In 1928, chemists invented a type of molecule that seemed perfect.

By the seventies, these molecules were in all kinds of things, making companies billions of dollars a year. Then, in 1974, a pair of scientists discovered that these miracle molecules were destroying a part of Earth’s atmosphere that keeps us alive.

It took years to convince the world they were right, and another three decades for the planet to finally start healing. But these scientists didn’t even set out to save the Earth, they were just studying what happens on Venus. Those seemingly perfect molecules were chlorofluorocarbons, or CFCs, which have chlorine, fluorine, and carbon atoms bonded together in different ways.

The great thing about CFCs was that almost nothing would react with them or break them apart. The carbon-fluorine bonds in a CFC like Teflon, for example, are so strong that the strongest acids in the world can’t break them. Plus, CFCs were non-toxic, fireproof, and relatively easy to make for lots of different purposes.

So CFCs were all over the place by the seventies: refrigerators, fire extinguishers, aerosols, solvents, insulation, you name it. A million metric tons of gaseous CFCs were released into the atmosphere every year, and at the time scientists just assumed that CFCs would keep being their wonderfully inert selves in the atmosphere and not cause any problems. CFCs were so unreactive down here at the surface that scientists working on them didn’t bother checking what happened when they got higher up.

So they missed something important: CFCs were destroying the ozone layer, the part of Earth’s atmosphere made up of three-atom oxygen molecules that absorb a lot of harmful UV radiation before it reaches the surface. And it took researchers working on a completely different planet-wide conundrum, on a totally different planet, to figure that out. Venus’s atmosphere is mostly carbon dioxide, with a tiny bit of nitrogen, water vapor, carbon monoxide, and hydrochloric, hydrofluoric, and sulfuric acids.

Basically, it’s not somewhere you’d wanna go on vacation. All that carbon dioxide confused scientists in the early 1970s, because they didn’t see an ozone layer on Venus. UV light breaks molecules apart, and they knew that without ozone to block it, UV light from the sun should’ve broken apart a lot of Venus’s CO2, leaving behind much more oxygen and carbon monoxide than they were seeing.

Then, groups led by environmental scientist Michael McElroy figured out what was probably happening: The same UV light that would break down CO2 would break down hydrochloric acid at the same time, freeing up hydrogen atoms. Then, those hydrogen atoms would act like ruthless matchmakers, ripping oxygen atoms off of molecules so that they combined with carbon monoxide to make carbon dioxide again. That’s why they saw so much carbon dioxide in Venus’s atmosphere, even though it wasn’t being protected by an ozone layer.

When UV radiation broke apart CO2, a small army of hydrogen atoms would just put it back together again. It took decades to prove that this was actually happening in the Venusian skies, along with a similar process that involved chlorine instead of hydrogen. But in the meantime, a pair of chemists named Mario Molina and Sherwood Rowland realized that something similar was happening on Earth, too, which meant we had a major problem.

CFCs might be totally unreactive down here on the surface, but they realized that would change once the molecules reached the upper atmosphere and got bombarded by that powerful UV radiation. The UV could break CFCs apart and leave chlorine atoms free to rip oxygen off of anything nearby, just like hydrogen does on Venus when it’s putting carbon dioxide back together. So all those CFCs in the upper atmosphere would lead to a lot of chlorine stealing oxygen atoms from ozone molecules, which meant that there were fewer ozone molecules around to block UV light.

And a single chlorine atom could tear apart a hundred thousand ozone molecules before permanently bonding to something. So Molina and Rowland were saying that those supposedly unreactive CFCs were eating away at the ozone layer that protects us from things like skin cancer, and protects the entire world from widespread crop failures. But everyone making CFCs wasn’t just going to stop because a couple chemists said so.

Molina and Rowland were studying Venus, not Earth. And besides, everyone knew that CFCs didn’t react with anything. That’s what made them so great!

But soon, the evidence started piling up. In 1985, scientists discovered a hole in the ozone layer above Antarctica that was getting bigger every year. Some people were still skeptical, though, until researchers found chlorine and fluorine in the hole itself, and others found that the ozone layer was getting thinner all over the world.

Today, there is no doubt about it: Molina and Rowland were right. And in 1995, they won the Nobel Prize in Chemistry for their discovery. Since the eighties, international agreements have just about stopped CFC production around the world, and corporations have been looking for safer alternatives.

So far, they haven’t found many good ones, and the ones they have found generally still aren’t great for the environment. But at least they’re better than CFCs. And in just the last couple of years, scientists have finally seen the hole over Antarctica start to shrink, more than forty years after a technical mystery in the Venusian skies accidentally saved the planet.

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