Why do most meteorites come from comets?
What are asteroids, meteorites and comets?
On some nights you can observe a special moment in the sky: it looks like a star is falling from the sky. Superstitious people even think that whoever sees such a shooting star could wish for something. But what is really behind it and where do the shooting stars come from?
In our solar system there are 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, hence they are called minor planets or Asteroids. Like their big siblings, they circle the sun in regular orbits. Most asteroids can be found in the "asteroid belt" between the orbits of Mars and Jupiter.
Every now and then two of these asteroids collide. A crash like this creates a lot of debris and splinters. These fly away from the previous orbit, across the solar system. Some of them get close to the earth, are attracted to it and fall to the earth. These falling chunks are also called meteorite.
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 to the side quickly enough. The air in front of the falling rock is compressed and therefore extremely hot. The air begins to glow and the meteorite begins to evaporate. We can then see that as a glowing streak that moves across the sky - a shooting star.
Most meteorites are so small that they burn up completely as they travel through the air. The trail then simply ends in the sky. Larger debris also lose mass on the way, but does not completely evaporate. They reach the ground and strike there.
What these meteorites do to Earth depends on how big they are. Small meteorites a few centimeters in diameter, for example, just leave a dent in a car roof.
The largest known meteorite hit about 65 million years ago. It was several kilometers in diameter and tore a crater 180 kilometers in diameter. The impact threw so much dust into the air that the sun was eclipsed for hundreds of years. As a result, plants and animals all over the world died out - this was the end of the dinosaurs.
Fortunately, such large meteorites are very rare so we don't have to worry. In addition, unlike the dinosaurs, we can observe the sky with telescopes and discover such large asteroids long before the impact.
While a shooting star burns up in a few seconds, another phenomenon remains visible longer: Comets with its tail there are days or weeks in the sky. In the past, people also attributed many properties to them - as divine signs, heralds of calamity or harbingers of happy events. But the truth is a little less spectacular.
Astronomers also call comets "dirty snowballs". They come from the outer solar system, far from the warming power of the sun. It's so cold there that water immediately freezes to ice. This is how lumps of ice and dust form - dirty snowballs.
Even a comet 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 gets closer to the sun and over time receives more and more light and warmth. This will cause the frozen surface to begin to thaw and even to evaporate. This creates an envelope of water vapor and dust around the comet.
At the same time, the comet gets to feel the “solar wind” - tiny particles that fly out of the sun at high speed. They hit the comet's vapor envelope. This will blow away the comet's vapor envelope, 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 comet's tail.
The comet makes an arc around the sun and then moves away again. When it is far enough away from the sun, thawing and evaporation will also stop. The tail disappears and the comet moves like a dirty snowball through the vastness of the outer solar system. Depending on the comet's orbit, it will take many decades or even centuries before it comes close to the sun again.
A catastrophe of unimaginable proportions: almost 15 million years ago, a gigantic explosion shook southern Germany. It was stronger than a hundred thousand atomic bombs combined and killed all living things within a radius of at least one hundred kilometers. There weren't any people back then, but all animals and plants were killed by a huge shock wave or buried under the rubble. The bang could still be heard on the other side of the earth.
It was triggered by a meteorite about 1,500 meters in size, which raced towards the earth at about 20 kilometers per second. The impact of this chunk caused an earthquake of magnitude eight and tore a 15-mile crater into the surface of the earth. 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 inside out: 150 cubic kilometers of rock were hurled into the air and some flew 450 kilometers into what is now the Czech Republic.
Fortunately, impacts from meteorites of this kind, which weigh several tons, are very rare, they only happen once every few million years.
The little brother
The crater remnants of the Nördlinger Ries can be clearly seen on an elevation map. But you can see something else there: about forty kilometers southwest of the Nördlinger Ries is a similar but significantly smaller crater, the Steinheim Basin, about three and a half kilometers in diameter. This crater can also be traced back to a meteorite impact - however, this metorite was only about 150 meters tall.
Two meteorite craters so close together - can that be a coincidence? No, say the scientists who examined the rocks of the craters: Both craters were formed at the same time. Either a small part of the meteorite broke off while approaching the earth and hit separately, or the asteroid had a small companion who fell with it to earth for a long time.
What is our solar system and how did it come about?
The earth is not alone in space: people have been observing the sun, moon and stars in the sky for a long time. They discovered early on that some stars are moving. These wandering stars were observed and their paths followed. But for a long time they did not understand their movements - until about five hundred years ago a man by the name of Nicolaus Copernicus solved the riddle: The earth and the "wandering stars" are actually planets that all orbit the sun at different distances.
