The Earth´s oceans were formed by water from comets.

The Earth´s oceans were formed by water from comets

Comets filled with ice and not stony asteroids bombarded the Earth and Moon 3.85 billion years ago and made the craters on the Moon and the Earth´s oceans, according to new research lead by astronomer Uffe Gråe Jørgensen from the Centre for Star and Planet Formation. The new results are published in the scientific journal Icarus in collaboration with a team of Danish and Japanese scientists.

Five years ago Uffe Gråe Jørgensen was part of an expedition to Isua on the Godthåb Fjord in Greenland along with a team of geologists. When the sediments at Isua formed, the Earth and the Moon were in the middle of the most violent cosmic bombardment of all geological times, and Isua is the only place on Earth where rocks as old as 3.8 billion years are still preserved. The goal was to find out what happened on Earth during that time – could traces of the bombardment from space still be found at Isua?

Bombardment from space
The Earth and the Moon were formed in the Solar System at the same time 4.5 billion years ago and share history. The Moon is covered with gigantic craters from huge impacts. During the Apollo missions, the astronauts collected lots of Moon-rocks formed during these impacts. They were all between 3.8 and 3.9 billion years old. All the craters must therefore have formed in a colossal bombardment within a period of just 100 million years. – but was it stones of meteorites that slammed into the Moon, or was it a shower of giant ice blocks from comets?

The same bombardment hit the Earth and over 1000 tons of cosmic material must have fallen on every square meter of the Earth’s surface then. If it was ice blocks from outer space it could have had tremendous effects on the evolution of life on Earth. The aim of Uffe Gråe Jørgensen’s expedition to Greenland was to search for traces of the cosmic intruders. The element iridium is the strongest evidence of cosmic material. On Earth all original iridium was vacuumed away from the crust shortly after its formation.

Cosmic trace
”We brought many rocks home with us to examine and I was sure that we would find a lot of cosmic material mixed into the rocks. At first we found nothing”, explains Uffe Gråe, who simply could not understand the results. Such a massive bombardment should have left behind obvious traces. In Japan, the team identified a laboratory that could examine the material in much better detail than laboratories in Denmark. By dumping the material into a nuclear power station it would be possible to measure each individual atom of cosmic material. That gave results. The Japanese measurements showed that there were 150 iridium atoms per 1000 billion other atoms. That is to say, very little, but still a clear cosmic trace of the bombardment in the rocks from Greenland.

The Apollo-rocks from the Moon had already been examined – and shown absolutely no iridium. How could one and the same bombardment have left behind cosmic traces on Earth, but not on the Moon?

After intense modeling, the team realised that if the bombardment was not of stony asteroids, as many scientists had previously thought, but rather by comets, it could explain the strange results.

”Comets come from the outer reaches of space at great speeds. When a comet collides with the Moon, the explosion is so powerful that it creates a large crater, and the material itself shoots out into space again. The collision leaves nothing other than the crater. On Earth, some of the material remains, because the Earth has a stronger gravitational force”, explains Uffe Gråe Jørgensen. Consequently, iridium and water from the comets would stay on the Earth but not on the Moon.

Suddenly it all added up. The iridium content was understood. The scanty amount of iridium found in the pieces of rock from Isua in Greenland matched exactly what one would expect from comets.

At the same time, it appeared that the amount of water that fell to the Earth with the comets corresponded to the amount of water found in the oceans today. Comets are typically 80 percent ice and 20 percent small rocky particles where the iridium sits. The number of craters on the Moon also fits with an intense bombardment of comets. Finally, the models with comets were able to explain why the oceans cover 70 percent of the Earth’s surface. The Earth could never have become completely covered with water, even if many more comets had hit the Earth.

”All of our measurements and modelling point to that it was a bombardment of comets that created the Moon’s craters and the Earth’s oceans and in that way contributed to life on the Earth”, explains Uffe Gråe Jørgensen.

A copy of the full manuscript accepted for publication in the scientific journal Icarus can be downloaded here.