The Earth´s oceans were formed by water from comets – Niels Bohr Institute - University of Copenhagen

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04 September 2009

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 by astronomer Uffe Gråe Jørgensen from the Niels Bohr Institute at the University of Copenhagen. The results are published in the scientific journal, Icarus.

Five years ago Uffe Gråe Jørgensen was part of an expedition to Isua on the Godthåb Fjord in Greenland along with geologists from the Geological Survey of Denmark and Greenland, GEUS. This is the only place on Earth with the oldest intact rocks dating back to 3,8 billion years. The goal was to find out what happened on Earth during that time period – could traces of a bombardment from space be found?

Bombardment from space

Comets consist primarily of water
and a small amount of small rocky
particles. They come with great
speed from the outer regions of the
Solar System. Hubble Space Telescope

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 giant craters that were formed by large impacts from space, but when? The astronauts from the Apollo missions to the Moon brought a lot of rocks back with them. It turned out that they were all between 3,8 and 3,9 billion years old. All the craters were formed in a colossal bombardment within a period of 100 million years. – but was it an impact of stony or iron meteorites or ice? 

The same bombardment hit the Earth and over 1.000 tons of cosmic material fell on every square meter of the Earth’s surface, which was nearly smashed. But what was it? – that was the aim of Uffe Gråe’s expedition to Greenland, to find traces of the cosmic material. The element iridium is evidence of cosmic origin.

Cosmic trace
”We brought many rocks home with us to examine and I was sure that we would find a lot of iridium. We found nothing”, explains Uffe Gråe, who simply could not understand the results. Such a massive bombardment must have left behind obvious traces, but the measurements in Denmark showed nothing. In Japan, they found a laboratory that could examine the material in an atomic reactor and measure each individual atom. That gave results. The Japanese measurements showed that there were 150 iridium atoms per 1.000 billion other atoms. That is to say, very little, but still a clear cosmic trace in the rocks from Greenland.

New research results show that it was
bombardments of comets that formed the Moon’s
craters and the Earth’s oceans 3,8 billion years

The Apollo-rocks from the Moon were also examined – here there was absolutely no iridium. What could the explanation be for that the same bombardment left behind cosmic traces on Earth, but not on the Moon?

Suddenly it hit Uffe Gråe, that if the bombardment was not of stony asteroids, as one had previously thought, but rather comets, which consist of 80 percent ice and 20 percent small rocky particles, it could be the explanation. 

”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, but the material itself shoots out again. On Earth, some of the material remains, because the Earth has a stronger gravitational force”, explains Uffe Gråe. Consequently, iridium from the comets would stay on the Earth and not on the Moon.

Suddenly it all added up. The iridium content was understood, as the scanty amount of iridium found in the pieces of rock from Isua in Greenland match what one would expect from comets, where 20 percent is small rocky particles and the rest is water. 

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. The number of craters on the Moon also fits with an intense bombardment of comets. 

”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.