Where did all the water come from?
Water, water everywhere. It covers 70 percent of Earth’s surface and earns it the nickname “the blue planet.” And yet… where did it come from? How is it so abundant on our planet when it’s almost nonexistent throughout the rest of our solar system? Most scientists believe that when planet Earth formed, about four and a half billion years ago, it was a dry, rocky planet. The most popular scientific theory states that the H2O arrived in the form of several enormous asteroids filled with ice. Another suggests that the water actually has been around since Earth’s formation, and that the forming Earth retained it from the cloud of gas and dust that formed the solar system. However it happened, though, it’s certainly worked out well for Earth’s life forms. These are the most baffling unsolved mysteries about the universe.
What about all the oxygen?
Another thing that’s proved very advantageous for the creatures living on Earth is the planet’s oxygen. And while we actually do know how it originated—about 2.4 billion years ago, microscopic creatures called cyanobacteria released oxygen as a waste product, filling the atmosphere with it—the happenings of the next few eons are a little less clear. After that, the level of oxygen in Earth’s atmosphere went wildly up and down until it finally stabilized around 540 million years ago. Since then, it’s remained at about the breathable level we experience today. But what caused it to suddenly stabilize? This remains one of the biggest scientific mysteries about our home planet.