A hundred thousand times Hiroshima

A catastrophe of unimaginable proportions: Almost 15 million years ago, a gigantic explosion shook southern Germany. It was more powerful than a hundred thousand atomic bombs combined and killed all living creatures within a radius of at least one hundred kilometres. Humans did not exist at that time, but all animals and plants were killed or buried under the debris by a huge blast wave. The bang could still be heard even on the other side of the earth.

It was triggered by a meteorite about 1500 metres in size, which was hurtling towards the earth at about 20 kilometres per second. The impact of this chunk caused an earthquake of magnitude eight and tore a 24-kilometre crater into the earth’s surface. The crater can still be seen today – the Nördlinger Ries between the Swabian and Franconian Alb. The landscape around the impact crater was completely turned upside down: 150 cubic kilometres of rock were hurled into the air, some of which flew 450 kilometres as far as today’s Czech Republic.

Fortunately, impacts of such meteorites weighing tons are very rare, they happen only once every few million years.

The little brother

The crater remnant of the Nördlinger Ries is clearly visible on an elevation map. But there is something else you can see there: about forty kilometres southwest of the Nördlinger Ries lies a similar, but much smaller crater, the Steinheimer Becken, with a diameter of about three and a half kilometres. This crater, too, goes back to a meteorite impact – but this metorite was only about 150 metres in size.

Two meteorite craters so close together – can that be a coincidence? No, say the scientists who studied the rocks of the craters: Both craters were formed at the same time. Either a small part of the meteorite broke off on its approach to Earth and hit separately, or the asteroid had a small companion for a long time that crashed to Earth together with it.

What are asteroids, meteorites and comets?

Some nights you can observe a special moment in the sky: It looks as if a star is falling from the sky. Superstitious people even think that whoever sees such a shooting star can make a wish. But what is really behind it and where do shooting stars come from?

In our solar system there is not only the sun, planets and moons. Many small pieces of rock and metal have also been discovered. They are much smaller and not as nicely round as planets, so they are called minor planets or asteroids. Like their larger siblings, they orbit the sun in regular orbits. Most asteroids are found in the “asteroid belt” between the orbits of Mars and Jupiter.

Every now and then, two of these asteroids collide. Such a crash creates lots of debris and splinters. These fly away from their previous orbit, right across the solar system. Some of them come close to the Earth, are attracted by it and crash into the Earth. These falling chunks are also called meteorites.

On Earth, they would literally fall like a stone from the sky – if it weren’t for the atmosphere. Because the meteorites are so fast that the air cannot move sideways fast enough. The air in front of the falling chunk of rock is compressed and thus extremely hot. The air starts to glow and the meteorite begins to vaporise. We can then see this as a glowing streak moving across the sky – a shooting star.

Most meteorites are so small that they burn up completely on their way through the air. The tracer then simply ends in the sky. Larger debris also loses mass on the way, but does not evaporate completely. They reach the ground and hit it.

What these meteorites do to the Earth depends on how big they are. Small meteorites with a diameter of a few centimetres, for example, leave just a dent in the roof of a car.

The largest known meteorite struck about 65 million years ago. It had a diameter of several kilometres and ripped a crater 180 kilometres in diameter. The impact threw so much dust into the air that the sun was eclipsed for hundreds of years. This caused plants and animals to die out all over the world – this was the end of the dinosaurs.

Fortunately, such large meteorites are very rare, so we don’t have to worry. Besides, unlike the dinosaurs, we can observe the sky with telescopes and detect such large asteroids long before they hit.

While a shooting star burns up in a few seconds, another phenomenon remains visible for longer: comets with their tails stand in the sky for days or weeks. In the past, people also attributed many properties to them – as divine signs, heralds of disaster or harbingers of joyful events. But the truth is somewhat less spectacular.

Astronomers also call comets “dirty snowballs”. They come from the outer solar system, far from the warming power of the sun. It is so cold there that water immediately freezes into ice. This is how clumps of ice chunks and dust form – dirty snowballs, in other words.

A comet also initially travels far away from the sun – until it is deflected by a collision and flies in the direction of the inner solar system. It comes closer to the sun and, over time, receives more and more light and heat. As a result, the frozen surface begins to thaw and even evaporate. This creates a shell of water vapour and dust around the comet.

