Ocean

As Earth formed in a cloud of gas and dust more than 4.5 billion years ago, a huge amount of lighter elements, including hydrogen (H) and oxygen (O), became trapped inside the planet as the gases condensed and formed molten rock. Materials of different densities separated out; in the young planet's molten interior, heavy elements sank and light elements rose.

Gases rose through thousands of miles of molten and melting rock, to erupt on the surface through volcanoes and fissures.

Within the planet and above the surface, oxygen combined with hydrogen to form water (H₂0). Enormous quantities of water—enough to fill oceans if it were liquid—shrouded the globe as an incredibly dense atmosphere of water vapor. Near the top of the atmosphere, where heat could be lost to outer space, water vapor condensed to liquid and fell back into the water vapor layer below, cooling the layer. This atmospheric cooling process continued until the first raindrops fell to Earth's surface and flashed into steam.

Many geologists argue this process may have happened several times, because planetoids (rocks the size of moons or asteroids) were still colliding with the early earth until about 3.9 billion years ago. Monstrous planetoid impacts would have vaporized all the water on the planet's surface. Earth has been changed so much by plate tectonics that no vestige of its original appearance remains. Unlike Earth, the faces of the moon, Mars, and Mercury bear the marks of the turbulent earliest history of the solar system. Hellas Basin on Mars and Caloris Basin on Mercury are the scars of planet-shattering impacts. One of these giant craters can be seen from anywhere on Earth: the moon's large round dark "eye," called Mare Imbrium.