Moon, Mars, impacts and collisions

She is Earth’s faithful companion in space. But how did Earth acquire such honor? Did Earth capture her? Were they both formed together, at the same time?

"Neither, nor", according to a team of astronomer from North-Rhine-Westphalia and Lower Saxony. The Moon has been ripped out of Earth during a collision with another celestial body. The scientists bolstered their theories with proofs appearing in the American scientific magazine Science.

The geologists are basing their theories on samples of lunar rock which were brought down to Earth by the Apollo astronauts in their six missions to the Moon, in the 1960s and 70s. Even today, 43 years after the last human moon-landing, the rocks hold surprises for us.

As it turns out, they probably carry the key to the answer to he question of how the Moon came to exist. And it now appears that, in the world of Solar System planets, this has been a unique process.

All the other moons of the Solar System were captured by their planet. „Our Earthmoon is very unusual,“ according to Daniel Herwartz from the Institute of Geology and Mineralogy at the University of Cologne. 

The hypothesis that Earth too might once have grabbed its Moon and forced it into orbit around itself, as Mars did, for instance, with ist two satellites, Phobos and Deimos, is now excluded by astronomers. The Moon would be much too large and heavy for this to be possible.

That both Earth and Moon could have been constituted together in the same gas and dust cloud – there’s no clue that speaks in favor of this hypothesis, either. So that scientists have come to favor the so-called colision-theory. It's an impact from outside that would then be responsible for the Moon’s existence. The Moon would have formed as the result of the collision of Earth with a body about the size of Mars, some 4.5 billion years ago.

Had these two Ur-planets hit each other frontally, nothing much would have been left of Earth. Scientists surmise therefore that the collision occurred sidewise, as a sort of a "glancing blow." The Mars-sized planet on a collision course was named Theia.

Both celestial bodies would have hit upon each other at a speed of up to 40,000 km/hour. When objects of such size collide, millions of rock pieces are torn from Earth and hurled into the Earth orbit, actually at a speed which is sufficient to overcome the Earth’s force of gravity. As time passes, they clump together – the Moon is formed. 

As a result, the Moon is made up of part Ur-Earth, part Theia. Just as Earth today is made up of a piece of Ur-Earth and a piece of Theia. The mighty collision has wildly mixed and swirled the stony material of both bodies but in such a way that both end results of this collision – the Moon as well as today’s Earth – carried away pieces of Theia.

Computer simulations show that most of the matter in the disk of rocks circulating Earth came from the impacting body. The Moon therefore must be constituted of a much higher proportion of matter coming from Theia, the Earth only by an estimated 10 percent.

The speed of the collision, the angle and the mass of the impactor would have caused more of the mass of Theia to contribute to the smaller Moon than the larger Earth, something which is confirmed by the mineralogist Andreas Pack from the Geoscience Center of the University of Göttingen in Germany.

"The Moon originated in great part from the material which impacted Earth. As a consequence, one would expect the Moon and Earth not to be constituted identically.“ 

But of what use are the most beautiful models and simulations, when reality speaks another language? For up to now, examinations of the moon rocks from the „Apollo“ missions had yielded far different results, which led to a different story, Daniel Herwartz is forced to admit.

"Unfortunately, up until now it has always been asserted that Moon and Earth are constituted the same, chemically.“ What then happened with Theia? „We have subjected the Apollo samples to a new analysis, of a higher accuracy than had been possible in earlier studies,“ says Herwatz. And lo and behold! „We have now indeed discovered the expected difference!“

That difference came to the fore through the presence of oxygen in the Moon rocks. Normally, that is, in almost 99,8 percent of cases, one atom of oxygen contains eight neutrons and eight protons. Chemists refer to it therefore as O16.

But one oxygen atom in some 40.000 possesses one more neutron. It is then referred to as Oxygen17 and ranks as an isotope. And it is precisely these exotics which have now come to the help of the geologists from Göttingen and Cologne to put together their reconstitution of the Moon’s origin.

"These isotopes of oxygen occur on Earth as well as on the Moon,“ says Andreas Pack. "What we studied were the minimal variations in the occurences of the ratios of isotopes amongst themselves.“

The German research team claims that these isotope ratios on the Moon differentiate themselves ever so slightly from those on Earth.

And to say „slightly“ is an exaggeration: for one million atoms of oxygen, there occur in the Moon rocks a mere 12 Oxygen17 atoms more than in Earth rocks. But nevertheless: the difference exists. The very rare isotope with nine neutrons is definitely more common on the Moon than it is on Earth.

"From the fact that there occurs on the Moon a little bit more O17 than on Earth, we know that Theia had more O17 than the Moon and Earth today,“ concludes the Cologne geologist Daniel Herwartz. The proportion of O17 must therefore be traced back to the early small planetoid Theia. As more remnant pieces of Theia must be occurring on the Moon than on Earth, this could explain the greater occurence of the O17 isotope in the Moon rocks.

