Sunlight might have put oxygen in Earth’s early air

Laser experiments suggest how the sun might have seeded Earth's early air with oxygen gas

The thin layer of Earth’s atmosphere hasn’t always contained oxygen. An experiment that used lasers to zap carbon dioxide suggests ultraviolet light from the sun might have helped boost oxygen gas levels in Earth's ancient atmosphere.

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Breaking up isn’t always hard to do — at least for some chemicals, such as carbon dioxide. A blast of ultraviolet light may be all it takes, new tests show. The finding suggests scientists may have been wrong about how Earth’s atmosphere got enough oxygen to sustain species (like us) that need this gas to breathe. Sunlight may have kick-started the buildup, not photosynthesis.

In a new experiment, researchers used a laser to decouple a molecule of carbon dioxide, or CO2. It yielded both carbon and oxygen gas, also known as O2.

Air hasn’t always been rich in oxygen. Billions of years ago, other gases dominated. Carbon dioxide was one of them. At some point, algae and plants developed photosynthesis. This allowed them to make food from sunlight, water and carbon dioxide. One byproduct of this process is oxygen gas. And that’s why many scientists had argued photosynthesis must have been behind the buildup of oxygen in Earth’s early atmosphere.

But the new study suggests that ultraviolet light from the sun could have cleaved oxygen from carbon dioxide in the atmosphere. And this could have converted CO2 to carbon and O2 long before photosynthetic organisms evolved. The same process also may have produced oxygen on Venus and other lifeless planets rich in carbon dioxide, the researchers say.

The researchers “have made a beautiful set of challenging measurements,” says Simon North. A chemist at Texas A&M University in College Station, he did not work on the study. Scientists had suspected that the atoms in carbon dioxide could be decoupled to produce oxygen gas, he notes. But it has been difficult to prove that. That’s why the new data are so exciting, he told Science News.

How the process may work

In a molecule of carbon dioxide, a carbon atom sits between two oxygen atoms. When carbon dioxide breaks apart, the carbon atom usually escapes still attached to one oxygen atom. That leaves the other oxygen atom alone. But scientists had suspected a high-energy blast of light might allow other outcomes.

For their new tests, researchers assembled several lasers. These fired ultraviolet light at carbon dioxide. One laser broke up the molecules. Another measured the leftover debris. And it showed lone carbon molecules drifting around. That observation suggested the laser must also have produced oxygen gas.

The researchers are not sure exactly what happened. But they have their ideas. A blast of laser light might link the molecule’s outer oxygen atoms to one another. This would turn the carbon dioxide molecule into a tight ring. Now, if one oxygen atom lets go of the carbon atom next to it, the three atoms would align in a row. And the carbon would sit at one end. Eventually the two oxygen atoms might break free from their carbon neighbor. That would form a molecule of oxygen (O2). 

Cheuk-Yiu Ng is a chemist at the University of California, Davis, who worked on the study. He told Science News that high-energy ultraviolet light may trigger other surprising reactions. And the newfound reaction could happen on other planets. It might even seed the atmosphere of distant, lifeless planets with trace amounts of oxygen.

“This experiment opens up many possibilities,” he concludes.

Power Words

atmosphere   The envelope of gases surrounding Earth or another planet.

atom   The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and neutrally charged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

bond  (in chemistry) A semi-permanent attachment between atoms — or groups of atoms — in a molecule. It’s formed by an attractive force between the participating atoms. Once bonded, the atoms will work as a unit. To separate the component atoms, energy must be supplied to the molecule as heat or some other type of radiation.

carbon dioxide (or CO2)  A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter (including fossil fuels like oil or gas) is burned. Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.

chemistry  The field of science that deals with the composition, structure and properties of substances and how they interact with one another. Chemists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances. (about compounds) The term is used to refer to the recipe of a compound, the way it’s produced or some of its properties.

debris  Scattered fragments, typically of trash or of something that has been destroyed. Space debris includes the wreckage of defunct satellites and spacecraft.

laser  A device that generates an intense beam of coherent light of a single color. Lasers are used in drilling and cutting, alignment and guidance, in data storage and in surgery.

molecule  An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

oxygen  A gas that makes up about 21 percent of the atmosphere. All animals and many microorganisms need oxygen to fuel their metabolism.

photosynthesis  (verb: photosynthesize) The process by which green plants and some other organisms use sunlight to produce foods from carbon dioxide and water.

radiation    Energy, emitted by a source, that travels through space in waves or as moving subatomic particles. Examples include visible light, ultraviolet light, infrared energy and microwaves.          

species  A group of similar organisms capable of producing offspring that can survive and reproduce.

ultraviolet  A portion of the light spectrum that is close to violet but invisible to the human eye.

Venus   The second planet out from the sun, it has a rocky core, just as Earth does. However, Venus lost most of its water long ago. The sun’s ultraviolet radiation broke apart those water molecules, allowing their hydrogen atoms to escape into space. Volcanoes on the planet’s surface spewed high levels of carbon dioxide, which built up in the planet’s atmosphere. Today the air pressure at the planet’s surface is 100 times greater than on Earth, and the atmosphere now keeps the surface of Venus a brutal 460° Celsius (860° Fahrenheit).

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