Water was created
Conditions for life on earth were created very late
Cologne - Without liquid water, our home planet would most likely be a barren, dead world. What brought us this life-giving wealth and when this happened in the history of the earth is still controversial in the professional world. For example, a study from the previous year based on molybdenum isotope analyzes came to the conclusion that the water came from a celestial body about the size of Mars called Theia, which collided with the young Earth about 4.4 billion years ago. Our moon was also formed in the cosmic frontal crash. However, the hypothesis put forward by most researchers is that the water was brought to Earth very early on by asteroids and comets.
But now an international team has found indications that a large part of the components important for the formation of oceans and life, such as water, carbon and nitrogen, were added to the earth relatively late in its history. This refutes the also popular theory that these elements were already present at the beginning of the earth's growth. Most of the water only came to our planet when it was almost completely formed.
Earth's mantle rocks now and then
"We have now been able to narrow down the question of time more precisely," says Mario Fischer-Gödde from the University of Cologne, first author of the study presented in the journal "Nature". "To do this, we compared the composition of the oldest, around 3.8 billion year-old mantle rocks with the composition of the asteroids from which it was formed, and with the composition of today's mantle."
In order to make the temporal process comprehensible, the researchers determined the isotope abundances of the very rare platinum metal ruthenium contained in the archaic mantle. Ruthenium was suitable as a reference because the rare platinum metal, like a fingerprint, is a suitable indicator of the earth's late growth phase.
Traces of rare platinum metals
"Platinum metals such as ruthenium have an extremely high tendency to combine with iron. When the earth was formed, ruthenium must therefore have been completely discharged into the metallic earth core," says Fischer-Gödde. "If we can still find traces of the rare platinum metals in the earth's mantle, this shows that they were only added to the earth's mantle after the formation of the nucleus - that is, through subsequent collisions of the earth with asteroids or smaller planetesimals," adds co-author Carsten Münker.
Scientists refer to these very late building blocks of the earth that were added by the collisions as "late veneers". If ruthenium was added in this phase, it is distributed and well mixed in today's mantle. The old archaic mantle, on the other hand, still has almost the same composition as it was at the sampling points of the research team in Greenland. "The rocks from Greenland, which are up to 3.8 billion years old, are the oldest preserved mantle rocks. They allow us to look through a window into the early history of the earth," says Fischer-Gödde. The oldest mantle of the earth lies open on the surface in southwest Greenland, so that rock could be removed directly from there.
From inside the solar system
The ruthenium examined here most likely comes from the inner part of the solar system, report the two geologists. It is believed to be the same material from which Mercury and Venus were mainly formed. The ruthenium comparison values of the asteroids came from meteorites found on Earth. "Our new results suggest in particular that water and other volatile elements such as carbon and nitrogen did indeed come to earth very late in the 'late veneer' phase", concludes Fischer-Gödde. (red, March 12th, 2020)
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