Where Did Water Come From?

Civilisation has historically dominated rivers and water bodies. Mesopotamia, the cradle of civilisation, was situated between the Tigris and Euphrates. Ancient Egypt was dependent on the Nile. Rome too was founded on the banks of the Tiber. Water has no odour or colour yet it has been important for all living organisms despite not providing energy or organic multi-nutrients. Water covers 71% of the Earth's surface, with seas and oceans contributing to most of it. Groundwater as well as ice caps also contribute to this percentage. Water exists in solid, liquid and gaseous forms. It is an inorganic compound with the chemical formula H2O.

We all know about the water cycle where water moves continually through evaporation, transpiration, condensation, and precipitation. Water evaporates from water bodies to form clouds in the atmosphere. These clouds later, upon getting heavy, lead the water to fall back to Earth in the form of rain, snow, sleet or hail. This is how we get water now but how did Earth get its water in the first place? 

What is The Theory Behind Water on Earth?

Billions of years ago, Earth collided with another early planet, roughly the size of Mars, beginning the Hadean Eon. This was named after Hades, the Greek god of the underworld, in reference to the state of Earth being similar to the fiery hells. The planet’s surface was once around 2000 degrees Celcius. Over time, Earth did cool off which led to the presence of liquid water but how? It may seem nothing out of the ordinary to have water on Earth as it is even referred to as the Blue Planet for being majorly covered by water. But keep in mind that the Earth is too close to the Sun. It is the third planet from the Sun. Mercury and Venus have little to no water and though Mars has ice, it lacks liquid water and Mars is further away than Earth. 

Earth is a rocky planet, primarily composed of iron, silicon, magnesium, and oxygen. Though Earth had an abundant amount of oxygen at first, it lacked hydrogen. Water in the solar system isn't uncommon, it is just more common further away. Planets like Jupiter and Saturn are gas giants with an abundance of hydrogen atoms. But how did it reach Earth and when? Though debated, water coming from meteorites is a very popular theory. There’s a specific type of meteorite known as the chondrite. Both individual hydrogen and complete H2O molecules have become part of the minerals that make up the chondrite. The minerals namely serpentine, chlorite, and smectite.

Rocks are said to hold 18 times more water than the water bodies. To remove this water, rocks will have to be melted. The meteorites just landing on Earth’s surface would do the job as mentioned earlier, the planet was in the Hadean Eon. At that time, everything that touched the surface would melt straight into the magma oceans. This is where the lonely Oxygen atoms finally found Hydrogen. The newly formed water molecules would turn into vapour. But this process didn’t last long as the magma was cooling. Once the magma formed a solid lid of rock, no water could escape. As the rocky layer was formed, Earth got its liquid oceans. Much of the water couldn’t escape the earth’s surface when the magma was cooling, causing the water to get trapped in the mantle.

What Debunks This Theory? 

Though the concept of a meteorite with the missing ingredient of water reaching Earth and thus melting due to the hot temperature makes total sense, there was an issue with this theory. The chemical composition of hydrogen in the chondrite meteorites doesn’t match the chemical composition of hydrogen found in the vast majority of water in the rocks (the oceans aren’t a good representation of water from that time as they’re sure to have changed). When we refer to chemical composition, we refer to isotopes or types of hydrogen. One type of isotope only has one proton and electron while another, also known as the Deuterium, has a proton, electron and neutron. The additional neutron makes it “heavier” than the first previously mentioned type of hydrogen. The hydrogen in the rocks is much lighter than the hydrogen in the meteorites. Thus, questioning this theory. Though scientists have found a type of meteorite, known as enstatite, it is believed that there aren’t enough of them to contribute to the extensive oceans.

What is An Alternative Theory?

In 2021, scientists discovered intriguing samples from an asteroid that could potentially be the answer to this question. The samples had a uniquely high level of hydrogen but to claim anything, they had to mimic the process of their theory. They began by measuring the hydrogen and water content of olivine crystals before and after they were exposed to what was equivalent to solar wind. The result of their experiment proved that the olivine crystals were just like the asteroid samples. They had a crust built of water and light hydrogen.

This light hydrogen is said to have arrived directly from the Sun. Solar winds, composed mainly of protons, interact with space dust, capturing an electron and forming light hydrogen. Light hydrogen trapped in a rock with oxygen results in light water. Moreover, the rock is a meteorite that falls to Earth and comes in contact with the magma oceans. 

Though fascinating, neither of the theories discussed above has been declared as concrete evidence of the origin of water on Earth. The Earth we love is not remotely close to what it was during the Hadean Eon and neither were the early oceans. For instance, they aren’t 230 degrees Celcius hot now. The planet as well as the oceans had to cool down extensively to support life. The theory of water’s origin on earth is highly complex but it highlights how the simple things in life have a deeper explanation lying somewhere in space. Even if we don’t know how exactly water arrived, it is here, and it's our duty to preserve it.