How long do they live

We would not recognize the earth immediately after its formation. It was a most uncomfortable planet: there were neither continents nor oceans, but a seething surface of red-hot, viscous magma. Why no earth crust could form for a long time?

A glowing ball of rock: the freshly formed earth
Source: Colourbox

A good 4.5 billion years ago, comets, asteroids, gas and dust condensed to form our planet. The own gravity pressed these individual parts together, so that they were exposed to a 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. To 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 was unable to cool and solidify.

Meteorite impacts heat up the young planet
Source: Colourbox

To understand the reason for this, 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. One assumes therefore that also the earth was exposed at the same time to a regular rock bombardment from the universe. The boulders crashed with high speed on the earth – and the impacts were accordingly violent: Even lumps of a few hundred tons could easily cause an explosion as powerful as an atomic bomb!

Volcanic eruptions show that the earth’s interior is still hot and liquid.
Source: Colourbox

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

The earth ages

Since its formation, the Earth has changed a lot: Mountains, oceans and continents have formed and passed away, 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 drastic events: Within a few thousand years, environmental conditions changed drastically.

Eons: The great chapters of Earth’s history
Source: SWR

For the scientists who study 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 Aeons called.

At the beginning, 4.5 billion years ago, the earth was completely uninhabitable. It was formed as a hot ball of glowing molten rock surrounded by hot, corrosive and toxic gases. This sounds like a description of hell – and from the Greek word "Hades" for hell also comes the name of this time: Hadaic. 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.

In the Hadaikum the earth was a ball of liquid rock
Source: Colourbox

The Earth continued to cool, allowing liquid water to collect on the crust: Oceans were formed. And in these seas life began about 3.8 billion years ago – but at first only in the form of the simplest bacteria. The Greek word for origin or beginning is in the name of this time: Archaic. An important climatic change about 2.5 billion years ago marked the transition to the next epoch: primitive life began to influence the environment. They produced oxygen, which until then had been almost absent from the atmosphere.

Early unicellular life forms became more complex over time, forming cell nuclei. Later, some also began to work together permanently in alliances – this eventually became the first multicellular organisms. However, they had still no firm shells or skeletons, so that from this time hardly fossils are preserved. This epoch owes its name to this time before fossils were formed: Proterozoic.

The Proterozoic ended 550 million years ago with an explosion of life: Within a short time, an enormous diversity of species developed from the primitive life forms. These species were much more complexly built – and some already had hard shells, preserved for the first time as fossils. Therefore, for scientists, the history of life only becomes really visible from this point onwards. And after the Greek term for "visible" this epoch is also named: Phanerozoic.

This age of life has been going on for 550 million years until today. However, the development of life did not proceed evenly either: After the explosive spread of life, there were two devastating mass extinctions. These mark further important cuts in the history of the Earth, so scientists divide the age of life, the Phanerozoic into three sections, Eras called, divide.

The sections of the Phanerozoic
Source: SWR

The oldest era of the Phanerozoic began 550 million years ago with the mass emergence of new species. It is called the Palaeozoic or Paleozoic. At first life took place only in the oceans. Then plants colonized the land, and later the animal world followed suit: First developed the amphibians, which could already venture a little on land, and finally also reptiles, which became independent of the water and conquered the land. The Earth’s Palaeozoic ended about 251 million years ago with the greatest mass extinction of all time: More than 90 percent of all animal and plant species became extinct, especially in the oceans. The reason is not finally clarified until today. 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, the Earth Middle Ages or Mesozoic at. Above all, it is the age of the dinosaurs: giant lizards evolved and dominated life for almost 200 million years. But the Mesozoic era 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.

The Middle Ages were the time of the dinosaurs, like Stegosaurus .
Source: Colourbox

. and Tyrannosaurus Rex.
Source: Colourbox

Small mammals were the main beneficiaries of this period, as they were best able to adapt to climate change. They had already evolved in the middle ages of the earth, but had remained in the shadow of the dinosaurs. Now they could spread rapidly, conquer the most different habitats and develop further and further. Humans are also descended from this group. This youngest age continues until today and is therefore also called the Earth’s new age or Cenozoic called.

