Early Earth's chlorine blown away by giant impacts

Element has long puzzled scientists because modern levels are so low

2:40pm, April 24, 2013
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Earthlings may owe a debt of gratitude to the enormous miniplanets that smashed into the planet in its youth. Such collisions might have knocked away much of the supply of chlorine concentrated on the planet’s surface, geochemists propose. Had that loss not occurred, the world’s oceans would have been too salty for complex life to thrive, they suggest.

The scenario may explain why Mars, which suffered fewer large impacts, may have more than twice as much chlorine as Earth does, the researchers report April 16 in Earth and Planetary Science Letters.

“The story seems to hang together pretty well,” says James Brenan, a geologist at the University of Toronto who wasn’t involved in the study. “Life, probably over a fairly long time, might have been able to adapt to this environment, though certainly things would be different than today.”

One snag is that the idea is “a very difficult thing to test,” says geochemist Ray Burgess of the University of Manchester in England.

The composition of ancient meteorites, which are remnants of the raw material that built the planets, indicates that Earth should have 10 times as much chlorine as it does. The missing chlorine has perplexed scientists for decades. In 1995, geochemist William McDonough suggested that chlorine was dragged to Earth’s center by iron, nickel and other metals that formed the planet’s core.

Normally, chlorine and other elements known as halogens don’t readily dissolve in metals or often combine with other elements to form minerals found in rocks. But perhaps under the intense heat and pressure of the core, chlorine might have become more willing to mix with metal. “I wasn’t happy with putting it in the core,” says McDonough, of the University of Maryland in College Park. But he didn’t know what else to do with it. “I was scratching my head,” he says.

The new work suggests that, in fact, the core is not where chlorine went. In lab tests, Zachary Sharp of the University of New Mexico in Albuquerque and David Draper of the NASA Johnson Space Center in Houston approximated the conditions of the core and observed chlorine’s behavior. They added iron metal, rocks typical of the mantle and a chlorine compound to a capsule heated to 1900° Celsius under pressures about 150,000 times higher than Earth’s atmospheric pressure. The result: Chlorine still didn’t dissolve in iron. That means chlorine probably isn’t hiding out in the core, Sharp says.

So he and Draper looked elsewhere for a solution. After ruling out the possibility that Earth never accumulated chlorine in the first place, the pair concluded that the incipient Earth rammed into giant planetary bodies more than 4 billion years ago and the repeated impacts blew the element away.

The explanation hinges on the peculiarity of chlorine. Unlike elements that mostly end up in rocks and metals, most of Earth’s chlorine is in salt deposits and brines or dissolved in the ocean. Because the element is concentrated on the surface, giant impacts in the past would have stripped away a good chunk of Earth’s chlorine supply, Sharp and Draper say.

Had the early impacts not happened, Sharp says, “the Earth would have been a halogen-poisoned planet.” The oceans would be as salty as the Dead Sea, and high salinity would reduce precipitation. With less rain, there would be less erosion on land and fewer nutrients washing into the sea. In such a world, he says, “it would be much more difficult for [complex] life to evolve.”

McDonough acknowledges that the new work disproves the idea that chlorine is trapped in the core. However, he’s not yet convinced that cosmic crashes removed the element. Even with the massive collision that created the moon, the pull of gravity returned to Earth most of the material that had been kicked into space, he says. “But I don’t have a better idea.”

To strengthen the argument, planetary geochemist Mikhail Zolotov of Arizona State University in Tempe suggests that the team develop simulations to assess how impacts could have affected elements in the young Earth’s atmosphere, oceans and crust. The team could also investigate whether other elements preferentially found on the surface are also lower than expected.  

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