The atmosphere we enjoy today is radically different from the atmosphere that formed with the Earth billions of years ago. And yet, the Earth’s early atmosphere somehow transformed into the life giving atmosphere we enjoy today.
The Earth formed with the Sun 4.6 billion years ago. At this point, it was nothing more than a molten ball of rock surrounded by an atmosphere of hydrogen and helium. Because the Earth didn’t have a magnetic field to protect it yet, the intense solar wind from the young Sun blew this early atmosphere away.
As the Earth cooled enough to form a solid crust (4.4 billion years ago), it was covered with active volcanos. These volcanos spewed out gasses, like water vapor, carbon dioxide and ammonia. This early toxic atmosphere was nothing like the atmosphere we have today.
Light from the Sun broke down the ammonia molecules released by volcanos, releasing nitrogen into the atmosphere. Over billions of years, the quantity of nitrogen built up to the levels we see today.
Although life formed just a few hundred million years later, it wasn’t until the evolution of bacteria 3.3 billion years ago that really changed the early Earth atmosphere into the one we know today. During the period 2.7 to 2.2 billion years ago, these early bacteria – known as cyanobacteria – used energy from the Sun for photosynthesis, and release oxygen as a byproduct. They also sequestered carbon dioxide in organic molecules.
In just a few hundred million years, this bacteria completely changed the Earth’s atmosphere composition, bringing us to our current mixture of 21% oxygen and 78% nitrogen.
We have written many articles about the Earth for Universe Today. Here’s an article about how the Earth’s early atmosphere was very different from the one we see today, and an another that describes how Titan’s atmosphere is probably similar to the Earth’s early atmosphere.
We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.