Where Did Earth's Water Come From? Researchers Offer Exciting Explanation

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An artists rendering of the sun, solar winds and the itokawa asteroid.
An artist's rendition of the sun, solar winds and the Itokawa asteroid. Credit: Curtin university
Gid'on Lev
Gid'on Lev
Gid'on Lev
Gid'on Lev

From the dawn of history, human beings have looked to the heavens in admiration. The early inhabitants of the British Isles expended colossal efforts to build Stonehenge, with openings positioned to capture the rays of the sun on the longest day of the year as well as its shortest. Thousands of kilometers to the east, the ancient Egyptians worshiped their chief deity, Amun-Ra, and build the magnificent set of temples at Karnak, which are also directed toward the sun on the longest day of the year.

One can understand the major importance that people of those generations attributed to the sun, which supplies the light and heat (radiation) without which life wouldn’t be possible. According to new research, despite its intuitive connection with burning heat, the sun also played a central role in creating another essential component without which life on Earth wouldn’t be possible – water.

Water covers most of the surface of the Earth, and even some of its depths – the planet’s core and crust are estimated to hold at least a few oceans’ worth of water. So where did that water come from? Despite a number of attempts to answer the question, it remains a mystery.

An artist's rendition of the interaction between solar winds and the Itokawa asteroid. Credit: University of Glasgow / Curtin University

Water is composed of two atoms of hydrogen and one of oxygen. Hydrogen has two stable isotopes – the common H (protium) isotope, whose nucleus consists of a proton, and the rare D (deuterium) isotope, whose nucleus contains a proton and a neutron and is twice as heavy as the H isotope. In the water on the Earth’s surface, the D isotope occurs once for every 156 million H isotopes. Water with a higher ratio is considered to have a heavy isotopic signature, and water with a lower ratio has a light isotopic signature. This helps us in assessing the contribution of various sources for the water on Earth.

According to one theory on the origins of the Earth’s water, it was formed here after water-rich comets collided with the planet. But it turns out that the ratio of isotopes in the water in comets is very different from that of the water on Earth – meaning that they can’t be the main source of water on our planet.

The second and more accepted theory is that the water arrived on Earth at the final stages of the accretion of the planet, as a result of the collision of C-type asteroids. Such asteroids, which account for about 75 percent of all known asteroids, contain relatively large quantities of carbon. It is also known that they contain relatively large amounts of water. But isotope measurements have revealed that the water found in C-type asteroids is heavier than the water on Earth, and leaves us again with little explanation as to the source of our lighter water.

A rock from the Itokawa asteroid.Credit: University of Glasgow / Curtin University

Now researchers from the University of Glasgow in Scotland and Curtin University in Australia are suggesting that a possible source of the light water is a rarer type of asteroid, the S-type, which contains silicate – stony minerals. Such asteroids don’t have naturally occurring water, but the researchers suggest that a reaction between grains of dust on them and solar winds from when the solar system was formed created the light water in S-type asteroids. During the period in which the Earth was accreted, this water mixed with the other sources of water. Later in the Earth’s geological history, this process made life possible. The findings of the study, “Solar wind contributions to Earth’s oceans,” were published this week in the journal Nature Astronomy.

Solar wind is composed of particles of hydrogen that were quickly discharged from the sun’s corona, where the temperature is about 2 million degrees Celsius. In an experiment conducted by the researchers, they analyzed individual atoms from tiny samples from an S-type asteroid known as Itokawa, which a Japanese spacecraft obtained in 2005 and brought back to Earth in 2010. They bombarded the grains of dust with radiation similar to the solar winds that would have been blowing at the time when the solar system took shape.

With the help of sophisticated imaging equipment, they were able to get a close look at the 50 outer nanometers of the surface of the grains of dust from Itokawa. (A nanometer is a billionth of a meter). They discovered that water had been formed there in quantities comparable to 20 liters for every cubic meter of rock. And an isotope analysis of the water revealed that it was very light.

If heavy water from C-type asteroids were indeed an important source of the water on the surface of the Earth, as has been assumed, the researchers said it would have had to have been offset by large quantities of light water. According to their estimation, 52 to 76 percent of the water on the Earth is light water. It could be the result of the reaction created by the solar wind and the S-type asteroids.

“The article describes an experiment that identified the process by which light water is created on S-type asteroids,” Prof. Boaz Lazar, a geochemist from the Hebrew University of Jerusalem, noted. “The researchers demonstrated that there is a way to obtain a substantial quantity of very light water on the outer skin of silicate particles through interaction with hydrogen laden by solar winds.”

The discovery, Lazar said, contributes to the continuing search for the origins of the light water on the Earth’s surface, but at this stage it doesn’t provide conclusive proof that a large quantity of water reached the Earth from S-type asteroids.

The authors of the article, including lead authors Luke Daly, Martin R. Lee and Lydia J. Hallis, said that beyond an explanation for the source of water on the face of the Earth, their discovery might have implications for space research. One of the major obstacles to long-distance space travel is providing water to astronauts. The researchers suggest that the same processes that made it possible to create water on Itokawa also created water on other airless celestial bodies, such as the moon. That would enable astronauts to collect large quantities of water from the dust in such places.

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