Most astronomers believe a rogue planet collided with Earth about 4.5 billion years ago. The impact sent molten debris into orbit around Earth, some of which coalesced to form the moon.
Under this scenario, the heat of the impact should have vaporized light elements, including the hydrogen necessary for water to form.
The impactor, sometimes named Theia
, is thought to have been a little smaller than the current planet Mars
. It could have formed by accretion of matter about 150 million kilometres from both the Sun and Earth, at their fourth or fifth Lagrangian point
. Its orbit may have been stable at first, but destabilized as Theia’s mass increased due to the accretion of matter. Theia oscillated in larger and larger orbits around the Lagrangian point until it finally collided with Earth about 4.533 Ga
Models show that when an impactor this size struck the proto-Earth at a low angle and relatively low speed (8–20 km/sec), much material from the mantles (and proto-crusts) of the proto-Earth and the impactor was ejected into space, where much of it stayed in orbit around the Earth. This material would eventually form the Moon. However, the metallic cores of the impactor would have sunk through the Earth’s mantle to fuse with the Earth’s core, depleting the Moon of metallic material. The giant impact hypothesis thus explains the Moon’s abnormal composition. The ejecta in orbit around the Earth could have condensed into a single body within a couple of weeks. Under the influence of its own gravity, the ejected material became a more spherical body: the Moon.
The radiometric ages show the Earth existed already for at least 10 million years before the impact, enough time to allow for differentiation of the Earth’s primitive mantle and core. Then, when the impact occurred, only material from the mantle was ejected, leaving the Earth’s core of heavy siderophile elements untouched.
The impact had some important consequences for the young Earth. It released a gigantic amount of energy, causing both the Earth and Moon to be completely molten. Immediately after the impact, the Earth’s mantle was vigorously convecting, the surface was a large magma ocean. Due to the enormous amount of energy released, the planet’s first atmosphere must have been completely blown off. The impact is also thought to have changed Earth’s axis to produce the large 23.5° axial tilt that is responsible for Earth’s seasons (a simple, ideal model of the planets’ origins would have axial tilts of 0° with no recognizable seasons). It may also have sped up Earth’s rotation.
Because the Earth lacked an atmosphere immediately after the giant impact, cooling must have been fast. Within 150 million years a solid crust with a basaltic composition must have formed. The felsic continental crust of today did not yet exist. Within the Earth, further differentiation could only begin when the mantle had at least partly solidified again. Nevertheless, during the early Archaean (about 3.0 Ga) the mantle was still much hotter than today, probably around 1600°C. This means the fraction of partially molten material was still much larger than today.
Steam escaped from the crust, and more gases were released by volcanoes, completing the second atmosphere. Additional water was imported by bolide collisions, probably from asteroids ejected from the outer asteroid belt under the influence of Jupiter‘s gravity.
The asteroid belt is a ring of rocky debris between Jupiter and Mars, thought to have been created when Jupiter's mass made the area too unstable for planet formation. The debris is not evenly distributed, and the belt has zones where there are far fewer asteroids than expected, said Minton. Some of those gaps, called Kirkwood gaps, are in zones where Jupiter or Saturn's gravitational influence destabilises the asteroids so much that they are ejected from the belt, but many are in areas that are currently stable. Jupiter is thought to have formed slightly further away from the Sun than it is today, and Saturn, Uranus and Neptune were once closer, Minton said. The planets were subsequently dragged into their present positions by the gravity of large objects ejected from the Kuiper belt, a ring of icy debris lying beyond the planets. Once the Kuiper belt was depleted of large objects, the planets settled into their current orbits. The researchers designed a computer simulation that described the changing gravitational influence of the planets on the asteroid belt, and found that many of the unexplained gaps matched areas that would have been destabilised by Saturn or Jupiter's gravity during the planetary relocation. The destabilised asteroids would have been ejected from the belt, and many would have become projectiles that bombarded Earth and the other inner planets. The effects of all those impacting asteroids would have been quite dramatic and violent.[2
The large amount of water on Earth can never have been produced by volcanism and degassing alone. It is assumed the water was derived from impacting comets
that contained ice. Though most comets are today in orbits farther away from the Sun than Neptune
, computer simulations show they were originally far more common in the inner parts of the solar system. However, most of the water on Earth was probably derived from small impacting protoplanets, objects comparable with today’s small icy moons of the outer planets. Impacts of these objects can have enriched the terrestrial planets (Mercury
, the Earth and Mars) with water, carbon dioxide
and other volatiles
. If all water in the Earth’s oceans was derived from comets alone, a million impacting comets are required to explain the oceans. Computer simulations show this is not an unreasonable number.
As the planet cooled, clouds formed. Rain created the oceans. Recent evidence suggests the oceans may have begun forming by 4.2 Ga, or as early as 4.4 Ga.
In any event, by the start of the Archaean eon the Earth was already covered with oceans.
The National Geographic released a full, 1 hour version of this documentary in 2008, which is a must watch for anyone interested in the history of our planet. It is being broadcasted today on France 5 in the program ‘Superscience‘, for viewers in France.
Sources:  National Geographic  Cosmos Magazine  Wikipedia