How did Earth get its oceans?

This question has only been answered recently. At one time the theory is that all of the water for Earth's oceans came from comets, but there are a few things wrong with that theory. One is that the amount of heavy water (deuterium dioxide) in comets is not the same as that of our oceans. So, it was back to the drawing boards, so to speak. They determined this disparity by observing the spectrum of a comet's tail.

Hydrogen and deuterium have slightly different spectra. All water contains heavy water. Actually, they call it semi-heavy water because deuterium and hydrogen atoms get exchanged between water molecules. In other words, you have a water molecule with HDO, H2O and D2O in dynamic equilibrium.

The main problem is that when our star ignited about 4.7 billion years ago, it blew the ice in the accretion disc (the ring of dust and ice that the star formed out of) into what cosmologists call the 'Snow Line'. This means that the ice orbiting the new star was far enough away not to be vaporized, which means that the stuff (dust and rocks) that the Earth formed out of was dry. Or was it?

The new theory says that Jupiter formed from the leftover gases and water that the new star didn't use. The problem is that Jupiter had to plow through rocks and ice on its orbit around the new star. This caused a drag that made Jupiter fall closer to the sun, causing ice to come closer in to the inner solar system. What happened next is strange, but it makes sense. The ice became part of the asteroids that formed in the region between Jupiter and Mars. This asteroid belt formed because Jupiter's gravity was too strong for the debris to form a planet, or if it did form it was torn apart. In other words, the asteroids have enormous amounts of water in them. The asteroid belt should have been a fifth rocky planet (Mercury, Venus, Earth and Mars are the other four).

The theory goes that before Saturn, Neptune and Uranus formed, Jupiter brought enough ice into the inner solar system so that Earth got its ocean from asteroids loaded with water during the late heavy bombardment period around 4.1 to 3.8 billion years ago. Jupiter came in to around Mar's orbit before being pulled back to where it is now by the gravitational effect of Saturn and Neptune.

Evidence for the late heavy bombardment is etched into the Moon's surface. All those craters show the violence of that period. The Earth being larger received many more collisions. The reason we don't see them on Earth is because of climatic erosion over time. When the asteroids hit the Earth, they made the crust melt again and the water in the asteroids (in the form of hydrated salts) was heated enough to release the water as vapor that formed clouds. Storms developed to rain the water down on the surface. If you find a meteorite from that period and heat in in a test tube you'll see how it releases its hydrated water.

The reason why this idea is accepted is because the ice in asteroids has the correct amount of heavy water. It matches that in our ocean water.
However, around 300 miles down into Earth's mantel, there is enough water to equal many oceans. This has been revealed from volcanic eruptions. This demonstrates that the Earth did have lots of water, even oceans, before a Mar's sized planet collided with it, resulting in the formation of the Moon. This collision blew lots of the water away, and it probably killed off any life that had formed. This is another reason to believe that the oceans were brought back during the late heavy bombardment period.

These theories are not complete. They will be adjusted as new data is discovered. No one knows for sure what happened. It's just based on observations in space, on the Moon, and here on Earth.

Thanks for reading.