Where Did Earth's Water Come From? Scientists have long debated whether the Earth's water was here when the planet formed or whether it arrived later. A study suggests much of the water originated in rocks from which Earth is built. Show Research NewsAugust 27, 20204:03 PM ET
Scientists have long debated whether the Earth's water was here when the planet formed or whether it arrived later. A study suggests much of the water originated in rocks from which Earth is built. AUDIE CORNISH, HOST: Water is everywhere on Earth - the clouds, the rain, the oceans and rivers, even our own bodies. Where all that water originally came from is a bit of a mystery. NPR's Nell Greenfieldboyce reports that scientists may have found the answer inside some rare meteorites. NELL GREENFIELDBOYCE, BYLINE: The Earth formed 4 1/2 billion years ago. Compared to other planets, it emerged pretty close to the sun. There, hot temperatures would mean no water ice, no ice to join with the swirling bits of rock and dust that were running into each other and building up our young planet. LAURETTE PIANI: That's why we do not know exactly where the water on Earth come from, why we need to find a source of water on Earth. GREENFIELDBOYCE: Laurette Piani works at a French research lab called CRPG. She says that source of water could have been farther out in the solar system, like maybe icy comets or water-rich asteroids that hit the newly formed Earth and watered it. This has long been the prevailing view. PIANI: To explain the presence of the Earth's ocean and of the Earth's water in general. GREENFIELDBOYCE: She wondered, though, if water could have been there at the start. So she and some colleagues recently took a close look at a rare kind of meteorite. It doesn't look like anything special. PIANI: It's a bit like a gray rock. GREENFIELDBOYCE: But it's also thought to have formed near the sun and is the same kind of primordial stuff that glommed together to create our planet. And it turns out it contains plenty of hydrogen. That's an indicator of its ability to contribute water to a planetary mix. In fact, if you built a planet out of this material, you'd have at least several oceans' worth of water. These findings, described in the journal Science, made Anne Peslier feel really happy. ANNE PESLIER: I was happy because it makes it nice and simple. GREENFIELDBOYCE: She's a planetary scientist at NASA's Johnson Space Center in Houston, Texas, who wasn't part of the research team. She says this old idea that Earth's water came from the outer solar system would have required something unusual, like Jupiter having a little trip through the inner solar system to send water-rich asteroids headed our way. PESLIER: So here, we just don't need Jupiter. We don't need it to do anything weird. We just grab the material that was there, where Earth formed, and that's where the water comes from. GREENFIELDBOYCE: Still, she says, even if most of the water was there at the beginning, comets and such probably did deliver some of Earth's water later on. Nell Greenfieldboyce, NPR News. (SOUNDBITE OF HOORAY FOR EARTH SONG, "KEYS") Copyright © 2020 NPR. All rights reserved. Visit our website terms of use and permissions pages at www.npr.org for further information. NPR transcripts are created on a rush deadline by an NPR contractor. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.  Earth is rich in water, and has been for a few billion years, but scientists are still debating just where all that life-sustaining liquid came from. At least some of it was thought to have been brought here by comets or asteroids, but that idea still falls short in explaining how so much water ended up on Earth’s surface – and deep below, as well. Now, a team of scientists at Arizona State University (ASU), led by Peter Buseck, has come up with a new proposal. The new peer-reviewed paper was published in the Journal of Geophysical Research: Planets on October 9, 2018. The new research suggests that Earth’s water came from both rocky material, such as asteroids, and from the vast cloud of dust and gas remaining after the sun’s formation, called the solar nebula. The 2019 lunar calendars are here! Order yours before they’re gone. Makes a great gift. Earth’s ocean water is similar to that found in asteroids. That’s one reason scientists have long thought that most earthly water came from an asteroid bombardment in the days of the early solar system. The ratio of deuterium – a heavier hydrogen isotope – to normal hydrogen is a unique chemical signature in various water sources. In the case of Earth’s oceans, the deuterium-to-hydrogen ratio is close to what is found in asteroids. But, according to Steven Desch, also at ASU and one of the team members:
Jun Wu at ASU is lead author of the study. He added:
The hydrogen in Earth’s oceans may not represent the hydrogen throughout the planet as a whole, however. Samples of hydrogen from deep inside the Earth, close to the boundary between the core and mantle, have notably less deuterium – indicating that this hydrogen may not have come from asteroids, after all. The noble gases helium and neon, with isotopic signatures inherited from the solar nebula, have also been found in the Earth’s mantle. How to explain these differences? The researchers needed to develop a new theoretical model of Earth’s formation to answer that question. According to the model, Earth was the largest of many planetary embryos – aka protoplanets – in the early solar system. Essentially, their model shows large, waterlogged asteroids eventually forming into planets like Earth through collisions. The surface of the very young Earth was initially an ocean of magma. Hydrogen and noble gases from the solar nebula were drawn to the planetary embryo, forming the first atmosphere. Nebular hydrogen, which contains less deuterium and is lighter than asteroidal hydrogen, dissolved into the molten iron of the magma ocean. Hydrogen was then drawn toward the center of the Earth – a process called isotopic fractionation. Hydrogen was delivered to the core through its attraction to iron, while much of the heavier isotope, deuterium, remained in the magma which eventually cooled to form the mantle. Impacts from smaller planetary embryos and other objects continued to add additional water and mass until Earth reached its final size. The end result was that Earth had noble gases deep in its interior, with a lower deuterium-to-hydrogen ratio in its core than in its mantle and oceans. Most of Earth’s water did come from asteroids, but some also came from the solar nebula. As Wu noted:
So what about comets, since they have so much water-ice in them? According to Desch:
Also, comets have higher deuterium-to-hydrogen (D/H) ratios, so they are actually not good sources for Earth’s water. The D/H ratio of hydrogen gas in the solar nebula was only 21 ppm, too low to have supplied most of the water on Earth. Asteroids are a much better match, along with the solar nebula. Finally, the new results have implications for rocky exoplanets orbiting other stars. Many such worlds have now been discovered, and if there is a greater chance for some of them to also have liquid water, that also increases the chances of those planets being habitable. According to the researchers:
Bottom line: The origin of Earth’s water has been debated for a long time, but this new study points to a source – the solar nebula, or cloud of gas and dust left after the sun’s formation – that had been previously mostly overlooked. The new work, based on computer modeling, may have implications for rocky worlds orbiting distant stars. Source: Origin of Earth’s Water: Chondritic Inheritance Plus Nebular Ingassing and Storage of Hydrogen in the Core Via Arizona State University Paul Scott AndersonView Articles About the Author: Paul Scott Anderson has had a passion for space exploration that began when he was a child when he watched Carl Sagan’s Cosmos. While in school he was known for his passion for space exploration and astronomy. He started his blog The Meridiani Journal in 2005, which was a chronicle of planetary exploration. In 2015, the blog was renamed as Planetaria. While interested in all aspects of space exploration, his primary passion is planetary science. In 2011, he started writing about space on a freelance basis, and now currently writes for AmericaSpace and Futurism (part of Vocal). He has also written for Universe Today and SpaceFlight Insider, and has also been published in The Mars Quarterly and has done supplementary writing for the well-known iOS app Exoplanet for iPhone and iPad. When did water first appear on Earth?Geological evidence also helps constrain the time frame for liquid water existing on Earth. A sample of pillow basalt (a type of rock formed during an underwater eruption) was recovered from the Isua Greenstone Belt and provides evidence that water existed on Earth 3.8 billion years ago.
How was water first formed in the universe?Water was first created in deep space, after hydrogen and oxygen first formed. All hydrogen atoms were created about 380,000 years after the big bang. Oxygen was created much later in the nuclear furnaces of stars, towards the end of their life cycles. Stellar lives begin by converting hydrogen into helium.
How did the water start?PathWater co-founders Shadi Bakour and Ali Orabi were in a hurry. It was 2015 and the friends had came up with an idea for a refillable, reusable and recyclable water bottle one day.
|