Today we know eight planets. To remember their names in the correct order, the first letters of the sentence "M.a Vater eclarifies mir jEden S.monday uurens Nachthimmel. “- or in short: M-V-E-M-J-S-U-N.
M.Erkur is the planet that orbits closest to the sun. Then come Venus, E.rde and M.ars. These four inner planets have a solid surface made of rock and are still relatively close to the sun - only a few hundred million kilometers.
They are circling further out, at a distance of about one to 4.5 billion kilometers from the sun outer planets: Jupiter, S.aturn with his rings, Uranus and all the way outside Neptun. They are made of gas (mostly hydrogen and helium) and are much larger than the inner planets. Jupiter and Saturn are about ten times the size of the earth, that's why they are also called that Gas giants.
And finally there are asteroids, comets, and clouds of dust that also orbit the sun. The gravitational pull of the sun holds all these heavenly bodies together and forces them to fly in a circle like on a long line. Everything together is called that Solar system. The moons are one of them - but they are held in place by the gravitational pull of the planets.
But why does the sun even have planets? This has to do with how the sun came into being: a cloud of gas and dust contracted by its own gravity and became a star. But not all of the material in this cloud was "built into" the star - around one percent was left over. And when the sun began to shine, the radiation pushed the remaining matter back outwards.
The light gases were pushed far outwards, the heavier dust and rocks remained close to the sun. From these clouds of dust and gas, the planets emerged over time. Therefore there are the gas planets outside in the solar system, further inside the rock planets - including our earth - and in the very center the sun. It contains 99% of the mass of the solar system and holds everything together with its gravity.
The geological ages
The earth has changed a lot since it was formed: mountains, seas and continents have arisen and passed, animal and plant species have spread and become extinct. Most of these changes happened very slowly, over many millions of years. But every now and then there were decisive events: within a few thousand years the environmental conditions changed drastically.
For the scientists studying the history of the earth, these drastic changes are like a new chapter in a book: they divide the earth's history into different sections, the Eons to be named.
At the beginning, 4.5 billion years ago, the earth was completely uninhabitable. It emerged as a hot ball of glowing molten rock, surrounded by hot, caustic and poisonous gases. That sounds like a description of hell - and the name of this time comes from the Greek word "Hades" for hell: Hadaikum. It ended about four billion years ago with the first big change: The earth had cooled down so much that the surface became solid - the earth got a crust.
The earth continued to cool, so that liquid water could collect on the crust: seas were formed. And life began in these seas around 3.8 billion years ago - but initially only in the form of the simplest bacteria. The Greek word for origin or beginning is in the name of this time: Archean. An important climate change about 2.5 billion years ago marked the transition to the next epoch: primitive living things began to influence the environment. They produced oxygen that was previously almost completely absent from the atmosphere.
The early unicellular life forms became more complex over time, they formed cell nuclei. Later, some began to work together on a permanent basis in alliances - this ultimately resulted in the first multicellular organisms. However, they did not yet have solid shells or skeletons, so that hardly any fossils have survived from this period. This epoch owes its name to this time before the fossils were formed: Proterozoic.
The Proterozoic era ended with an explosion of life 550 million years ago: within a short period of time, the primitive forms of life developed into an enormous biodiversity. These species were built much more complex - and some already had hard shells, which were first preserved as fossils. Therefore, the history of life only becomes really visible to scientists from this point in time. And this epoch is named after the Greek term for "visible": Phanerozoic.
This age of life has lasted for 550 million years until today. However, life did not develop evenly either: After the explosive spread of life, there were two devastating mass extinctions. These mark further important turning points in the history of the earth, so that scientists divide the age of life, the Phanerozoic, into three sections, Eras called, divide.
The oldest era of the Phanerozoic began 550 million years ago with the mass emergence of new species. They are called that Antiquity or Paleozoic. At first life only took place in the oceans. Then the plants colonized the land, later the animal world followed suit: first the amphibians developed, which could already feel their way a little on land, and finally also reptiles, which became independent of water and conquered the land. The ancient world ended about 251 million years ago with the greatest mass extinction of all time: Over 90 percent of all animal and plant species died out, especially in the oceans. The reason has not yet been finally clarified. Scientists suspect that an ice age was to blame, possibly as a result of a meteorite impact.
When the surviving animal and plant species had to get used to their new environment, it broke Earth Middle Ages or Mesozoic at. It is primarily the age of the dinosaurs: giant lizards evolved and ruled life for almost 200 million years. But the Middle Ages also ended with a decisive event: about 65 million years ago a large meteorite hit the earth. So much dust and ash was thrown into the air that the sky darkened and the climate changed for a long time. The dinosaurs and many other species became extinct.
