Earth should have a lot more gold and silver than it does. These elements are strewn throughout the solar system, but they're relatively rare here. Now scientists say they think they know why: because of our planet's hellish birth.
In a paper published Wednesday in the journal Nature, Oxford geologist Bernard Wood suggests our planet's missing riches evaporated when Earth was young and still molten. Gold and silver, along with other elements known as “moderate volatiles,” turn to gas at moderately low temperatures. When Wood melted a mock version of early Earth in a furnace, he found that moderate volatiles were lost from the rock like steam from a cake.
A second Nature study, led by University of Bristol geochemist Remco Hin, found that evaporation also explains why Earth is dominated by a heavier brand of the element magnesium than other places in the solar system. According to Hin, our planet may have lost up to 40 percent of its mass when these elements were vaporized and blown into space.
Some 4.6 billion years ago, the solar system was little more than a newborn sun surrounded by a swirling mass of gas and dust. Slowly, the material in that “protoplanetary disk” clumped together, forming pebbles, then rocks, then mid-sized “planetesimals,” then full-fledged planets. During a fiery period aptly named the “Hadean” after the Greek god of the underworld, the molten mass of early Earth slammed into and absorbed other bodies. Slightly later, after the Earth had cooled a bit, another impact may have remelted the planet and splashed some molten material into space, creating the moon.
The leftovers from this violent beginning still float around the solar system, much like flour and sugar scattered around a kitchen in which the baker has failed to clean up. These leftovers fall to Earth as meteorites that scientists call “chondrites,” and they provide clues about the conditions in which the planets formed.
Yet when it comes to moderate volatiles, Earth's composition is weirdly different from that of the chondrites. It's as though scientists found cocoa powder all over the messy kitchen, but the cake on the table is vanilla. Many have suggested that the depletion is a result of the way these elements were incorporated into Earth as it solidified — perhaps the moderate volatiles all sank to our planet's core, where neither miners nor scientists can reach them.
By Wood's calculation, the amount of missing gold alone is enough to gild Earth's entire surface in a layer almost 20 inches thick. A planet with a hidden golden core sounds wonderful — about as wonderful as a vanilla cake with a secret chocolate center, and about as complicated to engineer. Wood thought evaporation offered a simpler explanation.
So he and graduate student C. Ashley Norris set about trying to bake up a model of early Earth. They heated a furnace to 1,300 degrees Celsius — about three times as hot as a pizza oven, a quarter as hot as the surface of the sun — to replicate the scorching conditions of the Hadean. They pumped the furnace full of carbon dioxide and carbon monoxide gas — the chief components of our planet's first atmosphere.
Then they stirred together the ingredients found in the protoplanetary disk — melted basaltic rock (imported from Iceland!) spiced with powdered versions of volatile elements, like zinc oxide — poured about half a teaspoon of the mixture into a metal cup, and stuck it in the furnace.
After baking, the sample was dropped into a bucket of water to “quench,” or rapidly cool. When Wood and Norris examined the sample, they found that its volatiles were gone.
“Our experiment shows that melting processes explain the pattern [of volatile depletion] perfectly,” Wood said.
The scientists suggest that this evaporation could have happened on the planetesimals that smashed together to form Earth, or during the collision that led to the creation of the moon.
Unknown to Wood, over at the University of Bristol, Hin was working on a very similar problem. For years, researchers have puzzled over inconsistencies in Earth's ratio of magnesium isotopes (isotopes are forms of the same element that carry varying numbers of neutrons, which gives them different weights). Rocks from Earth seemed to have more of a heavy magnesium isotope than the chondrites do — but perhaps that was just a result of mistakes made in the lab. Hin wanted to know for sure.
Using an instrument called a mass spectrometer, which sorts materials into their component isotopes, he and his colleagues tested an array of Earth rocks, chondritic meteorites and space rocks from other bodies in the solar system. They found that Earth and other large bodies, like Mars and the asteroid Vesta, are dominated by the heavy form of magnesium. Because this element doesn't dissolve in metallic cores, the “vanilla cake with a hidden chocolate center” model could not explain where all the magnesium went. The best explanation was that the magnesium had been vaporized, Hin concluded, then either sucked back into the sun or blown out into the void of space.
In calculating how much magnesium — a relatively abundant element — and other elements might have evaporated, Hin was startled to discover that early Earth should have lost 40 percent of its mass through this process. Disbelieving, he double- and triple-checked his math, but always wound up with the same result.
“From a gut feeling that’s hard to believe,” Hin said. But science is based on data, not gut feelings. “In the end I asked myself the question, what scientific argument do I have not to believe, and I basically couldn't come up with one.”
Then, while attending a conference in the south of France, Hin heard Wood give a talk about his experiments. Afterward, the two chatted about their research over glasses of wine. (Planetary science sounds like a pretty nice gig, doesn't it?)
“It's always good to know that your colleagues go through the same thought process and end up with the same conclusion,” Hin said.
In an analysis for Nature, University of California at Los Angeles geologist Edward Young noted that evaporation is not a cure-all interpretation for oddities in our planet's composition. Earth and Mars have different silicon ratios, for example, which Wood and Hin's findings can't explain. Nor are these studies the first to suggest evaporation as the reason for Earth's missing elements. But, Young wrote, “their relative success should encourage further exploration of the potential role of collisions in determining the chemical and isotopic compositions of planets.”
Next he hopes to analyze the evaporation of early Earth's more difficult-to-detect elements, like chlorine, bromine and iodine. He's also working with a postdoctoral fellow to use the furnace to model the makeup of exoplanets — worlds that orbit suns far beyond our own.