How the Moon arose has long stumped scientists. Now Dutch geophysicists argue that it was created not by a massive collision 4.5 billion years ago, but by a runaway nuclear reaction deep inside the young Earth. ...
Photo Caption: Spectacular force: A georeactor deep in the ancient Earth's D''-layer (dark orange layer near core) goes supercritical - suddenly increasing temperatures to 13,000ºC. This turns rock into vapour, creating a rising bubble which pushes mantle, crust and atmosphere into space in a giant eruption.
IT HAPPENED 4.5 BILLION YEARS AGO, when the Earth was barely 50 million years old. Life didn't exist; the planet was a violent, boiling fireball.
Then, without warning, the unimaginable happened. Deep within the core, a tremor started. The young Earth shuddered and erupted. From its bowels spewed a trillion-tonne column of molten and vaporised rock. On this day the Moon was born.
In his renovated Saxon farmhouse in Peize in the north of the Netherlands, retired nuclear geophysicist Rob de Meijer vividly paints a picture of the cataclysm: "The material in the Earth's mantle heated up some 8,000ºC and was completely vaporised.
This huge bubble of gas forced itself up through the still liquid mantle," he says. "As a result, part of the Earth's mantle and crust were blown away, as well as the early atmosphere. From the debris, the Moon could have formed rather quickly."
The Earth ejecting the Moon: isn't that a rather fanciful scenario? Not according to de Meijer and his colleague, petrologist Wim van Westrenen from the Free University in Amsterdam. They argue that this hypothesis is the logical consequence of new data, and also that an erupting Earth solves a number of unsolved and profound astronomical mysteries...
Before man set foot on the Moon almost 40 years ago, there were still three theories to explain its origin. The first stated that the Moon, like the Earth, was the result of the accretion of cosmic dust into ever more massive chunks. The second argued that the Moon formed elsewhere in space and was later captured by Earth, without impact. Under the third hypothesis, the primordial Earth was spinning so fast that the matter that formed the Moon flew from Earth, by the apparent centrifugal force.
This last one, the 'fission hypothesis', was proposed as early as 1880 by George Darwin, son of the famous father of evolution, Charles Darwin. As evidence, he put forward the Pacific Ocean. This gaping hole, he suggested, was visible evidence that a large mass was missing from the Earth. It didn't take Apollo missions to refute Darwin's idea. The discovery of plate tectonics provided a more plausible explanation for the Pacific Ocean. Also, around 1930 other scientists calculated that although a day would have lasted just 2.5 hours, the early Earth was spinning too slowly to eject so much matter. "The centrifugal force was insufficient for a Moon to escape," says de Meijer. ...
"The density of the Moon turned out to be much lower than the Earth's density," says van Westrenen. "That excludes the accretion model, which says that the Earth and the Moon were formed from the same primordial material. [They] are simply too different."
Paradoxically, the second hypothesis – that the Moon was formed elsewhere in the Solar System and later captured – was excluded for exactly the opposite reason.
"In that respect, the Earth and the Moon are too similar," adds van Westrenen. "The ratio of isotopes oxygen-17 and oxygen-18 are identical in terrestrial and lunar rock, and deviates strongly from the isotope ratio in meteorites from Mars." These isotopes carry information about the distance from the Sun the rock formed, and indicate that the Moon and Earth must have formed at roughly the same distance. ...
"At first, the impact hypothesis seemed to solve all our problems," he says. "It gives you an Earth and Moon with the correct size, in the correct orbit."
A quarter of a century later, though, things have changed. In particular, computer simulations of the impact have become more advanced and detailed. And in turn, they have generated new mysteries. ... the main puzzle that came from the simulations – detailed in the British journal Nature in 2003 – was that 'successful' impacts produce a Moon of about 80 per cent mantle material from the impactor.
"If the Moon is mainly made of impactor material, than it is much less likely that the ratio of oxygen isotopes for Moon and Earth would be the same," says van Westrenen. "That is only possible if the Earth and Theia were formed at the same distance from the Sun, which means that they have been chasing each other in more or less the same orbit until they collided." The problem is that we don't see anything like that elsewhere in the entire Solar System. ... "We calculated the conditions for the impact velocity. This indicates that the impactor originated somewhere between the present orbits of Mars and Venus." That is, somewhere near the orbit of the Earth. ... ould something similar happen today? Researchers don't think so; the Moon drew so much energy from the Earth in that blast that too little is left for an encore. However, de Meijer believes that the georeactors in the D''-layer remain active.
"There could still be places where fissile isotopes reach the critical concentration for forming a georeactor. Remember that georeactors are fast breeders that produce more fuel than they use," he says.
He also knows how to find out. Both natural decay of uranium and thorium produce antineutrinos, tiny particles that can fly straight through the Earth and you and me. Considering the inaccessibility of the D''-layer, an antineutrino detector is the only way known to prove the existence of a georeactor. But such a detector must be able to detect the energy and direction of the antineutrinos, because artificial nuclear reactors produce them, too.
De Meijer has been working on precisely this type of direction - sensitive detector. "We made a design that looks feasible. The next step is building a prototype. We hope to start soon." - cosmos
Read the entire article on Cosmos. Very interesting with lots of history and geochemistry.
No comments:
Post a Comment