Small mammals in particular benefited from this, as they were best able to adapt to climate change. They had already developed in the Mesozoic, but remained in the shadow of the dinosaurs. Now they could spread rapidly, conquer the most varied of habitats and keep developing. Humans also descend from this group. This most recent age continues to this day and therefore becomes the Earth New Age or Cenozoic called.
This rough classification of the earth's history is based on very drastic changes in life: explosive multiplication or mass extinction. In between, however, there were further upheavals due to various other influences - changes in the seas and continents due to continental drift, climate change between ice ages and warm periods, the composition of the air and much more. The new conditions always favored individual species and disadvantaged others. So the three sections of the Phanerozoic (Age of Life) can each be divided into several periods.
The beginnings of the earth
We would not recognize 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 couldn't the 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. Its own gravity pressed these individual parts together so that they were subjected to strong pressure. This pressure was naturally highest in the core of the earth, on which the weight of the entire outer layers weighed. As a result of the high pressure, the rock was heated up and melted. Outwardly, the pressure and thus also the temperature became less. Even so, the surface of the earth remained very hot for several hundred million years and could not cool down and solidify.
In order to understand the reason for this, the scientists had to look at the moon: Ancient lunar craters from the time the solar system was formed tell us that the moon was hit by numerous meteorites when it was young. It is therefore assumed that the earth was also exposed to a real rock bombardment from space at the same time. The lumps fell to the earth at high speed - and the impacts were correspondingly violent: Even lumps of a few hundred tons could easily cause an explosion the strength of an atomic bomb!
So the earth's surface continued to heat up for a long time, stirred up again and again and remained so fluid. Only when the impacts gradually subsided after a few hundred million years did the temperatures on the earth's surface drop. The rock could slowly solidify and form an earth crust that grew thicker and thicker over the course of millions of years. But to this day it is only a very thin layer that floats on a viscous, hot interior of the earth.
Why does the moon have spots?
The man in the moon - known from songs, films and stories. Indeed, there are conspicuous dark spots on the lunar surface, and with a little imagination you can see a face in them. But what are these spots really?
At first, scientists thought the dark spots were seas. But at least since the first visit to the moon in 1969 it has been clear: the moon is dust-dry, the entire surface of the moon consists of fine gray rock powder. And the dark spots are great plains that are simply filled with darker dust. This makes the moon appear speckled light and dark. But how did these plains come about?
The plains are almost as old as the moon itself.When, in the early days of the solar system, 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 plains. Lava rock is darker than the crustal rock, so the plains appear darker.
There are now hardly any large asteroids hitting the moon, but still lots 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, the force of the impacts is only enough to crumble some rock and stir up a bit of dust, which quickly sinks back to the ground. Therefore, the surface of the moon today consists of rock dust, mainly of light crustal rock and, in the lowlands, of darker lava rock. From the earth it looks like spots, seas - or a face.
How did the water come to earth?
About two thirds of the earth is covered with water - a unique selling point: the earth is the only planet in the solar system on which there is liquid water. Life originated in water, and water is also vital for us humans. But where does the water on earth actually come from?
Scientists suspect that the water comes from comets. These lumps of ice and dust originally formed on the edge of the solar system. But some also got into the interior of the solar system on orbits and became part of the newly emerging planets.
Initially, the young planets were very hot - so hot that the rock melted and formed a liquid ball. And the ice on the comets not only melted, it even evaporated. Because the water vapor was much lighter than the molten rock, it bubbled up towards the surface. There it escaped into the atmosphere through volcanoes.
As the earth slowly cooled, the steam turned back to liquid water. To put it more clearly: It started to rain. Those first downpours must have been stronger than any thunderstorm we can imagine today. And it must have rained for a very long time - tens of thousands of years. Large parts of the young earth's surface were flooded - in some places up to ten kilometers high. This is how the oceans came into being.
And what happened to the water on the other planets? Why are there no oceans there? Mercury doesn't have enough gravity to hold an atmosphere at all - like all gases, water vapor simply escaped into space. The same thing happened on the moon. The solar radiation on Venus is so strong that the water has also evaporated into space. On the other hand, it is too cold on Mars, but there are suspected large deposits of ice beneath the surface. And the gas planets have no solid surface on which seas could form. One suspects an ocean of water on Jupiter's moon Europa, but the surface is frozen. So the earth remains the only celestial body in the solar system with seas.
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