At the same time, the comet feels the “solar wind” – tiny particles that fly out of the sun at high speed. They hit the vapour envelope of the comet. This blows the vapour envelope of the comet away, forming an elongated cloud that points away from the Sun. When this cloud is then hit by sunlight, it appears as a glowing streak – the tail of the comet.

The comet flies an arc around the sun and then moves away again. When it is far enough away from the sun, the thawing and evaporation also stops. The tail disappears and the comet moves through the vastness of the outer solar system as a dirty snowball. Depending on the comet’s orbit, it takes many decades or even centuries until it comes close to the sun again.

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Why does it look different on Earth than on the Moon?

It doesn’t look very inviting on the moon: The surface is dry and covered with a grey layer of dust. Meteorite impacts have torn huge craters into the ground, which filled with lava from the interior of the moon. Around these lava basins, kilometre-high crater rims tower up as mountain rings.

Our blue planet is completely different – if only because three quarters of it is covered by water. But water not only covers a large part of the earth, it also shapes its land mass: rivers, glaciers and the surf of the sea work the rock, break it up and rearrange it. This is how valleys, coasts and ever new layers of rock are formed.

The interior of the moon today is solid and rigid. The Earth, on the other hand, has a liquid mantle on which movable plates float. The movement of the Earth’s plates causes mountains to fold up, deep-sea trenches to form and volcanoes to spew fire and ash.

Unlike the moon, the earth has an air envelope, the atmosphere. It is in this envelope of air that the weather is created. Wind, rain and snow have worked and shaped the earth’s surface over millions of years. The atmosphere also acts as a protective shield, slowing down meteorites and preventing them from burning up.

Why does the moon have spots?

The man in the moon – well-known from songs, films and stories. Indeed: there are conspicuous dark spots on the surface of the moon, and with a little imagination you can make out a face in them. But what are these spots really?

At first, scientists thought the dark spots were seas. But at the latest since the first visit to the moon in 1969, it has been clear: the moon is dry as dust, the entire lunar surface consists of fine grey rock powder. And the dark patches are large lowlands that are simply filled with darker dust. This makes the moon appear brightly and darkly spotted. But how did these deep plains form?

The lowlands are almost as old as the moon itself. In the early days of the solar system, when the surface of the moon had already solidified into a crust, large asteroids repeatedly hit the moon and tore holes in the fresh crust. There, lava ran out of the still hot, liquid interior of the moon and filled the lowlands. Lava rock is darker than the crustal rock, so the lowlands appear darker.

In the meantime, hardly any large asteroids hit the moon, but there are still plenty of smaller ones. Since the Moon (unlike the Earth) has no atmosphere, they do not burn up but hit the surface. Most of the time, however, the force of the impacts is only enough to crumble some rock and kick up a little dust, which quickly sinks to the ground again. That’s why the lunar surface today consists of rock dust, mainly light-coloured crustal rock and, in the lowlands, darker lava rock. From Earth, this looks like spots, oceans – or even a face.

The beginnings of the earth

We would not recognise the Earth immediately after its formation. It was an extremely uncomfortable planet: there were neither continents nor oceans, but a seething surface of glowing hot, viscous magma. Why could no earth’s crust form for a long time?

A good 4.5 billion years ago, comets, asteroids, gas and dust condensed to form our planet. Their own gravity pressed these individual parts together so that they were exposed to strong pressure. Of course, this pressure was highest in the Earth’s core, on which the weight of the entire outer layers rested. As a result of the high pressure, the rock was strongly heated and melted. Towards the outside, the pressure and thus also the temperature became less. Nevertheless, the Earth’s surface remained very hot for several hundred million years and could not cool down and solidify.

To understand why, scientists had to look at the moon: Ancient lunar craters from the time of the formation of the solar system tell us that the moon was hit by numerous meteorites in its young years. It is therefore assumed that the Earth was also subjected to a veritable rock bombardment from space at the same time. The chunks fell to Earth at high speed – and the impacts were correspondingly violent: Chunks as small as a few hundred tonnes could easily cause an explosion as powerful as an atomic bomb.

Thus, the Earth’s surface continued to be heated for a long time, repeatedly stirred up and thus remained liquid. Only when the impacts gradually subsided after a few hundred million years did the temperatures on the Earth’s surface drop. The rock was able to solidify slowly and form an earth crust that became thicker and thicker over the course of further millions of years. But to this day, it is only a wafer-thin layer floating on a viscous, hot interior of the Earth.