New York Times,
With the unofficial start of summer on Monday, many people will get up close and personal with the element that carries 17 protons.
I speak, of course, of chlorine.
Over the next few months, chlorine will ensure that countless swimming pools don’t turn into microbe-choked petri dishes. That’s only one of many uses we’ve found for the element. We sprinkle it on our food as table salt — a.k.a. sodium chloride. We pump water through pipes made of polyvinyl chloride. Perchlorate, a combination of chlorine and oxygen atoms, fuels rockets and ignites fireworks.
But in other incarnations, chlorine is a bane of our existence. In World War I, the German army unleashed clouds of chlorine gas and killed or injured thousands of enemy -soldiers. The Hudson River is burdened with cancer-causing dioxins, chlorine-bearing compounds dumped from factories along its banks.
Still, chlorine’s threats today are nothing compared with its menace on the early Earth.
According to a study recently published in the journal Earth and Planetary Science Letters, the entire planet was poisoned with the stuff. Fortunately, the planet got rid of most of its chlorine. If it hadn’t, we might not be here today.
“It would be tough to have any complex life,” said Zachary D. Sharp of the University of New Mexico, an author of the study.
Researchers like Dr. Sharp track the history of elements like chlorine to learn about how the solar system formed and how Earth became a habitable planet. The chlorine atoms floating in swimming pools today were forged billions of years ago in ancient stars. They drifted into a dusty cloud that would eventually become our solar system.
As Earth formed some 4.6 billion years ago, it swept up some of that chlorine and incorporated it into its rocks. To trace the history of chlorine on Earth, Dr. Sharp and David S. Draper of NASA Johnson Space Center recently tallied up how much of the stuff our planet contains.
The salty ocean contains a lot — about 20 million billion tons, in fact. More chlorine can be found in places like Death Valley, Calif., which formed from ancient evaporated seas.
It’s harder to know how much chlorine is hidden deeper underground. Some clues come from diamonds, which formed hundreds of miles underground before being pushed up to the surface. They also contain traces of chlorine.
But deeper down, things get more mysterious. “We have no samples from the core,” said Dr. Sharp.
We do know, however, that the Earth’s core formed when molten iron sank to the center of the planet. Lighter rock, like basalt, floated up to higher levels. So Dr. Sharp and Dr. Draper ran an experiment to see how chlorine would behave in such an environment.
They combined iron, basalt and chlorine and burned them under intense pressure. They found that all of the chlorine combined with the basalt, and none with the iron. Those results, Dr. Sharp and Dr. Draper concluded, mean that Earth’s iron core is chlorine-free.
All told, Dr. Sharp and Dr. Draper estimate the concentration of chlorine on Earth is 17 parts per million. And that’s a very puzzling number. Scientists have found that the meteorites that formed at the birth of the solar system have vastly higher levels of chlorine. Based on those studies, Dr. Sharp and Dr. Draper estimate that the early Earth had 10 times the amount of chlorine than it has today.
“A huge fraction of chlorine that should be on the Earth is gone,” said Dr. Sharp.
Dr. Sharp believes that most of Earth’s chlorine was wiped out by the huge meteorites that slammed into the planet during its first few hundred million years. Those impacts vaporized some of the oceans, sending chlorine out into space.
If those impacts hadn’t gotten rid of all that chlorine, Earth would be radically different. The oceans would have been loaded with as much salt as the Dead Sea. Such salty water wouldn’t be able to dissolve oxygen. Much less vapor would rise from it, leading to much less rain. The continents would have become parched, preventing nutrients like phosphorus from flowing from the
land to the sea. “It would be a very nasty place,” said Dr. Sharp.
So nasty, in fact, that animals and other complex life-forms might not have been able to survive. A chlorine-laced swimming pool may be a pleasant place to spend a summer day, but a planet covered in a global Dead Sea would have been no vacation at all.
Copyright 2013 The New York Times Company. Reproduced with permission.