How Did Water Get on the Earth?

The Earth now has vast oceans, but when it first formed, it was a dry rock, and to find out where our oceans and lakes got their water, keep reading below.

In the fields of planetary science, astronomy, and astrobiology, the origin of water on Earth is a hot topic.

The only planet known to contain seas of liquid water on its surface is Earth, which is unusual among the rocky planets in the Solar System. 

Liquid water, which is necessary for life as we know it, continues to exist on the Earth's surface because the planet is in the habitable zone, which is a distance from the Sun far enough that it does not lose its water to the runaway greenhouse effect, but not far enough that all water on the planet freezes due to low temperatures.


How Did Water Get on the Earth?

A protoplanetary disk—a vast disc of gas and dust spinning around our freshly created Sun—is the starting point for our current model of planet formation. As the dust and ice grains in the disc contact with one another, they begin to form larger and larger clumps. These clumps eventually merge into planetesimals, the basic components of rocky and massive planets.

However, at the time when our solar system was forming, that disc was significantly hotter at the location where our Earth currently rests. Even though water molecules were most likely present in the tangle of debris that made up the disc, the temperature was too high for water to form into a liquid, so it evaporated instead.

Furthermore, because the early Earth lacked an atmosphere, any liquid water droplets were more easily blasted into space. As a result, we're left with a bit of a puzzle. How did they get here if the Earth couldn't have formed from the disc with its oceans intact?

However, a recent study suggests that hydrogen from within the Earth had a role in the creation of the oceans. Water was brought to Earth by impacts from ice planetesimals comparable in composition to asteroids in the asteroid belt's outer reaches, thus the two theories aren't mutually incompatible.

Water Formation In Early Earth

The very early Earth's surface was once a magma ocean. The first atmosphere was formed when hydrogen and noble gases from the solar nebula were attracted to the planetary embryo. The magma ocean's molten iron dissolved nebular hydrogen, which has less deuterium and is lighter than asteroidal hydrogen.

The hydrogen was then pulled into the Earth's core, a process known as isotopic fractionation. The heavier isotope, deuterium, stayed in the magma that eventually cooled to form the mantle, while hydrogen was transported to the core by its attraction to iron. Earth's eventual size was determined by impacts from smaller planetary embryos and other objects, which continued to contribute water and mass.

Comets and Asteroids.

Comets also have higher deuterium-to-hydrogen (D/H) ratios, making them poor sources of water for Earth. The D/H ratio of hydrogen gas in the solar nebula was only 21 ppm, which is far too low to have provided most of Earth's water. Asteroids, as well as the solar nebula, are a much better match.


The origin of the Earth's water has long been debated, but a recent study suggests that the solar nebula after the sun's formation, as well as asteroids, maybe the reason why we have water on Earth. 

However, in the future, we may discover much more information about the planet's formation and the solar nebula, which will help us to confirm these studies.

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