The oort cloud
The oort cloud is theoretically thought to be a cold, spherical cloud around the sun, with a maximum radius of about a light-year and full of inactive comets. Scientists suspect that the earth's water came from comets.
Water on earth contains two kinds of water, each 9,997 of every 10,000 molecules is "normal water", or H2O, and the rest is D2O (a compound of deuterium and oxygen, deuterium d o being an isotope of hydrogen, known as heavy hydrogen), which is called heavy water because the molecule has about 11% more mass than water.
If comets are the main source of water on earth, the European space agency's space vehicle in 1986 successfully approached the core of Halley's comet from the oort cloud at a distance of 596 kilometers and became the first spacecraft to observe a comet up close. The analysis shows that the comet, which formed 4.5 billion years ago, was made of volatile material (mostly ice) that gathered on interstellar dust particles. Halley's comet is made up of 80 percent water, but it turns out the comet is twice as heavy as the water on earth.
The kuiper belt
Another comet comes from the kuiper belt, the European space agency's 2004 probe that landed on a comet in 2014, becoming the first spacecraft to land on a comet's nucleus and bringing back a wealth of valuable data. If the kuiper belt were the main source of earth's water, the deuterium-hydrogen ratio on earth would be much higher than it is now.
Asteroids in the solar system
Most asteroids today have almost no water, but were probably rich in water billions of years ago, bringing a lot of water to earth after they hit, but now they've lost most of it because of the heat from the sun. In future studies of icy asteroids, scientists may find more information to prove the point.
The earth itself produces water
Small amounts of water may come from volcanic activity, which occurs when water vapor in the erupting atmosphere condenses into rain, slowly filling earth's ocean basins. Some land water may be produced by REDOX reaction and photosynthesis during the great oxidation process. In the 1930s, scientists discovered sulfide-dependent chemoautotrophs - purple sulfur bacteria - that use hydrogen sulfide and carbon dioxide to fix carbon and synthesize water as a byproduct of the photosynthetic pathway. Few modern organisms use this method of photosynthesis.
Water may have existed when the earth was born, according to new research. A thousand kilometers deep in the earth's rock layer, it locks up a huge amount of water, enough to fill all the oceans on earth today, but in minerals like hydrous olivine. This completely refutes the hypothesis that all the water originated from outer space, but more evidence is needed to prove it. After all, human exploration of the earth is just scratching the surface.
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