From this ratio of the oxygen isotope, the geologists have now been able to deduce the portion of the Moon mass  which was contributed by Theia to the satellite. Previous estimates oscillated between a mere 8 up to 90 percent.
Now scientists are considering a 50:50 ratio. The Moon is composed half with material from the Ur-Earth, half from remnants of Theia.

Still, the new findings do not suffice to prove for the impact theory. There still exist two competing models. According to one of them, Earth and Moon would have been constituted in the same protoplanetary disk of gas and dust.

This would explain their pratically identical composition. Or then, the Moon could have been torn by escape velocities out of a much faster spinning Earth.

"These theories are not completely dead, yet!" concedes Daniel Herwartz. Theoretically, it would be possible that the ratio of oxygen isotopes of Earth was subsequently altered by the deposit of cosmic dust in such a way that it is no longer is identical to the one occurring on the Moon. It’s possible – but unlikely.

Guido Meyer
Translated from the German by Anne-Marie de Grazia
Die Welt, 06.05.2014
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When one looks at the Earth from space, one sees a rather regularly structured surface. Oceans and continents alternate, on the Northern as well as on the Southern hemisphere.

But our neighbor Mars looks as if it had two faces, according to which hemisphere one is considering. Its Northern hemisphere is flat and regular, whereas the Southern hemisphere is dominated by high plains, mountains and volcanoes.

Geologists have been seeking trying for a long time to understand why this should be so. Scientists from Switzerland have come up with a possible explanation. They suspect Mars, shortly after ist formation, to have collided with another planet.

Let’s take a trip back in time into the solar system of some four billion years ago: at a short distance from the Sun, outside the orbit of Earth, another small planet is very slowly forming. In its first millions of years, this planet, Mars, looks like crème brûlée.

In the case of the dessert, a soft layer of vanilla creme is topped with a caramel crust. In the case oft he young planet, the inside is made of a slushy layer of magma and the crust of a thin layer of stone. Mars cools down and hardens.
"At the time, many objects were flying around in the area, which constantly collided, " according to  Paul Tackley of the Geophysics Institute at the Eidgenössisch Technische Hochschule (ETH) Zürich. Tackley believes that such a collision gave it's shape to Mars.

He has played out his hypothesis with a the help of a model, a celestial body of a diameter of ca 2,000 m. He let this mini-planet crash into Mars, which was still soft and malleable inside.

"Such an impact would have melted almost the whole Southern hemisphere of Mars,“ Tackley points out. According to his theory, things developed thus: the Southern hemisphere melted into a sea of magma, which cooled and crystallized.

In this way, a crust was formed, which was thicker than the original surface, which had been destroyed in the impact. The crust was even thicker than the crust of Mars is in the Nothern hemisphere, to this day.

The theory matches the observations of geologists concerning the surface of Mars. On the one side, flat plains dominate, the other is covered with a thick, mountanous layer, crossed by high plateaus.

The impact created a gigantic sea of magma

These high plateaus could be the remnants of the gigantic sea of magma which must have covered the whole Southern hemisphere of Mars after the collision. Giovanni Leone, a geophysicist who also is doing research at ETH, shares this view. „Such an impact would not just create a crater, but a true-to-God ocean made of magma,“ he says.

The celestial body, which must have hit on Mars, must have been about one tenth of the mass of Mars, researchers are speculating. This would be close to the size of the Moon.

The impact must have created temperatures around 2,000 degrees Celsius. The heat brought the early, thin crust of stone covering Mars to melting. The impacting object itself must have melted during the collision and transformed itself into magma. 

"Despite the fact that the sea of magma has cooled down, the Southern hemisphere is still to this day hotter than the Northern one,“ according to Leone. According to the theory, fountains of magma, still sizzling under the surface, should break through to the surface, especially in the South.

Indeed, far more volcanoes are to be found in the Southern hemisphere than in the Nothern. The volcano Olympus Mons, situated on the Northern hemisphere of Mars, with a height of 22,000 meters the highest volcano of the solar system, is an exception, which changes nothing to the basic phenomenon.

The theory of an impact of a celestial body could solve another mystery of Mars. For a long time, scientists have been puzzling what might have happened to Mars‘ magnetic field. It disappeared four billion years ago, shortly after the creation of Mars. 

The heat of the collision must have cooled down very quickly, Leone says, according to the first 200 million years of the model. At some time, the point must have been reached when the inner heat was no longer high enough to keep a magnetic field going.

But one question finds no explanation in the theory presented by the Swiss – the one of water on Mars. Did seas exist there, even a whole ocean? Some scientists think so.

The Swiss geophysicist are of the opinion that their theory speaks against this. The volcanoes, which must have erupted after the collision of Mars and of the mini-planet, would have spewed out magma. Lava contains the mineral olivin. Olivin would have changed into serpentine if it had come into contact with water.
But no serpentine has yet been found yet on the surface of Mars, neither in the North nor in the South. All mysteries are not yet solved...

Guido Meyer
Translated from the German by Anne-Marie de Grazia
Die Welt, March 16, 2015

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