Earth’s water may have originated from both asteroids and gas left over from the formation of the Sun according to new research. The discovery has profound implications for both the origins of life on Earth and the possibility of life elsewhere in the Universe.

The study, published in Journal of Geophysical Research: Planets, casts doubt on previously held belief of the source of Earth’s water by suggesting that the hydrogen which comprises the majority of the water molecule may have come from the solar nebula, clouds of gas and dust left over from our star’s formation.

previous models have suggested that Earth’s water was delivered via asteroid impact

Previous models have connected the water found on Earth to that found on asteroids implying that the majority of water delivered to the planet’s surface was a result of impacts from these objects. The ration of deuterium, a heavier isotope of hydrogen, in seawater is similar to the ratio found on asteroids. Scientists have traditionally used this method of tracing hydrogen to discover water’s origins simply because it is the most abundant element in the universe.

Earth’s horizon as seen onboard the International Space Station (NASA)

To conduct their investigation the team led by Peter Buseck, Regents’ Professor in the School of Earth and Space Exploration and School of Molecular Sciences at Arizona State University, took a different approach.

Steven Desch, a professor of astrophysics in the School of Earth and Space Exploration at Arizona State University in Tempe, Arizona and co-author of the new study, explains: “When people measure the [deuterium-to-hydrogen] ratio in ocean water and they see that it is pretty close to what we see in asteroids, it was always easy to believe it all came from asteroids.”

The authors of the paper went further, suggesting that hydrogen in Earth’s oceans does not represent hydrogen throughout the entire planet. Samples of hydrogen from deep inside the Earth, close to the boundary between the core and mantle, have notably less deuterium, indicating this hydrogen may not have come from asteroids. Noble gases helium and neon, with isotopic signatures inherited from the solar nebula, have also been found in the Earth’s mantle.

Their new theoretical model aims to explain these discrepancies.

A New Model of Earth’s Origins

According to the new model suggested in the paper, several billion years ago, large waterlogged asteroids began developing into planets while the solar nebula still swirled around the Sun.

These asteroids, known as planetary embryos, collided and grew rapidly. Eventually, a collision introduced enough energy to melt the surface of the largest embryo into an ocean of magma. This largest embryo would eventually become Earth.

Artist’s conception of the dust and gas surrounding a newly formed planetary system (NASA)

Gases from the solar nebula, including hydrogen and noble gases, were drawn in by the large, magma-covered embryo to form an early atmosphere. Nebular hydrogen, which contains less deuterium and is lighter than asteroidal hydrogen, dissolved into the molten iron of the magma ocean.

Through a process called isotopic fractionation, hydrogen was pulled towards the young Earth’s centre. Hydrogen, which is attracted to iron, was delivered to the core by the metal, while much of the heavier isotope, deuterium, remained in the magma which eventually cooled and became the mantle, according to the study’s authors. Impacts from smaller embryos and other objects then continued to add water and overall mass until Earth reached its final size.

This new model would leave Earth with noble gases deep inside its mantle and a lower deuterium-to-hydrogen ratio in its core than in its mantle and oceans.

The authors used the model to estimate how much hydrogen came from each source. They concluded most was asteroidal in origin, but some of Earth’s water did come from the solar nebula.

“For every 100 molecules of Earth’s water, there are one or two coming from the solar nebula,” said Jun Wu, assistant research professor in the School of Molecular Sciences and School of Earth and Space Exploration at Arizona State University and lead author of the study.

A more Insightful model?

As we clearly understand the importance of liquid water to life, this model may well offer scientists new perspectives on the development of life and other planet’s ability to support. It suggests that despite not having access to a supply of water provided by asteroid impacts, exoplanets may still have liquid water as a result of left-over gas solar nebulas in their systems.

Wu said: “This model suggests that the inevitable formation of water would likely occur on any sufficiently large rocky exoplanets in extrasolar systems.

“I think this is very exciting.”

Anat Shahar, a geochemist at the Carnegie Institution for Science, who was not involved with the study, was also impressed by this new model: “This paper is a very creative alternative to what is an old problem.

“The authors have done a good job of estimating what these different fractionation factors would be without having the experiments.”

As Shahar suggests, the next step for astrochemists is the development of experimentation to test this model. This may involve an investigation into the hydrogen fractionation factor, which describes how the deuterium-to-hydrogen ratio changes when the element dissolves in iron, something that is both currently unknown and difficult to measure.

Original research: “Origin of Earth’s Water: Chondritic Inheritance Plus Nebular Ingassing and Storage of Hydrogen in the Core” Wu, Desch, Schaefer, et al,(2018), Journal of Geophysical Research-Planets