Where did the building blocks of Earth's first water come from? At least in part, from a cloud of gas and dust circling the young Sun, new research suggests.
Water is made up of two hydrogen atoms and one oxygen atom. Most of this hydrogen was probably brought to Earth by rocky asteroids billions of years ago. However, new research suggests that the young Earth also received hydrogen from the solar nebula.
"About one in every 100 water molecules on Earth originated from the solar nebula," the researchers write. In the early solar system, this cloud - left over from the formation of the sun - contained large amounts of hydrogen.
To find out the origin of water on Earth, scientists studied its chemical traces, considering the ratio of hydrogen isotopes - variants of hydrogen with different numbers of neutrons and, therefore, different atomic masses.
The ratio of normal hydrogen to deuterium - the heavier isotope - in ocean water matches the ratio in water of asteroids, indicating that Earth's water has asteroid roots. However, water extracted from the planet's interior, near the region where the mantle meets the core, tells a different story. According to the study, the ratio of deuterium to hydrogen in it is lower compared to ocean water, indicating a source other than asteroids.
The earth formed billions of years ago when small asteroids collided and merged into a larger body. While the newborn planet was taking shape, it took dust and gas from the Solar Nebula. The nebula's hydrogen was drowning in molten magma, heading for its core, while hydrogen from the asteroids was trapped in what eventually became the mantle.
Asteroids bombarded the Earth and dispersed hydrogen with a higher deuterium ratio in the mantle and in the oceans. While this hydrogen makes up most of the planet's water, scientists have concluded that some of the water owes its formation to hydrogen from the nebula.
What's more, it is estimated that the amount of water hidden within the planet is about two oceans in the mantle and four to five oceans in the core, most of which probably originated from the material of the solar nebula, scientists say.
Promotional video:
The results also hint that similar processes may be behind the formation of water on distant exoplanets.
The study is published in the Journal of Geophysical Research: Planets.