How water came to earth

Where the water on Earth as oceans or clouds comes from has long been the subject of scientific debate. A considerable amount comes from the Earth's interior, blown into the atmosphere by volcanoes and then partially filled the first oceans as rain. But where does that water originate initially? The material from which Earth was formed did not contain as much water as the planet has now, so there must have been other sources. So was there also water added from outside and when? According to current research, millions of asteroid fragments from zones of the Solar System far away from the Sun, originally beyond Jupiter, brought a considerable amount of the water to Earth.

A recently published study now shows that this was only possible because water-rich primordial building blocks of the solar system formed later, more slowly and at lower temperatures than planetesimals further inland in the solar system, which could contain little or no water or ice.

"If this delay in the formation of planetesimals had not occurred, Earth would be a bone-dry planet today," says Dr. Wladimir Neumann from the Institute of Planetary Research at the German Aerospace Center (DLR) and the Institute of Geodesy at Technische Universität Berlin. He is the lead author of the study now published in the scientific journal Scientific Reports (Nature Portfolio). "To put it simply, the distance of the planetesimals from the Sun during their formation was decisive in determining which components were incorporated into them." The formation of planetesimals far outside the Sun was somewhat delayed and slower than in the inner solar system, but above all it was repeated again and again. "The late planetesimals did not get as hot and therefore did not lose the water they contained. Later, many of these water-rich planetesimals entered the inner solar system and are likely to have brought large quantities of water to Earth." This could also be how the outer neighbouring planet Mars got its water, which it has now almost completely lost, but whose traces we can still see today. It is also being discussed that Venus could have had water for several hundred million years in its early history.”

The building blocks of the planets were formed in just a few million years.

By astronomical standards, everything happened very quickly in the earliest times of the solar system. After the explosion of two or more "burnt-out" stars in one of the spiral arms of the Milky Way, our home galaxy, the gases from these supernovae remnants condensed to form a new star. Four and a half billion years ago, it had accumulated so much mass that hydrogen atoms were able to fuse into helium in its interior, generating energy in the process: The sun was born. It was orbited by a disk of dust and gas extending billions of kilometers into space.

This is where the building blocks of the planets were formed. Meteorites, fragments of parent bodies that were formed at that time, bear witness to this. Most meteorites are chondrites, about 86 percent. The chondrules, small spheres, were formed within a few million years in this protoplanetary disk by molten dust material forming droplets. They solidified and then agglomerated together with dust and gases, including water, to form larger bodies, the planetesimals.

Around four and a half billion years ago, planet formation was therefore completed relatively soon after the sun "ignited" 4.568 billion years ago. However, because countless small bodies still remained, these were very turbulent times in the solar system with asteroids and comets hitting the young planets many orders of magnitude more frequently. In particular, asteroids from the outer zone of the main belt between Mars and Jupiter, which formed beyond a distance from the sun known as the "snow line", are likely to have supplied the Earth with a large proportion of its water.

So how could parent bodies of meteorites form that were cold enough not to lose the volatile water molecule? Vladimir Neumann and his co-authors from the Institute of Geosciences at the University of Heidelberg, the University of Bayreuth and the Swiss Federal Institute of Technology in Zurich found the key to answering this question by studying some carbon-rich meteorites whose parent bodies must have formed far from the sun.

Among them was the 25-gram "Flensburg meteorite", which fell from the sky in northern Germany on September 12, 2019. It contains minerals, all of which could only crystallize in combination with water, and whose parent body was formed 2.7 million years after the formation of the accretion disk, i.e. after the time "zero". Records from other carbonaceous achondrite and chondrite meteorites and asteroid samples returned from Ryugu were also compared in the study. They were discovered at the Bavarian Geoinstitute at the University of Bayreuth by co-authors Ning Ma (now a doctoral student at ETH) and Prof. Dr. Audrey Bouvier.

In particular, Tafassites constitute a group of primitive achondrites (heated, partially melted) meteorites named after the Tafasset meteorite found in Niger in 2000. This new group of meteorites was established at BGI by co-authors Ning Ma (now doctorant at ETH) and Prof. Bouvier. From their chemical composition, these meteorites formed in the outer solar system (beyond Jupiter). They show that their parent bodies formed earlier than non-heated planetesimals formed in the same region of the solar system. It only took 1.1 million years old after time "zero" to be formed. Other heated meteorites formed even earlier at 0.6 million years after the Sun formation. Now compared against other meteorites from planetesimals that formed from the same dust material, it shows that there was repeated planetesimal formaiton in the region between 0.6 to 3.7 million years after formation of the Sun. The ones formed last preserved more water and water-bearing minerals than the ones formed early.