Why does the earth appear blue

The blue planet

Seen from space, the globe appears in a strong blue. This is because almost three quarters of the earth is covered with water. Small amounts of water are transparent, but from a certain depth onwards it becomes more and more blue. Because we see the mighty oceans blue, the earth is also called "the blue planet". The term south of the equator is particularly applicable. Because the southern hemisphere is almost completely covered by the sea, because a large part of the continents have migrated to the north due to plate movement.

The vast oceans contain almost all of the water on earth. There is a lot of salt dissolved in sea water, which is why it is not suitable as drinking water. The little fresh water on earth is frozen mainly in glaciers and ice caps. Only a tiny fraction of freshwater is found in groundwater, in lakes and rivers, or in the air.

But the view from the outside is deceptive: the earth's surface is largely covered by water, but measured by the diameter of the earth, the oceans are only a very thin layer. Therefore, the water makes up only a fraction of the earth's mass. For comparison: if the earth were the size of a basketball, all the water on earth would fit into a table tennis ball. And the drinking water would be proportionally even smaller than a single popcorn.


You have to search for a while in this picture: The sensation is a “pale blue dot”, a tiny light blue dot in the void. It is hard to imagine that this little dot should be our home!

This picture shows the earth. It was picked up by the Voyager 1 probe from the edge of the solar system - 6.4 billion kilometers from Earth. It is part of a unique group photo of our solar system, which is composed of a total of 60 individual photos and contains all planets except Mars and Mercury.

Although the picture has no scientific use, it shows a fascinating and eerie view of our planet: From this distance, the earth is just a tiny grain of sand in space, our island in the middle of an empty, hostile void.

The probe Voyager 1 and its identical sister, Voyager 2 were launched in 1977 to explore the outer solar system. In March 1979 she visited Jupiter, in November 1980 Saturn. It provided impressive close-ups of the moons and rings of both planets. On the further journey of the probe, scientists hope to obtain new, interesting measurement data from the edge of the solar system - and the area beyond it.

Before it finally left the solar system, however, the scientists activated the camera one last time for these recordings.

A record for aliens

As with previous probes, NASA has also equipped Voyager 1 and 2 with a message to aliens. For this purpose, a copper plate was attached to the probe and coated with gold. Instructions for use on how to reproduce the images and sounds on the back are engraved on the front. Like on a record, it contains greetings in 55 languages, animal voices and other noises from nature, music (including by Bach and Mozart) and a personal address by the then US President Jimmy Carter. In addition, photos of life on earth and scientific graphics are stored there.

The idea behind it: These probes will leave our solar system and fly out into the void of space. There is nothing there to damage or decompose the probes. Hence, they could be the man-made objects that have existed the longest - estimated up to 500 million years!

The researchers were drawn to the following idea: What if, in the distant future, far away from the solar system, extraterrestrial astronomers discover, capture and investigate one of the probes? Then they decided to give their extraterrestrial colleagues some information about the builders of the probe, a kind of cosmic message in a bottle.

However, space is unimaginably large and empty. Therefore, it is very unlikely that aliens will actually find the probe. And even if: The earth will then look completely different - and then probably no people will be alive either.

The world of the oceans

To this day, many secrets lie dormant in the depths of the oceans. Large parts of the world's oceans are still completely unexplored. We even know the moon better than the deep sea. But what we do know: Almost all of the water on earth - 97.5 percent to be precise - ripples in the five oceans.

The largest of all oceans is that Pacific. Its water surface measures a total of 180 million square kilometers! It makes up about half of all ocean areas. At the same time, the deepest point on earth is located in this ocean: it descends up to 11,034 meters into the Vitja Depth in the Mariana Trench, a deep-sea trench in the western Pacific.

The Atlantic is the second largest ocean. It was formed about 150 million years ago when the supercontinent Pangea broke up. With its 106 million square kilometers, it covers a fifth of the earth's surface.

The Indian ocean is mostly in the southern hemisphere. With an area of ​​almost 75 million square kilometers, it is a good deal smaller than the Atlantic and Pacific. Its deepest point is called Diamantina Depth, which is 8,047 below sea level.

The Southern Ocean is also called the Southern or Antarctic Ocean. It includes all marine areas south of the 60th parallel in the southern hemisphere. It is considered by seafarers to be the stormiest of all seas. The large tabular icebergs floating in its water are also typical of the Southern Ocean. They broke off the ice shelf that formed around the Antarctic continent.

That's all around the North Pole Arctic Oceanalso known as the Arctic Ocean. It is the smallest of the five oceans. About two thirds of the Arctic Ocean is covered with ice in winter. However, like the ice in the Southern Ocean, its ice cover continues to melt as a result of global warming.

Even if we live a few hundred kilometers away from them, oceans are very important to us. Their currents and the evaporation of sea water have an enormous influence on our weather. A large part of the air we breathe is also created in the world's oceans: algae that live here convert carbon dioxide into oxygen when exposed to sunlight.

How does the salt get into the sea?

Anyone who has swallowed water while bathing in the sea knows from their own experience: Sea water tastes salty. And when the water evaporates, a fine white layer of salt often sticks to the skin. This is because, on average, seawater consists of 3.5 percent salt. For one liter of sea water that is 35 grams or about one and a half heaped tablespoons of salt. But how does the salt actually get into the sea?

