Jason Davis • May 11, 2023
How did Earth get its oxygen?
There’s no place like home. Of all the wondrous worlds we’ve studied in our Solar System and beyond, none are quite like Earth. Our planet is covered in liquid water and harbors an oxygen-rich atmosphere that has allowed a multitude of plants and animals to evolve — including us.
It wasn’t always that way. If you could travel back in time to early Earth, your lungs would choke on an atmosphere devoid of oxygen. The gas that humans need to survive didn’t appear in significant quantities until roughly halfway through our planet’s current lifespan.
CRESCENT EARTH FROM ROSETTA Rosetta viewed Earth in a thin crescent phase as it approached for its November 13, 2009 flyby. This image is one in a series taken to make an animation of the rotating Earth over a 24-hour period.
Image: ESA / OSIRIS Team MPS / UPD / LAM / IAA / RSSD / INTA / UPM / DASP / IDA / Gordan Ugarković
So how did Earth’s atmosphere get its oxygen? The simple answer is that early microorganisms produced it using a process you may have learned about in elementary school: photosynthesis.
Photosynthesis is the process by which plants and other organisms use sunlight, water, and carbon dioxide to produce energy. As a byproduct, oxygen is released, which has over time given Earth an oxygen-rich atmosphere suitable for complex lifeforms.
Exactly how the Cosmos set the stage for this to happen is not known with complete certainty, but we know the basics. Let’s start at the beginning.
The care and feeding of early Earth
Our
Sun formed about
4.57 billion years ago from a cloud of collapsing dust and gas, in an unremarkable corner of the Milky Way galaxy. The material left over the Sun’s formation coalesced into everything else in our Solar System, including Earth, which was born around
4.55 billion years ago.
Earth was still practically a baby when a Mars-sized world named Theia is believed to have plowed into the planet about
4.51 billion years ago. Most of Earth’s mantle was
melted by the impact, and molten remnants from the collision formed the Moon. Vaporized rock and silicate clouds circled Earth, making our world shine like a small,
fiery Jupiter.