clump called chondrites. In carbonaceous chondrites, named for their carbon content, the water content can be as high as 20%. "It's not that when you touch a meteorite, it's wet," said Maria Valdes, a geologist at the Field Museum in the US. In fact, most of the water is locked up in minerals.

In a 2022 study published in Science Advances, Jin and colleagues used energy spectroscopy to analyze the Winscomb meteorite. They found that the meteorite's deuterium-to-hydrogen ratio matched that of Earth's oceans almost perfectly—a result that was particularly noteworthy given the speed with which they collected the meteorite samples.

"Meteorites and the atmosphere don't get along," said Denton Ebel, curator of geology at the American Museum of Natural History. Upon contact with the Earth's atmosphere, the minerals in the meteorite absorb water vapor like a sponge. But because the Winchcombe sample was obtained within 12 hours of the impact, it is much less contaminated with Earth's water than most samples.

A few months after the analysis of the Winshcombe meteorite was completed, Newcomb and his team further confirmed the carbonaceous chondrites in a study published this year in the journal Nature. The research team analyzed several new achondrites. Unlike carbonaceous chondrites, these come from asteroids or primitive rock aggregates that have undergone partial melting. Newcomb and co-authors found that, like cookie dough as it bakes, achondrites lose water during melting. "Everything we find, whether it's from the inner solar system or the outer solar system, is very, very dry," she said.

However, cosmochemist Laurette Piani of the University of Lorraine in France points out that this discovery does not mean that carbonaceous chondrites are the only source of water on Earth. "In my opinion, water on Earth could have come from several sources," she said. If it were just water in chondrites, the Earth's oceans would require a large number of meteorite impacts to form. However, today's carbonaceous chondrites Quite rare. Piani noted that roughly equal proportions of solar wind, comets, mantle water and chondrite sources combined would match the current isotope ratio of Earth's water. But whatever the exact recipe of Earth's water, its origins will shed light on how our planet formed and evolved the vibrant blue world we live in.