Building blocks for life on Earth arrived later than we thought

An international team of geologists has found evidence that a large proportion of the elements that are essential to the formation of oceans and life — such as water, carbon and nitrogen — only came to Earth very late in its history; much later than previously thought.

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, with many scientists believing that these elements were already there at the beginning of our planet’s formation.

Now, thanks to a study published in the journal Nature, geologists have been able to narrow down this time frame more precisely. This is because Earth’s oldest mantle is openly accessible in surface outcrops in southwest Greenland, allowing the scientists to easily collect rock samples.

“The rocks we analysed are the oldest preserved mantle rocks,” said Dr Mario Fischer-Gödde from the University of Cologne, first author on the study. “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 was already present in Earth’s mantle by Archean time. Like a genetic fingerprint, this rare platinum metal is an indicator for the late growth phase of the Earth.

“Platinum group metals like ruthenium have an extremely high tendency to combine with iron,” Dr Fischer-Gödde said. “Therefore, when the Earth formed all ruthenium must have been completely sequestered into the Earth’s metallic core.”

The pristine ruthenium preserved in the old mantle rocks most likely originates from the inner part of the solar system, the geologists report. It is presumably the same material that — for the most part — also formed Mercury and Venus. The reference values for the asteroidal ruthenium were previously obtained from meteorites found on Earth.

“If we still find traces of the rare platinum metals in the Earth’s mantle, we can assume that they were only added after the formation of the core was completed,” added co-author Professor Dr Carsten Münker, also from the University of Cologne. “They were certainly added during later collisions of the Earth with asteroids or smaller protoplanets, so called planetesimals.”

Scientists refer to these very late building blocks of the Earth, which were delivered by these collisions, as the ‘late veneer’. If ruthenium was added during this stage, it is distributed and well mixed into Earth’s mantle by now. The old Archean mantle relics in Greenland, on the other hand, have preserved Earth’s pristine composition.

“Our findings suggest that water and other volatile elements such as carbon and nitrogen did indeed arrive on Earth very late, during the ‘late veneer’ phase,” Dr Fischer-Gödde said.

Co-author Professor Martin Van Kranendonk, from UNSW, said the research relates directly to understanding the origins of life on Earth, how we humans came to be and whether we are truly alone 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 said.

“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.”

Image caption: Thin section of the 3.8-billion-year-old mantle rock from southwestern Greenland. Image credit: Julia van de Löcht.

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