Many of these salts come from the rocks of the earth's crust. Rainwater dissolves salts from the rock and takes them with it. It washes them into rivers and into the groundwater. This is how salts are washed into the sea. Because relatively little salt is transported, the river water is hardly salty. Only in the sea does the concentration increase. Because there are also salts from the ocean floor and submarine volcanoes. When the sea water evaporates, all of these salts are left behind. That is why washed-out salts have been accumulating in the oceans for millions of years.

The salinity is not the same in all seas. The more water evaporates, the more salty the water becomes. The Red Sea contains more salt than the Pacific. And the Dead Sea in the Middle East - actually a lake - is so salty with a salt content of around 30 percent that you can lie in it without sinking. The Baltic Sea, on the other hand, is rather poor in salt: because of the low temperature, very little water evaporates there. In addition, many rivers flow into the inland sea and feed it with fresh water. That is why the Baltic Sea is much less salty than the Dead Sea.

How sweet is fresh water?

It doesn't taste sweet at all, but it's called fresh water. In contrast to salt water, it contains no or only very small amounts of salt and therefore has hardly any taste. For this reason it is also well suited for the production of drinking water.

Fresh water is rare: only two to three percent of all water on earth is fresh water. Most of it is in the high mountains and at both poles. There it is stored as ice in glaciers. Only a very small fraction of the fresh water on earth flows in streams and rivers or splashes in lakes and groundwater. The water in clouds and precipitation is also "sweet".

Fresh water is vital to us. To stay healthy, a person needs about two liters of fluids per day; without water it can only survive five to seven days. In addition, we need a large amount of fresh water for showering, washing clothes or washing dishes. Plants and animals that we feed on also live from water. Freshwater is even a habitat for many living things: crayfish, pond and river mussels and freshwater fish such as trout, pikeperch and char.

What is a glacier?

Glaciers flow down from the mountains like white tongues. Others cover huge land masses as mighty ice sheets. Glaciers are primarily made up of ice and can be hundreds of meters thick and several kilometers long. Most of the fresh water on earth is frozen into ice! But how do such ice masses come about?

Glacier ice forms where it is very cold all year round. Such low temperatures prevail high up in mountains, for example in the Alps. The snow that falls there doesn't completely thaw, even in summer. The snow cover is therefore getting thicker and heavier. Under this load, the loose snowflakes are first pressed into granular firn and then into thick ice.

In the areas around the North Pole or South Pole, too, more snow falls throughout the year than can thaw again. Then, even in flat landscapes, glaciers form. The glaciers of the polar regions are thousands of meters thick. They are shaped like huge shields and are therefore called ice sheets.

Glaciers flow very slowly down the slope under the weight of their own weight. Melt water at their bottom makes it easier for them to slide across the ground. With their ice mass, they also drag sand and rocks with them that have been blown off the ground by frost.

If a glacier finally penetrates into warmer regions, its ice melts. The meltwater flows off in a trickle; a river forms when there is a large amount of water. If the meltwater collects in a basin, a glacier lake is created in it.

The water cycle

The water on earth is always on the move. Huge amounts of it are constantly moving - between sea, air and land - in an eternal cycle in which not a single drop is lost.

The motor of the water cycle is the sun: It heats the water of the seas, lakes and rivers so much that it evaporates. Plants also release water vapor into the atmosphere through tiny openings. The humid air rises, tiny water droplets gather in the air and form clouds. As rain, hail or snow, the water falls back into the sea or onto the earth. If it falls on the ground, it seeps into the ground, supplies plants or flows through the ground, over streams and rivers back into the sea. The eternal cycle of evaporation, precipitation and runoff starts all over again.

The water cycle has been around for almost as long as the earth has existed. He ensures that living beings on our planet are supplied with fresh water. And not only that: Without the water cycle, the weather as we know it would not exist.

How is the earth structured?

In the beginning, young earth was a hot ball of molten matter. All components were initially well mixed, just as they were distributed when the earth was formed: Metals, rocks, trapped water and gases and much more - a big mess.

But in the course of time that changed: The heavier substances sank down to the center of the earth - especially metals. Rocks, on the other hand, were a little lighter and rose, the lightest to the surface of the earth. There they slowly cooled down and froze.

So the material of the earth separated into the three spherical layers that we know today. You can imagine the structure of the earth like a peach: on the outside a wafer-thin “shell” made of light, solid rock - the Earth crust. On average, it is only 35 kilometers thick.

Under the crust is the "pulp" - the almost 3000 kilometers thick Mantle made of heavy, viscous rock. And inside the earth lies that Earth core from the metals iron and nickel.

The core of the earth itself consists initially of an outer layer about 2200 kilometers thick, the outer core. It is over 5000 degrees Celsius there, which is why the metal has melted and is as fluid as mercury.

That is right inside inner core, slightly smaller than the moon. At over 6000 degrees Celsius, it is a little hotter than the outer core - but surprisingly solid. This is because with increasing depth, not only does the temperature increase, but also the pressure. The outer layers that weigh on the earth's core compress its material so unimaginably that it cannot liquefy.

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 actually come from on earth?

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. On Venus, the solar radiation is so strong that the water has also evaporated into space. On the other hand, it is too cold on Mars, but large ice deposits are suspected there under 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.