Watery Earth 
Water covers more than two-thirds of Earth’s surface. But there’s a debate about where all that water came from. Some scientists say that Earth was born wet, while others argue that the water was dumped here later on.
One key bit of evidence is the ratio of normal hydrogen to heavy hydrogen in today’s oceans. The ratio varied in different regions of the early solar system. So comparing the ratio found in the oceans to that of other solar-system bodies could tell us where Earth’s water came from.
One possibility is a class of asteroids — mountain-sized boulders that contain a lot of water. Many of them pounded the young Earth, perhaps delivering much of the planet’s water.
The ratio of hydrogen to heavy hydrogen is about the same in some comets — giant balls of frozen water and gases mixed with rock. They, too, pelted the young Earth. But there’s also support for the idea that the water was created along with Earth itself.
The young Earth might have had a bloated atmosphere made of hydrogen. The hydrogen mixed with oxygen to make water. According to this idea, the ratio of the forms of hydrogen has changed over time — until it’s a match for that found in today’s oceans.
Wherever the water on Earth came from, though, there’s agreement that lots of water was sprinkled throughout the young solar system. In fact, astronomers recently discovered lots of water in another young star system. More about that tomorrow.
Script by Damond Benningfield, Copyright 2011