Where Did Earth's Water Come From? New Meteorite Study Finds A Clue
If you want to know which objects brought water to a once-dry planet, think small.
Water probably got to early Earth aboard small meteorites, not fragments of bigger protoplanets that had been blasted apart.
Our watery, life-sustaining planet was born dry and barren. Earth and the other inner planets of our Solar System formed too close to the Sun’s heat for the volatile chemicals that make up water and organic molecules to survive. And yet the planet we live on today is more than 70 percent water and teeming with life.
In a recent study, University of Maryland geologist Megan Newcombe and her colleagues examined the chemical makeup of meteorites to help narrow down exactly how that happened. They published their work in the journal Nature.
Where Did the Water Come From?
The dashed white line in this illustration marks the boundary between the inner and outer Solar System; asteroids on either side of that dividing line seem to have different chemical fingerprints. Near the top of the image, a rock with attached water molecules (extremely not to scale) shows how water might have reached Earth.
“Planet-building is a very chaotic process where we have lots of small things colliding together,” Newcombe tells Inverse. The first 700 million years of our Solar System’s history were definitely chaotic; the cratered surfaces of most of the rocky planets and moons reveal a period of constant collisions between asteroids, newly-formed and still-forming planets, and other chunks of space rock.
Some of the objects that found their way to Earth and Mars had originally formed in the outer reaches of the disk of gas, dust, and ice that circled the newborn Sun. Born beyond what’s called the “ice line,” these objects were far enough from the Sun’s heat that when they formed, they were made of not only rock, but also frozen water, methane, carbon dioxide, and other compounds. And enough of those objects, carrying enough water and other chemicals, crashed into Earth to fill entire oceans.
Planetary scientists agree on that general picture, but there’s still debate over exactly which ill-fated space objects — comets, asteroids, or something else — delivered most of Earth’s water and thus enabled life to emerge from a soup of very ambitious chemistry.
By examining the chemical makeup of seven meteorites, all of which formed in the Solar System’s earliest millennia but crashed into Earth relatively recently, Newcombe and her colleagues say they’ve ruled out one group of asteroids and pointed the finger at another.
Meteorite Forensics Uncovers a Surprise
Newcombe and her colleagues measured the amount of water trapped inside the structure of mineral grains in seven meteorites. Don’t picture droplets of water, or even slight dampness; instead, picture individual molecules, locked into the atomic structure of a crystal. Its presence offers a clue about how much water — or other combinations of oxygen and hydrogen, like hydroxide — the original asteroid contained.
Newcombe and her colleagues analyzed these tiny chips from seven meteorites; each chip is a few millimeters wide.
Some of the “wettest” meteorites ever recovered, a type called carbonaceous chondrites, contained enough water to make up about 20 percent of their weight.
But the seven meteorites that Newcombe and her colleagues studied had all originally been part of the crust or mantle of planetesimals. The seeds of future planets, planetesimals are rocky objects whose gravitational might managed to pull in enough nearby dust and gas to eventually grow into planets.
Early in our Solar System’s history, planetesimals contained a radioactive isotope of aluminum called aluminum-26, an aluminum atom with 26 protons and 26 neutrons. If there’s enough aluminum-26 in one place, like a big chunk of space rock, its radioactive decay can produce enough heat to melt the iron and rock around it. So large planetesimals in the early Solar System heated up, melted, and then settled into layers as they cooled, with the heaviest elements in the center (which is why Earth has an iron core and a light, rocky crust).
Sometime after that, in our Solar System’s chaotic Wild West days of things crashing into each other, some of those planetesimals shattered in devastating impacts — and some pieces of them eventually landed on Earth. Based on their chemical makeup, the meteorites Newcombe and her colleagues studied came from at least 5 different objects. Some of those objects originally formed in the inner Solar System, and others came from the outer Solar System, beyond Jupiter and the ice line.
“I expected the outer Solar System samples to be very wet, because you expect more ice to be present in the outer Solar System than in the warmer inner Solar System,” says Newcombe.
But every single one of the meteorite samples turned out to be completely dry.
Narrowing the Field of Contenders
On average, each of the seven meteorite samples contained about .0002 percent water, or about 2 parts per million. The best explanation, according to Newcombe and her colleagues, is that when aluminum-26 heated up the planetesimals and eventually melted them, any water they’d once contained just boiled away into space. Along with the water, other easily-evaporated chemicals like nitrogen, ammonia, methane, and carbon dioxide would also have gone up in a puff of space steam.
“Things that have melted, even if they formed in the outer Solar System in the presence of ice — because they've melted, they have very efficiently dried out,” says Newcombe.
And that means that meteorites like the seven Newcombe and her colleagues studied, which were once part of planetesimals (and are now a type of meteorite called achondrites) can’t have been the chunks of space debris that brought water to our once-parched, now-drenched planet.
“If we're accreting the Earth from a lot of these early-formed planetesimals, it’s likely that the Earth was pretty dry in its early stages,” says Newcombe.
Instead, Newcombe and her colleagues say we should be thanking chondrites, meteorites made of grains of dust and rock that have never been melted and reforged into other minerals. Chondrites are chunks of objects that never got hot enough to melt and lose all their water because they were never big enough to contain enough aluminum-26 to produce that much heat.
“I think unmelted material delivered water to the earth,” says Newcombe. “But that material could actually have come from the inner or the outer Solar System.” Figuring out exactly which group of unmelted chondrites we have to thank for water — and thus, life — on Earth is work for future studies of both meteorites here on Earth (and possibly on Mars and the Moon) and asteroids still drifting around in space.
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