Earth has long been a blue planet. Our globe is some 4.5 billion years old and supports a vast amount of water. And Earth's rocks reveal that for most of its history--during at least the last 3.9 billion years, and possibly much longer--the planet has had liquid water.
How did all the water we see around us get here? Most scientists argue for an origin in the planet itself. Water was present in the cloud of gases and dust from which Earth formed, and volcanic eruptions on early Earth must have brought much of it to the surface in the form of water vapor. Others speculate that some water was delivered from outer space, by comets or asteroids crashing into Earth.
The steam trail from this volcano in Alaska is condensed water vapor released from magma as it rose through Earth's crust. That water vapor becomes part of Earth's atmosphere. According to experts, there may be many oceans' worth of water locked up in the minerals that make up the deep Earth.
A Visitor From Space
During its first several hundred million years, Earth was slammed by a constant hail of space debris--comets and asteroids. Such impacts still happen today, though rarely. Tens of millions of meteorites and comets became part of the original mass of early Earth. As Earth cooled, massive volcanoes released that water as vapor, which condensed into liquid water as cooling continued. This process continues today.
The Wrong Water
Comets are largely mineral dust and ice, and one the size of Hale-Bopp can contain a trillion tons of water, giving rise to speculation that comets were Earth's water bearers. But when Hale-Bopp came close enough to Earth for detailed study, scientists realized comets probably didn't bring much water to Earth after the planet formed, because the hydrogen molecules in cometary water have, on average, heavier nuclei than the hydrogen in earthly water. Today, most experts think the contribution made by comets, if any, was relatively small.