Water likely arrived on early Earth aboard small meteorites, not through fragments of larger protoplanets that had been blown 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 over 70 percent water and teeming with life.
In a recent study, University of Maryland geologist Megan Newcombe and her colleagues examined the chemical composition of meteorites to narrow down exactly how this happened. They published their work in the journal Nature.
Where did the water come from?
“Planet building is a very chaotic process, with lots of little things colliding,” says Newcombe The opposite. The first 700 million years of our solar system’s history were definitely chaotic; The cratered surfaces of most rocky planets and moons reveal a period of constant collisions between asteroids, newly formed and nascent planets, and other chunks of space rock.
Some of the objects that made their way to Earth and Mars originally formed in the outer reaches of the disk of gas, dust and ice that orbited the newborn Sun. Born beyond the so-called “ice line,” these objects were far enough from the sun’s heat that when they formed, they were composed not only of rock but also of frozen water, methane, carbon dioxide, and other compounds. And enough of these objects, carrying enough water and other chemicals, fell to Earth to fill entire oceans.
Planetary scientists agree on this general picture, but there is still debate as to which ill-fated space objects — comets, asteroids, or something else — provided most of Earth’s water and thus gave rise to life 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 fell to Earth relatively recently, Newcombe and her colleagues say they ruled out a group of asteroids and associated them with the pointed fingers at someone else.
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Newcombe and her colleagues measured the amount of water trapped in the structure of mineral grains in seven meteorites. Imagine no water droplets or even slight moisture; Instead, imagine individual molecules locked into the atomic structure of a crystal. Its presence gives an indication of how much water — or other combinations of oxygen and hydrogen, such as hydroxide — the original asteroid contained.
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 were all originally part of the crust, or mantle, of planetesimals. The seeds of future planets, planetesimals, are rocky objects whose gravity has managed to attract enough nearby dust and gas to eventually grow into planets.
Early in our solar system’s history, planetesimals contained a radioactive aluminum isotope called aluminum-26, an aluminum atom with 26 protons and 26 neutrons. If there’s enough aluminum-26 in one place, like a large chunk of space rock, its radioactive decay can generate enough heat to melt the iron and rock around it. Planetesimals this large in the early Solar System warmed, melted, and then settled in layers as they cooled, with the heaviest elements at the center (which is why Earth has an iron core and a light, rocky crust).
Sometime after that, during our solar system’s chaotic wild west days when things crashed into each other, some of these planetesimals shattered in devastating impacts — and some pieces of them eventually ended up on Earth. Based on their chemical composition, the meteorites studied by Newcombe and her colleagues came from at least 5 different objects. Some of these objects originally formed in the inner solar system, 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 the outer solar system is believed to have more ice than the warmer inner solar system,” says Newcombe.
But every single one of the meteorite samples turned out to be completely dry.
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On average, each of the seven meteorite samples contained about 0.0002 percent water, or about 2 parts per million. According to Newcombe and her colleagues, the best explanation is that as aluminum-26 heated and eventually melted the planetesimals, any water they once contained simply evaporated into space. Along with the water, other easily evaporating chemicals such as nitrogen, ammonia, methane, and carbon dioxide would have risen in a plume of space vapor.
“Molten things, even if they formed in the outer solar system in the presence of ice — because they melted, they dried up very efficiently,” Newcombe says.
And that means meteorites like the seven Newcombe and her colleagues studied that were once part of planetesimals (and are now a type of meteorite called achondrites) couldn’t have been the chunks of space debris that dumped water into our once-dried, now waterlogged planet.
“If we accrete the Earth from many of these early formed planetesimals, it’s likely that the Earth was quite dry in its early stages,” says Newcombe.
Instead, Newcombe and her colleagues say we should thank chondrites, meteorites made of grains of dust and rock that have never been melted and transformed into other minerals. Chondrites are clumps of objects that never got hot enough to melt and lose all of their water because they were never big enough to contain enough aluminum-26 to generate that much heat.
“I think unmelted material added water to the earth,” says Newcombe. “But this material could actually have come from the inner or the outer solar system.” Figuring out which group of unmolten chondrites give us water – and therefore life – on Earth is work for future studies of both meteorites here on the planet Earth (and possibly Mars and the Moon) as well as from asteroids still floating in space.