- A new theoretical model explains how did water come to Earth.
- Water on Earth may not have solely came from asteroid matter.
- Nearly 2% of Earth’s water originated from the solar nebula.
The origin of water is a longstanding enigma. Uncovering this mystery is key to accessing the Earth’s progressive history and its modern structure. It could help us understand how much water other planets in the solar system accreted, and provide in-depth details about the development of exoplanets (planets outside our solar system) and their tendency to support life.
There are many mutually compatible theories that explain how water could have accumulated on the surface of Earth over the past 4.5 billion years. Most of them agree with the fact that Earth’s water has originated from the substance carried by asteroids.
However, a new study from Arizona State University says something different. It suggests that water on Earth may not have solely came from asteroid matter, but also from the solar nebula – clouds of hydrogen gas and interstellar dust left over after the formation of the solar system.
What Other Theories Say About Water Origin?
So far, many researchers have supported theories that say entire water on Earth originated from asteroids, mainly because the ratio of usual hydrogen to deuterium (one of two stable isotopes of hydrogen) is similar in sea and samples obtained from asteroids. Surely, chemical compositions of ocean match with asteroidal substance, but it might not be the complete story.
The recent studies indicate hydrogen in the sea doesn’t account for the whole hydrogen present throughout the Earth. Samples extracted from the boundary between the mantle and core, contain significantly less amount of deuterium, which indicates that hydrogen couldn’t have had originated from asteroidal materials.
The Earth’s mantle consists of noble gases like neon and helium, with isotopic compositions acquired from the solar nebula.
The New Model
According to the new theoretical model, a few billion years ago, massive waterlogged asteroids (called planetary embryos) started forming into planets while the solar nebula was still swirling around the Sun.
These asteroids had gone through multiple collisions and evolved quickly before ultimately transforming into the ocean of magma. This celestial body finally became Earth.
Hydrogen and noble gases from the solar nebula were later inherited by the massive embryo (covered with magma) to create an atmosphere. The hydrogen from nebula was way lighter than hydrogen obtained from asteroids, and it contained less deuterium. Gradually, nebular hydrogen diffused into the ocean’s molten iron.
Artistic impression of gas and dust surrounding a planetary system | Credit: NASA
Then hydrogen was dragged towards the center of the Earth via isotopic fractionation process. The ‘iron-attracted’ element was pulled to the core, while most of the deuterium resided in the magma that ultimately cooled down and turned into the mantle.
Small embryos and other celestial bodies continued colliding on Earth, which added more water and mass until the planet reached its final volume. The new model leaves the planet with a lower ratio of deuterium and hydrogen in the core compared to mantle and sea, and it leaves noble gases deep inside the mantle.
The model gives an approximate amount of hydrogen inherited from each source. Although most of the hydrogen came from asteroids, some of the water on Earth, as much as two percent, originated from the solar nebula.
The study enables us to see the development of exoplanets from new perspectives. Planets similar to Earth could also have gathered water/hydrogen through their own nebula.