Geophysicists propose new theory to explain origin of water


The idea that Earth’s water came only from asteroids is a ‘blind spot’, researchers say. Nick Carne reports.


Oceans and asteroids share a similar chemical signature.

Oceans and asteroids share a similar chemical signature.

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Earth’s water may not have originated solely from material carried by asteroids, according to new US research.

A study in the Journal of Geophysical Research: Planets led by Steven Desch from Arizona State University challenges widely-accepted ideas about the asteroidal origin of hydrogen in the planet’s water by suggesting it came in part from the solar nebula – clouds of dust and gas left over after the formation of the Sun.

And the researchers believe their findings could provide new insights about the development of other planets and their potential to support life.

To date, many scientists have supported a theory that all of Earth’s water came from asteroids, primarily because the ratio of deuterium, a heavier hydrogen isotope, to normal hydrogen is similar in ocean and asteroidal samples.

Certainly the ocean chemical signature is close to what is found in asteroids, but that may not be the whole story, explains Desch.

“It’s a bit of a blind spot in the community,” he says. “When people measure the 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.”

Desch and his colleagues say more recent research suggests hydrogen in Earth’s oceans does not represent hydrogen throughout the entire planet. Samples taken from deep underground, close to the boundary between the core and mantle, have notably less deuterium, indicating this hydrogen may not have come from asteroids.

The noble gases helium and neon, with isotopic signatures inherited from the solar nebula, have also been found in the Earth’s mantle.

In the new study, the researchers developed a theoretical model which suggests that 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 until eventually a collision introduced enough energy to melt the surface of the largest one, creating an ocean of magma. This object eventually became Earth.

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 Earth’s centre. The element, , which is attracted to iron, was delivered to the core by the metal, while much of the deuterium remained in the magma, which eventually cooled and became the mantle. 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, perhaps as much as 2%, did indeed come from the solar nebula.

The authors say their study offers new perspectives about the development of other planets. Earth-like planets in other solar systems may not all have access to asteroids loaded with water, they say, but could still have obtained water through their system’s own nebulae.

“This model suggests that the inevitable formation of water would likely occur on any sufficiently large rocky exoplanets in extrasolar systems,” says co-author Jun Wu. “I think this is very exciting.”

  1. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018JE005698
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