If the new research turns out to be true, it could help scientists figure out where life came from.
According to new research, the Earth used to be mostly water, with little or no land in sight. And that could mean a lot about how life started and how it changed over time.
New research shows that Earth was a true ocean world about 3 billion years ago, even though about 70% of its surface is covered by water now. At this point, only a few archipelagos broke the salty surface of our world’s ocean. That is, if there were any land.
The scientists’ conclusions were based on unique rocks they found in the Panorama district of Western Australia. Researchers figured out that the rocks were made in a hydrothermal vent system on the sea floor about 3.24 billion years ago. This is because rocks have marks that show where they were made. Over time, the rocks turned on their sides and were exposed. This made it easy for scientists to study Earth’s watery past from dry land. This led them to think that the ancient Earth might have been mostly water with no big landmasses.
“An early Earth without emerging continents may have looked like a ‘water world,'” the authors of the new study, which came out March 2 in Nature Geoscience, wrote. “This is an important environmental constraint on the origin and evolution of life on Earth, as well as its possible existence elsewhere.”
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Lots and lots of water
Even though there are a lot of oceans on Earth now, there are still a lot of questions about where they came from. Did the Earth always have water on it, or did it come later? How much later, if later? And did comets, asteroids, or something else bring the water?
Scientists still think about these and other questions. This is because there is clear evidence, like ancient minerals called zircons that seem to have formed in a watery environment, that Earth had water right after it formed, about 4.4 billion years ago. That’s a long history of the ocean.
But it’s not as clear how much water the Earth had back then. And the researchers were able to look into this question by looking at their piece of the old seafloor.
The network for oxygen
When rocks are made in water, the water leaves its mark on the rock. Hydrogen and oxygen are always what make up water, or H2O. But the type of oxygen in the water, called an isotope, also tells us something about the environment where the water formed. For example, how hot it was or how water moved back and forth between the land, the sea, and the air over time.
There are two types of oxygen that are commonly used. Oxygen-16 (O16) is a lighter version with eight protons and eight neutrons. And oxygen-18 (O18), which is heavier and has eight protons and ten neutrons. These two extra neutrons make O18 heavier, which means that water molecules with O16 can evaporate faster than those with O18. Also, rocks and dry land are more likely to pick up and store O18, taking it out of the sea.
When the people who wrote the new study looked at their piece of ancient seafloor, they found a lot of O18, on average more than is found in our oceans today. And because dry land is a huge source of heavy oxygen, the fact that there was a lot of O18 in the early Earth suggests that there wasn’t any dry land. The researchers figured out that the most likely reason why their sample had more heavy oxygen than usual was that land hadn’t yet come up from the ancient ocean.
What it means for life
Scientists often disagree about where the first single-celled organisms on Earth came from. Did life start near hydrothermal vents in the ocean, where there was both heat and water with lots of minerals? Or did life start on land, maybe near what Darwin thought was a warm little pond? There are many different ideas, and scientists don’t know for sure yet.
But if more research shows that the early Earth really was completely covered in water, that could help scientists make their theories about how life started even better.
Boswell Wing, a geology professor at the University of Colorado Boulder, said in a statement, “The history of life on Earth follows available niches.” “If you have a water world, one that is mostly ocean, you won’t be able to find dry niches.”
In other words, life couldn’t have started on land at all if the Earth was covered in water when it first started. And if that turns out to be true, it would mean that exoplanets that are covered in water all the way to the surface might be the best places to look for alien life. But let’s not get too far ahead of ourselves.
Even though this seafloor sample from Australia only shows one moment in time, it covers a large area that has been well preserved. So, the researchers hope to do similar research on rock samples from all over Earth’s history to figure out how the continents formed. These samples are in Africa, Canada, New Mexico, and Arizona. They are spread out over a few billion years on Earth. Together, they will tell the story of when Earth stopped being an ocean and became the dry land we live on now.