Last updated March 12, 2020 at 11:00 am
Billion-year-old rocks show the elements necessary for the evolution of life on Earth didn’t come until very late in the planet’s formation.
Why This Matters: Ancient rocks on Earth can shed insight into life on other planets.
A large proportion of the elements essential to the formation of oceans and life – such as water, carbon and nitrogen – only came to Earth very late in its history, researchers have found.
Many scientists previously believed that these elements had already been there at the beginning of our planet’s formation. However, the investigations have shown that most of the water only came to Earth when its formation was almost complete.
Seeing the history of life on Earth through a rock window
Volatile elements such as water originate from asteroids, the planetary building blocks that formed in the outer solar system. There has been a lot of discussion and controversy in the scientific community around when precisely these building blocks came to Earth.
Mario Fischer-Gödde, who led the work at the University of Cologne, says we are now able to narrow down the timeframe more precisely.
“The rocks we analysed are the oldest preserved mantle rocks. They allow us to see into the early history of the Earth as if through a window.
“We compared the composition of the oldest, approximately 3.8 billion-year-old, mantle rocks from the Archean Eon with the composition of the asteroids from which they formed, and with the composition of the Earth’s mantle today.”
To understand the temporal process, the researchers determined the isotope abundances of a very rare platinum metal called ruthenium, which the Archean mantle of the Earth contained.
Like a genetic fingerprint, the rare platinum metal is an indicator for the late growth phase of the Earth.
“Platinum metals like ruthenium have an extremely high tendency to combine with iron. Therefore, when the Earth formed, ruthenium must have been completely discharged into the Earth’s metallic core,” says Fischer-Gödde.
Understanding the origins of life on Earth, and in the Universe
Martin Van Kranendonk, the UNSW scientist who was part of the research, says the reason why this is of such interest relates directly to understanding the origins of life on Earth, how we humans came to be, and in fact, to whether we might be alone, or have neighbours in the universe.
“This is because the results show that Earth did not really become a habitable planet until relatively late in its accretionary history,” he says.
“If you combine this with the evidence for very ancient life on Earth, it reveals that life got started on our planet surprisingly quickly, within only a few hundred million years. Now this might sound like a lot of time, and it is, but it is far different from what we used to think, that life took half a billion, or even a billion years to get started.”
“And this gives hope for finding life on other planets that had a shorter geological history and period of ‘warm and wet’ conditions than Earth, because if life could get started quickly here, then perhaps it got started quickly elsewhere.”
Carsten Münker, also at the University of Cologne, adds: “The fact that we are still finding traces of rare platinum metals in the Earth’s mantle means that we can assume they were only added after the formation of the core was completed – they were certainly the result of later collisions of the Earth with asteroids or smaller planetesimals.”
Scientists refer to the very late building blocks of Earth, which arrived through these collisions, as the ‘late veneer’.
“Our findings suggest that water and other volatile elements such as carbon and nitrogen did indeed arrive on Earth very late in the ‘late veneer’ phase”, Fischer-Gödde says.