Earth’s modern era belongs to mammals
Source: Colourbox

The new earth era belongs to the mammals
Source: Colourbox

The modern era of the earth belongs to the mammals
Source: Colourbox

This rough classification of Earth’s history is based on very drastic changes in life: explosive proliferation or mass extinctions. In between, however, there were further upheavals due to various other influences – changes in the oceans and continents due to continental drift, climate change between ice ages and warm periods, composition of the air, and much more. The new conditions always favored individual species and disadvantaged others. Thus the three sections of the Phanerozoikum (age of the life) can be divided still in each case into several periods.

Overview table of the earth ages
Source: SWR

What is our solar system and how did it come to be??

Earth is not alone in space: For a long time, people have been observing the sun, moon and stars in the sky. They discovered already early that some stars move. These wandering stars were observed and their paths followed. But for a long time, people didn’t understand their motions-until about five hundred years ago, when a man named Nicolaus Copernicus solved the mystery: the Earth and the "wandering stars" are actually planets, all orbiting the sun at varying distances.

The arrangement of the planets
Source: Colourbox

Today we know eight planets. To remember their names in the right order, the first letters of the sentence "Ma Vater erklart mir jeden Son Sunday unsider NEIGHT SKY." – or in short: M-V-E-M-J-S-U-N.

This is how you have to imagine the formation of the solar system
Source: NASA/JPL

Merkur is the planet that orbits closest to the sun. Then come Venus, Erde and Mars. These four inner planets have a solid surface of rock and are still relatively close to the sun – only a few hundred million kilometers.

Further out, orbiting at a distance of about one to 4.5 billion kilometers from the sun, the outer planets: Jupiter, Saturn with its rings, Uranus and far outside Neptun. They consist of gas (mainly hydrogen and helium) and are much larger than the inner planets. Jupiter and Saturn are about ten times the size of Earth, which is why they are called the Gas giants.

And finally there are asteroids, comets and dust clouds, which also orbit the sun. The sun’s gravitational pull holds all these celestial bodies together, forcing them to fly in circles as if on a long leash. All together this is called Solar system. Also the moons belong to it – but they are held by the attraction of the planets.

But why does the sun have planets at all? This is related to how the sun was formed: a cloud of gas and dust pulled together by its own gravity and became a star. But not all material of this cloud was "built up" in the star – about one percent remained. And then, when the sun began to shine, the radiation pushed the remaining matter back outward.

The light gases were pushed far outward, the heavier dust and rock remained near the sun. From these clouds of dust and gas, the planets formed over time. Therefore, in the solar system, there are the gas planets on the outside, the rocky planets further in – including our Earth – and the sun in the very center. It contains 99% of the mass of the solar system and holds everything together with its gravity.

What are asteroids, meteors 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, who would see such a shooting star, could wish for something. But what’s really behind it and where do shooting stars come from?

Asteroid 951 Gaspra
Source: NASA

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 big siblings, they orbit around the sun in regular orbits. Most asteroids are found in the "asteroid belt" between the orbits of Mars and Jupiter.

Barringer crater was formed by a meteorite impact
Source: USGS

Every now and then two of these asteroids collide. A crash like this creates lots 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 by it and crash to the earth. These crashing chunks are also called Meteorite.

Close-up of comet Hartley, you can see dust and vapor
Source: NASA/JPL-Caltech/UMD

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 can not evade fast enough to the side. The air in front of the falling chunk of rock is compressed and thus extremely hot. The air begins to glow and the meteorite begins to evaporate. We can then see this as a luminous streak moving across the sky – a shooting star.

The coment "NEAT
Source: NASA

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 meteors do to the Earth depends on how big they are. Small meteorites with a diameter of a few centimeters, 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 kilometers and ripped a crater 180 kilometers in diameter. The impact threw so much dust into the air that the sun was eclipsed for hundreds of years. This caused the extinction of plants and animals 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 about them. In addition, 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 longer: Comets with their tails are 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. There it is so cold that water immediately freezes to ice. Thus, clumps of ice chunks and dust are formed – dirty snowballs in fact.

A comet also initially travels far from the sun – until it is deflected by a collision and flies toward the inner solar system. It comes closer to the sun and receives more and more light and warmth with the time. This causes the frozen surface to begin to thaw and even evaporate. So a shell of water vapor and dust forms around the comet.

