Earth may have kept its own water rather than getting it from asteroids
by Julia Rosen
Carl Sagan famously dubbed Earth the “pale blue dot” for our planet’s abundant water. But where this water came from—and when it arrived—has been a longstanding debate. Many scientists argue that Earth formed as a dry planet, and gained its water millions of years later through the impact of water-bearing asteroids or comets. But now, scientists say that Earth may have had water from the start, inheriting it directly from the swirling nebula that gave birth to the solar system. If true, the results suggest that water-rich planets may abound in the universe.
“This is a great test of our canonical picture for how Earth got its water, and it suggests that things are not as simple as we first thought,” says Fred Ciesla of the University of Chicago in Illinois, who was not involved in the new study. To understand the origin of Earth’s water, scientists have fingerprinted potential sources, like asteroids and comets, using the ratio of light to heavy hydrogen isotopes. Then, researchers can compare the ratios with those found in water sources on Earth.
However, researchers don’t really know the true hydrogen isotopic composition of Earth’s water, says Lydia Hallis, a planetary scientist at the University of Glasgow in the United Kingdom and lead author of the new study. Scientists have often assumed that the isotopic signature of seawater is close to the true value, but Hallis thinks this has probably changed over geologic time, as Earth preferentially lost light hydrogen atoms to space and gained water from asteroid and comet impacts.
So Hallis and her colleagues went looking for vestiges of the early Earth that might preserve the original hydrogen isotope ratio of the planet. They found them in an unlikely place: Baffin Island in the Canadian Arctic. Here, massive eruptions—fueled by the hot spot that now sits beneath Iceland—produced lava that originated deep in the mantle. So deep, in fact, that this material was probably isolated from the surface for almost all of Earth’s history. The evidence lies in the fact that the lavas, now hardened into basalts, still contain a fair amount of light helium isotopes, which would have escaped to space had the rocks spent much time anywhere near the surface.
In the new study, the researchers report the hydrogen isotope ratios of water trapped in glassy inclusions inside the basalts. The results, published online today in Science, reveal that the inclusions have a much lighter isotopic signature than does the ocean, suggesting that the composition of seawater has indeed evolved over time. Although scientists were aware of processes that could cause an isotopic shift in surface waters, Hallis says, “until we made our measurements, we didn’t know whether that would be a measureable difference or not.”
The new data suggest that the difference is vast. And Hallis suspects that the deepest, most primitive material in the mantle should have an even lighter isotopic composition than the inclusions her team measured. That’s because the rising magma that produced the lavas probably mixed with upper mantle rocks, which have been contaminated with isotopically heavy surface water that got dragged down by subducting slabs of tectonic plates.
So what does all this mean for the origin of Earth’s water? For one, the new data throw a wrench in the conventional story that carbonaceous chondrites—a water-rich variety of asteroid—delivered water to an initially dry Earth after its formation. That scenario has been bolstered by similarities in the isotopic signatures of the asteroids and seawater. But the chondrite signatures are too heavy to explain the deep Earth samples, Hallis says. “The carbonaceous chondrites don’t really work.”
Instead, Hallis and her colleagues propose that Earth’s water came directly from theprotosolar nebula—the cloud of gas and dust that eventually clumped together to form the solar system. Based on measurements of Jupiter and the solar wind, which are thought to preserve the hydrogen isotopic ratio of the protosolar nebula, scientists think nebular water had an extremely light hydrogen isotopic signature—much closer to what the Baffin Island lavas suggest about the deep mantle’s water.
Traditionally, the main objection to this idea has been that the inner portion of the protosolar nebula, where Earth formed, would have been too hot for water to hang around. But Hallis’s team suggests that water floating around in the nebula snuck into our nascent planet by adsorbing to dust particles. They cite previous modeling work suggesting that this mechanism could allow a significant amount of water to survive the brutal temperatures and violent processes by which dust particles coalesced to form planets. Hallis says the discovery of a deep reservoir of material with protosolar isotope ratios supports the idea that the hot, early Earth somehow retained this water.
However, some scientists aren’t ready to abandon the asteroid hypothesis just yet. That’s because, on top of bringing water, they are also believed to have delivered much of Earth’s so-called volatile elements, namely, carbon, nitrogen, and noble gases, says Conel Alexander, a cosmochemist at the Carnegie Institution for Science in Washington, D.C. To explain the abundance of these elements, there would have had to have been enough impacts to also deliver Earth’s water, he says. “That still seems to me the simplest and most attractive explanation.”
Ciesla says that the new results will force scientists to re-evaluate the process of Earth’s formation. Perhaps the team’s adsorption model is correct, or perhaps water came to Earth aboard a kind of asteroid that hasn’t yet been found, or that no longer exists because it all went into making the Earth. “What we have to do is try to understand what fits and what doesn’t,” he says.
However, if Hallis and her colleagues turn out to be right, their hypothesis could have major implications for other planets. In the prevailing model of an initially dry Earth, hydrating the planet seemed like “more of a one-off event,” Hallis says. However, if the planet managed to keep water from the solar nebula before it evaporated away, there’s no reason other planets couldn’t do the same thing. Hallis says that her results could mean that water-rich planets like Earth are not so rare after all.
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