Imagine a superhot rocky planet. It’s so hot, all the rocks are a hot, glowing liquid. The pull of a giant star keeps the liquid moving in tides. It swirls around and around the center of the planet. Rain falls sometimes. But the heat turns the rainwater to steam.
This may sound like another planet, but that’s what the Earth was like—about 4.5 billion years ago. The giant star was the Sun.
Scientists aren’t sure where the oceans’ water came from. Some think it came from water vapor given off by the hot, melted rock. Others think the water flowed out of the rocks as they cooled.
Later, the rocks and land released salt into the water. Then, very, very slowly, tiny creatures began to form in the water. It took them millions of years to turn into today’s ocean life. Later, land creatures evolved from life forms in the oceans.
Today, Earth is home to more than 300 million cubic miles of seawater. Most seawater has the same amounts of salt and other chemicals.