At the same time, the comet gets to feel the "solar wind" – these are tiny particles flying out of the sun at high speed. You hit the vapor envelope of the comet. This blows the vapor envelope of the comet away, forming an elongated cloud pointing away from the sun. When this cloud is hit by sunlight, it appears as a luminous streak – the tail of the comet.

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

Why is the earth round?

"What happens if you always go in the same direction? At some point you come to the edge of the world or is the world infinitely large?"Already more than 2300 years ago the famous Greek scientist Aristotle was sure: Neither the one nor the other. Because the Earth is not flat like a disk, but a sphere – but why?

Already Aristotle knew: The earth is round!
Source: imago stock&people

To understand this, one must go back to the time when the earth was formed. The force that was responsible for this is gravity – all massive objects attract each other. This force made chunks of rock collide and combine to form a planet. And it gave the planet its shape. Because gravity acts equally in all directions.

After its formation, the Earth was hot and liquid and formed into a sphere.
Source: Colourbox

Since the earth was hot and liquid at the beginning, the material could flow into the form that gravity dictated. If somewhere a piece of earth material moved further out, it was attracted by the rest, until the surface was smooth and the same gravity worked at all places. And since the force of gravity is the same in all directions, the earth automatically took the form of a sphere – because only in a sphere are all points on its surface equidistant from the center of gravity.

But if you look closely at the shape of the Earth, you will see that it is not a perfect sphere: it is slightly flattened at its poles and somewhat bulbous at the equator.

The reason for this is the earth’s rotation: In the course of 24 hours, the earth rotates once around its axis. The rotational motion creates a force, the centrifugal force. We know this from the chain carousel, when we fly outward in the swings. With the earth the centrifugal force causes that the rock masses slip a piece from the axis of rotation outward, thus from the poles in the direction of the equator. There the diameter of the earth is about 41 kilometers larger than between the north and south pole.

Why is the earth warm inside?

The liquid interior of the Earth bubbles beneath our feet. Volcanic eruptions and geysers show the heat that prevails there – over 6000 degrees Celsius in the Earth’s core. But why is it actually so hot in the earth?

The earth was first a glowing sphere after its formation.
Source: Colourbox

A large part of the heat still comes from the infancy of the earth, when dust and rock fragments condensed to form a planet. However, the word "compress" sounds a bit too harmless: In reality, you have to imagine it like many big meteorite impacts – each impact a gigantic explosion that heated up the young planet and melted the material.

Meteorite impacts heated the young earth additionally.
Source: Colourbox

Since then, things have calmed down a bit and the earth is cooling down again. However, it does this extremely slowly, the heat in the earth’s interior can escape only very slowly into space. Hot magma flows in the tough mantle of the Earth transport the heat upwards. There it remains enclosed under the rigid earth’s crust like under a lid. Only slowly the crust rock releases the warmth into the universe.

Moreover, heat is still being produced inside the earth. This is due to the fact that the Earth has a lot of radioactive substances in its core, such as uranium. Since the formation of our planet, they decay and give off heat over a very long period of time. This "fuel" is enough for many billions of years.

How life arose?

The origin of life on earth has been puzzled over for a long time. It is known that simple bacteria evolved as early as 3.8 billion years ago. But how was that possible – can life come into being just like that??

The primordial atmosphere could have looked like this
Source: Colourbox

A student named Stanley Miller came up with an idea in 1953: He wanted to recreate the environmental conditions on Earth about 3.8 billion years ago in an experiment. For this purpose, he filled a glass flask with water as well as some gases that were probably components of the primordial atmosphere: Ammonia, methane and hydrogen. In this gas mixture he ignited electrical discharges to simulate the lightning of the thunderstorms of that time. The water was meant to mimic the natural water cycle. There was a heater where the water evaporated and cooling coils where it condensed again.

Lightning provided the energy for chemical reactions
Source: Colourbox

Miller let this experiment run for several days and examined the water afterwards. In it, he found a certain kind of chemical compound: Amino acids, an important component of the cells of all living things. In this way, Miller proved that the building blocks of life can be formed from simple gases.

That’s why scientists today assume that the gases in the primordial atmosphere reacted in a similar way to form organic substances. Rain washed them into the sea, especially in shallow waters high concentrations could accumulate. Whether by aggressive sunbeams or lightning – the particles must have reacted with each other again and again. A random combination of molecules then had for the first time a special property: it was able to multiply itself – the beginning of life.

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