Imperial College London researchers have uncovered the likely origin of Earth’s volatile chemicals, some of which form the building blocks of life. The results, published in Science, show that around half of Earth’s inventory of the volatile element zinc came from asteroids originating in the outer solar system, beyond the asteroid belt that includes the planets Jupiter, Saturn, and Uranus.
Previously, scientists thought that most of Earth’s volatiles came from asteroids that formed closer to the Earth. However, the new findings reveal that the outer solar system played a much bigger role than previously thought in supplying these vital elements.
Volatiles are elements or compounds that change from solid or liquid state into vapor at relatively low temperatures. They include the six most common elements found in living organisms, as well as water. As such, the addition of this material will have been important for the emergence of life on Earth.
“Our data show that about half of Earth’s zinc inventory was delivered by material from the outer solar system, beyond the orbit of Jupiter,” says Senior author Professor Mark Rehkämper, of Imperial College London’s Department of Earth Science and Engineering. “Based on current models of early solar system development, this was completely unexpected.”
To carry out the study, the researchers examined 18 meteorites of varying origins, eleven from the inner solar system, known as non-carbonaceous meteorites, and seven from the outer solar system, known as carbonaceous meteorites. They measured the relative abundances of the five different forms of zinc, or isotopes, and compared each isotopic fingerprint with Earth samples to estimate how much each of these materials contributed to the Earth’s zinc inventory.
The results suggest that while the Earth only incorporated about ten percent of its mass from carbonaceous bodies, this material supplied about half of Earth’s zinc. The researchers also suggest that material with a high concentration of zinc and other volatile constituents is also likely to be relatively abundant in water, giving clues about the origin of Earth’s water.
“We’ve long known that some carbonaceous material was added to the Earth, but our findings suggest that this material played a key role in establishing our budget of volatile elements, some of which are essential for life to flourish,” says first author on the paper, Ph.D. candidate Rayssa Martins, at the Department of Earth Science and Engineering.
The next step for the researchers is to analyze rocks from Mars and the moon. “The widely held theory is that the moon formed when a huge asteroid smashed into an embryonic Earth about 4.5 billion years ago. Analyzing zinc isotopes in moon rocks will help us to test this hypothesis and determine whether the colliding asteroid played an important part in delivering volatiles, including water, to the Earth,” says Professor Rehkämper.
This discovery not only provides important clues about how Earth came to harbor the special conditions needed to sustain life, but also opens the door to a deeper understanding of the early solar system and the role of outer solar system material in the formation and evolution of planets.