The blue planet

Seen from space, the earth’s sphere appears a strong blue. This is because almost three quarters of the earth is covered with water. Although water is transparent in small quantities, from a certain depth it takes on an increasingly strong blue shimmer. Because we see the mighty oceans as blue, the Earth is also called “the blue planet”. The term is especially true south of the equator. This is because the southern hemisphere is almost completely covered by sea, because a large part of the continents have migrated northwards due to plate movement.

The vast oceans contain almost all the water on earth. A lot of salt is dissolved in seawater, which is why it is not suitable as drinking water. What little fresh water there is on earth is mainly frozen in glaciers and ice caps. Only a tiny fraction of freshwater is found in groundwater, 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 against the diameter of the Earth, the oceans are only a wafer-thin layer. Therefore, the water makes up only a fraction of the Earth’s mass. By comparison, if the Earth were the size of a basketball, all the Earth’s water would fit into a ping-pong ball. And the drinking water would be proportionally even smaller than a single popcorn.

A small blue dot

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

This picture shows the Earth. It was taken by the Voyager 1 probe from the edge of the solar system – 6.4 billion kilometres 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.

While the image has no scientific value, it shows a fascinating and eerie view of our planet: From this distance, Earth is just a tiny grain of sand in space, our island in the middle of an empty, hostile nothingness.

The Voyager 1 probe and its identical sister, Voyager 2, were launched in 1977 to explore the outer solar system. It visited Jupiter in March 1979 and Saturn in November 1980. It provided impressive close-up images of the moons and rings of both planets. As the probe continues its journey, scientists hope to obtain new and interesting measurement data from the edge of the solar system – and the region beyond.

Before it left the solar system for good, however, the scientists activated the camera one last time to take these pictures.

A record for aliens

As with previous probes, NASA has also equipped Voyager 1 and 2 with a message to extraterrestrials. For this purpose, a copper plate was attached to the probe and covered with gold. On the front is engraved instructions on how to play the images and sounds on the back. This contains greetings in 55 languages, animal voices and other sounds from nature, music (including Bach and Mozart) and a personal address by the then US President Jimmy Carter, just like on a record. 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. Therefore, they could be those man-made objects that exist the longest ever – estimated up to 500 million years!

The researchers were attracted by the following idea: what if in the distant future, far away from the solar system, extraterrestrial astronomers discover, capture and study one of the probes? They then decided to give their extraterrestrial colleagues some information about the probe’s builders, a kind of cosmic message in a bottle.

However, space is unimaginably vast and empty. Therefore, it is very unlikely that aliens will actually find the probe. And even if they do: Earth will look very different then – and probably no humans will be alive then either.

The world of the oceans

Many secrets still 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. What we do know, however: Almost all the water on this earth – 97.5 per cent to be exact – laps in the five oceans.

The largest of all oceans is the Pacific. Its water surface measures a total of 180 million square kilometres! This means that it accounts for about half of all ocean surfaces. At the same time, this world ocean is home to the deepest place on earth: it descends up to 11,034 metres into the Witja Deep in the Mariana Trench, a deep-sea trench in the western Pacific.

The Atlantic Ocean is the second largest ocean. It was formed about 150 million years ago when the primeval continent of Pangaea broke apart. With its 106 million square kilometres, it covers one fifth of the earth’s surface.

Most of the Indian Ocean lies in the southern hemisphere. With an area of just under 75 million square kilometres, it is a good deal smaller than the Atlantic and Pacific Oceans. Its deepest point is called the Diamond Low, which lies 8,047 below sea level.

The Southern Ocean is also called the Southern or Antarctic Ocean. It includes all sea areas south of the 60th parallel in the southern hemisphere. Sailors consider it the stormiest of all seas. Typical of the Southern Ocean are also the large tabular icebergs that float in its waters. They have broken off from the ice shelf that has formed around the Antarctic continent.

Around the North Pole lies the Arctic Ocean, also 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. But its ice cover, like the ice of the Southern Ocean, continues to melt due to global warming.

Even if we live several hundred kilometres away from them: Oceans are of great importance to us. Their currents and the evaporation of seawater have an enormous influence on our weather. A large part of the air we breathe is also produced in the 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 ever swallowed water while bathing in the sea knows from personal experience: seawater tastes salty. And when the water evaporates, a fine white layer of salt often remains on the skin. This is because seawater consists on average of 3.5 percent salt. For one litre of seawater, that’s 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 in the earth’s crust. Rainwater dissolves salts from the rock and takes them with it. It washes them into rivers and groundwater. In this way, salts are washed into the sea. Because relatively little salt is transported, the river water is hardly salty. The concentration only increases in the sea. This is because salts from the ocean floor and from submarine volcanoes are added there. When the seawater evaporates, all these salts remain. This is why salts that have been washed out have been accumulating in the oceans for millions of years.

