Read out The Origin of the Moon. 4.5 billion years ago, a primeval planet collided with Earth. From the rubble formed our moon. Scientists are now tracking the consequences of the cosmic collapse. Our moon is one of the most enigmatic bodies in the solar system. Its mass is a good one percent of the Earth, while all other moons (with the exception of the Charon Pluto Moon) have less than 0.025 percent of the mass of their planet. In addition, the earth's moon has an amazingly low density for the inner solar system. The reason: it contains very little iron.

After all, its atomic composition differs significantly from the isotope ratios on Earth, and there is a striking similarity. This is shown by soil samples that astronauts brought with their Apollo flights from our cosmic neighbor.

The genesis of the moon bears the explanation of all these characteristics. But decoding that early time is extremely difficult. Jack J. Lissauer of NASA's Ames Research Center in Moffett Field, California recalls, "When I heard a lecture from Moon expert Irwin Shapiro 20 years ago, he joked that the best explanation was the admission of an observation error: the moon does not exist ".

For a long time it had been speculated whether the moon was once captured by the earth, whether it split off due to the rapid rotation of our planet or whether it - together with the earth - has condensed from the dust and gas of the Ur-nebula - as it did probably with most other satellites was the case. But none of these three hypotheses is readily compatible with the celestial laws and the cosmic boundary conditions, for example, the distance and angular momentum of the earth-moon system today, the small iron core of the moon and the composition of its surface rock. display

That's why a fourth hypothesis was developed in the mid-1970s that has since become accepted. It is based on a collision of the primordial earth with a primeval planet, which was completely destroyed in the cosmic billiards. His mass must have been about 20 percent of that of our earth. He was about twice as heavy as Mars. The relative speed of the two celestial bodies was a few kilometers per second. The iron core of the primordial planet remained stuck in the primeval earth, while its mantle and part of the mantle were exploded into space. From this rubble belt a new celestial body has formed - our moon. Computer simulations by Shigeru Ida of the Tokyo Institute of Technology in Japan along with Robin M. Canup and Glen R. Stewart of the University of Colorado at Boulder showed that this aggregation lasted at most a year. That was 4.51 billion years ago, according to a team led by Der-Chuen Lee from the University of Michigan at Ann Arbor.

The conclusion of the researchers: The moon came into existence only 50 million years after our solar system began to form - at a time when the formation of the primordial earth from the primeval mist probably had not yet ended.

Lee and his colleagues had measured the abundance ratios of radioactive hafnium-182 and its decay product tungsten-182 in lunar rocks. From this, his age can be determined. The rock comes from the Apollo missions or came as a meteorite to Earth. In the meantime, 17 of those tiny fragments of the moon have been known that once blew meteorites out of its surface and eventually captured Earth's gravity. "EET 96008", a 53-gram, 4.5-centimeter-long meteorite found in the Antarctic Elephant Moraine region in 1996, was revealed as basaltic debris from the Moon just a few months ago.

The computer simulations of Shigeru Ida and other scientists prove that a collision of collision debris in a tight orbit is possible. The collision hypothesis only works if the earth turned much faster then. In addition, the Moon should have initially circumnavigated it on a low equatorial orbit. Today it moves on a steeply inclined orbit 384, 000 kilometers away.

Jihad Touma of the University of Texas and Jack Wisdom of the Massachusetts Institute of Technology have now developed a model that makes this orbit understandable. Completely unexpected they also found an explanation of how the lunar highlands could have formed.

It has long been known that the tidal friction of the moon gradually slows the earth's rotation. As a result, the moon slowly moves away from the earth. But one must also consider the gravitational interaction between Earth, Moon, Sun and the other planets, especially Jupiter. That's what Touma and Wisdom did. They came across two resonances that must have led to a drastic change in the lunar orbit.

Resonances are integer orbital relationships between celestial bodies, which are either particularly stable or lead to rapid and strong orbital changes. Touma and Wisdom suggest that the orbit of the moon was subject twice to such abrupt changes.

According to her model, the first resonance threw the terrestrial satellite into an extremely elliptical orbit only 1000 years after its turbulent birth. As a result, terrestrial gravitation was alternately stronger and weaker. From these periodically fluctuating tidal forces, the moon was literally kneaded - a thousand times stronger than Io, the volcanically active innermost Greatmoon of Jupiter. There, the Galileo spacecraft recently measured the highest surface temperatures in the entire solar system - apart from the sun - over 1400 degrees Celsius.

Touma and Wisdom suggest that the earthly tidal friction has so heated the young moon that its surface has melted. This has led to the emergence of its low-iron crust, because the heavy metals sank into the moon's interior. The dark Maria, mighty frozen lava flows that cover 17 percent of the lunar surface, emerged later, about 3.9 to 3.1 billion years ago, by powerful volcanic eruptions or impacts of kilometer-sized planetoids.

The melting of the surface consumed so much energy that the moon escaped the first resonance and moved away from the earth in a spiral path. Soon after, however, he came into a second resonance, which brought him from the equatorial plane of the earth. He escaped this resonance only when his orbit was inclined twelve degrees to the equator. Since then, he has slowly moved away from the earth, making his days and nights ever longer. Touma and Wisdom had to work through different scenarios until they found a model that could describe all the important orbital parameters. In this model, a day on the primordial earth lasted only five hours and its axis of rotation was inclined 10 degrees to the orbital plane - today it is 23.5 degrees. The distance of the moon from the equator at that time was only 22000 kilometers.

These distant events are still of great importance today. Without the moon, the earth would chaotically wobble, because it has a stabilizing effect on the position of the earth's axis of rotation. It would fluctuate between 0 and 85 degrees within a few million years without a moon. Like the planet Uranus, the earth could then roll on its orbit around the sun. This was the result of computer simulations by Jacques Laskar and his colleagues at the Bureau des Longitudes in Paris five years ago.

Darren M. Williams and James F. Kasting of Pennsylvania State University have now shown with climate models for the Earth without the Moon that the axis fluctuations would have devastating effects for our planet. Tropical zones are always disappearing in the snow, and the polar regions would heat up to 80 degrees Celsius. Continents in the temperate latitudes would be exposed to enormous temperature fluctuations between minus 25 and plus 45 degrees Celsius within a few months. In a different position of the continents - 200 million years ago there was only one major super continent on our planet - the conditions would be even more extreme. The same would happen if the earth had less water.

Without the moon, earthly life would have developed completely differently or would have died out long ago. Maybe it would not have been created. We humans certainly would not exist.

=== R diger Vaas

science.de

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