Today’s Wonder of the Day was inspired by Maanit. Maanit Wonders, “How did water form on the Earth?” Thanks for WONDERing with us, Maanit!

Splash! There’s nothing like playing in the water on a hot summer day. And what could be better than a cool drink of water after working or playing? In fact, we use water in many, many ways to make our lives better. Life would be a lot harder if we didn’t have water to help us keep our clothes and bodies clean. And, of course, we couldn’t live at all if we didn’t have water to drink! It’s easy to forget how lucky we are to have access to water. But have you ever WONDERed, where did all of our water come from? Is Earth the only planet with so much water?

To find out where water came from, we have to think about what water is made of. You may already know that everything in our world is made from tiny building blocks. These tiny blocks are called atoms. When atoms join together, they make up molecules. Water is made of a molecule called dihydrogen oxide. That means it has two hydrogen atoms and one oxygen atom. You may have heard the short version of the name—H2O.

So, when we’re WONDERing about where water came from, we have to first think about where hydrogen and oxygen came from. To figure that out, we need to go all the way back to the beginnings of our universe! Hydrogen has been around since nearly the start of the universe. It is the simplest of the elements. Oxygen did not exist until much later—hundreds of millions of years later!

Inside the first stars, it was so hot that hydrogen began to form other elements. Those included oxygen. Then, after a long time, these stars died and exploded. These explosions spread new elements around the universe. In turn, these elements joined to form new substances. H2O, or water, was one of these. Water molecules were present from the beginnings of our solar system.

There were also water molecules inside the Earth as it formed. But does that account for the water that we see on Earth today? Scientists are not completely sure! There are several possible sources of water. Scientists theorize that both of these explanations may be partially true. The first is that water molecules that are deep inside the Earth slowly come to the surface over many years. In other words, the water that covers much of the Earth has been a part of the planet since the beginning!

Another theory is that water molecules came to Earth from other parts of space. In the early years of our solar system, many asteroids and comets crashed on the Earth. These brought more water to our planet. It is most likely that water deep in the Earth, along with water from asteroids and comets, combined to give Earth so much water. We have so much water, in fact, that over 70% of the Earth’s surface is covered with it!

You may be WONDERing, what about other planets. The Earth is not the only planet with plenty of H2O. In fact, we know that Neptune and Uranus both have a lot of water. The Earth is special because of our temperature. If we were closer to the sun, and our planet was much hotter, our water might all evaporate. If we were farther away from the sun, our planet would be much colder. Our water might freeze into ice, as it has on Neptune and Uranus. The Earth has a mix of warm and cold temperatures, so that some of our water is gas, some liquid, and some frozen.

The origins of water are amazing to think about! What do you think scientists will discover next about our universe?


Wonder What's Next?

Find out how the donkey and the elephant became the official symbols for two of America’s largest political parties.