The salt content is not the same in all seas. The more water evaporates, the more saline 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 about 30 per cent that you can lie in it without sinking. In contrast, the Baltic Sea is rather low in salt: because of the low temperature, only little water evaporates there. In addition, many rivers flow into the inland sea and feed it with fresh water.

How sweet is fresh water?

Although it does not taste sweet at all, it is called fresh water. Unlike 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 obtaining drinking water.

Freshwater is rare: Of all the water on earth, only two to three percent is fresh water. Most of it is found in the high mountains and at the two 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 for us. To stay healthy, humans need about two litres of liquid per day; without water they can only survive for five to seven days. In addition, we need a large amount of fresh water for showering, washing clothes or dishes. Plants and animals that we feed on also live on water. Freshwater is even the habitat for many creatures: crayfish, pond and river mussels and freshwater fish such as trout, pike-perch and char.

What is a glacier?

Glaciers flow down from the mountains like white tongues. Others cover huge land masses as mighty sheets of ice. Glaciers consist mainly of ice and can be hundreds of metres thick and several kilometres long. Most of the fresh water on Earth is frozen into ice! But how do such ice masses come about in the first place?

Glacier ice forms where it is very cold all year round. Such low temperatures prevail far up in mountains, for example in the Alps. The snow that falls there does not even thaw completely in summer. The snow cover therefore becomes thicker and heavier. Under this load, the loose snowflakes are pressed over time first into grainy firn and then into dense ice.

Even in the areas around the North Pole or South Pole, more snow falls throughout the year than can thaw again. Glaciers then form, even in flat landscapes. The glaciers of the polar regions are thousands of metres thick. They have the shape of huge shields and are therefore called ice sheets.

Glaciers flow downhill very slowly under the weight of their own weight. Meltwater at their bottom makes it easier for them to glide over the ground. With their mass of ice, they also drag sand and rock fragments that have been blasted off the ground by frost.

If a glacier eventually advances into warmer regions, its ice melts. The meltwater runs off in a trickle; if there is a large amount of water, a river forms. If the meltwater collects in a hollow, it forms a glacial lake.

The cycle of water

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 drop is lost.

The motor of the water cycle is the sun: it heats the water of the oceans, lakes and rivers so much that it evaporates. Plants also release water vapour into the atmosphere through tiny openings. The moist air rises upwards, 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 to earth, it seeps into the ground, feeds plants or flows through the ground, via streams and rivers back into the sea. The eternal cycle of evaporation, precipitation and runoff starts all over again.

The water cycle has existed almost as long as the earth has. It 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 even exist.

How is the earth structured?

In the beginning, the young Earth was a hot ball of molten matter. All the 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 over time this changed: the heavier substances sank downwards to the centre of the earth – especially metals. Rocks, on the other hand, were somewhat lighter and rose upwards, the lightest ones to the earth’s surface. There they slowly cooled and solidified.

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

Beneath the crust is the “flesh” – the almost 3000 kilometre thick mantle of heavy viscous rock. And inside the earth lies the earth’s core made of the metals iron and nickel.

The Earth’s core itself initially consists of an outer layer about 2200 kilometres thick, the outer core. It is over 5000 degrees Celsius hot there, so the metal is molten and as thin as mercury.

On the very inside is the inner core, somewhat smaller than the moon. At over 6000 degrees Celsius, it is even hotter than the outer core – but surprisingly solid. This is because with increasing depth, not only the temperature rises, but also the pressure. The outer layers that weigh on the Earth’s core compress its material so unimaginably strongly that it cannot liquefy

How did the water get to earth?

About two-thirds of the Earth is covered with water – a unique feature: the Earth is the only planet in the solar system that has 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 at the edge of the solar system. Some, however, also fell into the interior of the solar system on orbits and became part of the freshly formed planets.

In the beginning, the young planets were very hot – so hot that the rock melted and formed a liquid sphere. And the ice of the comets not only melted, but even evaporated. Because the water vapour was much lighter than the molten rock, it bubbled upwards towards the surface. There it escaped through volcanoes into the atmosphere.

Then, as the earth slowly cooled, the steam turned back into liquid water. More vividly, it began to rain. These first downpours must have been stronger than any thunderstorm we can imagine today. And it must have rained for a very long time – several tens of thousands of years. Large parts of the young earth’s surface were flooded – in some places up to ten kilometres high. This is how the oceans were formed.

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 – the water vapour simply escaped into space like all gases. The same thing happened on the moon. On Venus, the solar radiation is so strong that the water also evaporated out into space. On Mars, on the other hand, it is too cold, but large deposits of ice are suspected beneath the surface. And the gas planets have no solid surface on which seas could form. On Jupiter’s moon Europa, an ocean of water is suspected, but the surface is frozen. So the Earth remains the only celestial body in the solar system with oceans.