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From dust to life – the birth of the Earth

Posted in Geology, Prehistory, Science, Space on Tuesday, 30 August 2011

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This edited article about the Solar System originally appeared in Look and Learn issue number 1048 published on 10 April 1982.

Earth, picture, image, illustration

How the earth formed out of cosmic dust into the familiar world we know, with a cross-section of the earth’s crust showing tectonic plates

The planets of our Solar System formed from a collapsing cloud of dust and gas surrounding a young star, our Sun. The nature of each planet today depends on how far away the cloud of material it formed from was from the Sun, because the heat from the young star vaporised the lighter materials and blew them off into space.

Closest to the Sun, planet Mercury is small and dense, composed only of very heavy materials, since the fierce solar heat drove away everything else. Far from the Sun, planets like Jupiter and Saturn are great balls of gas, because there was never enough heat for the gas to be driven off. Our planet Earth lies somewhere in between; the light gases, especially hydrogen, were blown away by the heat of the young Sun, leaving a small, rocky planet. But Earth is bigger than Mercury, and contains some lighter materials that the solar heat did not evaporate. In fact, Mercury is like the dense metallic core of the Earth, without a surrounding shell of lighter rocks or an atmosphere.

At our distance from the Sun the materials which were stable in the primeval dust cloud included silicon compounds, and iron and magnesium oxides, with just a trace of all the other chemical elements we find on the Earth today. Small particles in the dust cloud collided and stuck together, building up larger lumps which collided in turn. One of these lumps grew until its gravitational pull began to attract other lumps onto itself. While this happened, the growing planet was heated in three ways. First, each lump that struck, ploughed a crater in the surface and gave up its energy of motion (kinetic energy) as heat. Then, as the planet grew, it was pressed continually into a ball by gravity, this pressure heating the interior in the same way as heat is generated when air is squeezed inside a bicycle pump. Finally, radioactive elements gave off heat energy when they fissioned into lighter elements.

So the young Earth was hot. The heavy elements, especially iron, settled down into the core as molten globules, while lighter material, rich in silicon, floated up to form a skin like that on a bowl of congealed custard. As the whole Earth cooled the crust set, and the layered structure of the Earth today was established.

The Solar System is 4,600 million years old. The oldest rocks known on the surface of the Earth are 3,900 million years old. So it took 700 million years for the material of the young Earth to “differentiate” into dense core and light crust, and for the crust to set. Today, there are two kinds of crust: relatively thick but light rock, called granite, which makes up the continents; and slightly heavier, but thinner, rock known as basalt which makes up the crust of the ocean basins.

Starting from the surface and working in, the Earth is a rocky ball which averages 6,371 kilometres in radius. It is slightly flattened at the poles because the speed of its rotation – once every 24 hours – makes the equator bulge. The crust itself makes up only 0.6 per cent of the volume of the planet, varying from an almost uniform 5 kilometres thickness beneath the oceans to 35 kilometres under low-lying continents and 80 kilometres under great mountain ranges such as the Himalayas.

We now know that new ocean crust is constantly being formed as molten rock is pushed out from deep inside the Earth and spreads out on each side of great cracks, called spreading ridges, as it sets. One such crack runs down the middle of the Atlantic Ocean; the volcanic island of Iceland is a site where this spreading ridge just breaks the surface of the sea.

To balance this creation of new crust, thin ocean crust is also destroyed in the deep ocean trenches, where it is pushed under the thicker continental crust. The best known site of such a destructive trench is along the western edge of the Pacific; the earthquakes and volcanoes of Japan have been thrown up as by-products of the destruction of ocean crust in the region.

It is now also known that the continents are slowly, but constantly, on the move. The Atlantic Ocean is widening at a rate of a couple of centimetres a year; Africa and Asia are being split apart by a spreading ridge in the Red Sea, which may one day form a new ocean as big as the Atlantic. And Africa and Europe are slowly being crushed together – building up the Alps, the Pyrenees and the mountains of the Balkans – while the Mediterranean Sea is squeezed inexorably out of existence.

This continental drift, as it is popularly termed (technically it is described as “plate tectonics”), is driven by the flow of sticky, fluid rock in the layers immediately below the crust. Most of the Earth – 82 percent by volume – is the mantle, immediately below the crust and extending to a depth of 2,900 km. The upper mantle, the key element in plate tectonics, is 370 km thick. Beneath it lies a transition zone about 600 km thick, and then the lower mantle, 1,900 km thick. Deeper still is the core; an outer core 2,100 km thick, and the solid inner core, 1,370 km in radius, at the centre of the Earth.

This layered structure has been worked out from studies of earthquakes. The waves from earthquakes reverberate around the Earth, and seismologists can estimate the structure of the planet’s interior by studying the patterns they make. However, apart from the basic divisions between crust, core and mantle, and the density measurements which show the core must be rich in iron, we really know very little about the deep interior of our planet.

Paradoxically, astronomers know more in some ways about the composition of distant stars than geologists do about the composition of the lower mantle and core. But from the point of view of life on Earth, it is not the interior but the surface, and especially the oxygen-rich atmosphere, that we must look at.

When the Earth was a young planet, 4,000 million years or more ago, it had no atmosphere. Any original gases in the inner Solar System were swept away by the heat and activity of the young Sun as it formed. But once the Earth had developed a solid crust, volcanic activity poured out gases to build up an atmosphere around the solid planet.

Although the Earth was much more active volcanically when young, there is no reason to assume that the gases released by volcanoes then were any different from the gases released by them today. Surprisingly, in view of their association with fire, the main “gas” produced by volcanoes is actually water vapour. After this, the next most common gas released is carbon dioxide.

Nitrogen, which today makes up 78 per cent of the volume of the atmosphere, represents only one or two per cent of the products of volcanic outgassing. But nitrogen is a relatively stable gas which does not dissolve in water or combine easily with many other elements. So that one or two per cent, added up over 4,000 million years or more, has produced our nitrogen-dominated atmosphere.

But it is oxygen, which makes up 21 per cent of the atmosphere today, that is the vital ingredient for animal life. Oxygen is not released freely by volcanoes at all – it is locked up with other elements in water (H2O) and carbon dioxide (CO2). Venus and Mars, similar to Earth in many ways, have retained atmospheres rich in carbon dioxide; the Earth’s oxygen atmosphere has been produced by the action of life – plant life – on the original carbon dioxide. Oxygen is essential for animal life, but it is a product of plant life. The presence of so much oxygen in the atmosphere of the Earth today singles it out as a home of life. The absence of oxygen, and presence of carbon dioxide, in the atmospheres of Venus and Mars hints that they are lifeless planets.

At first, the atmosphere on the young Earth consisted mainly of water vapour and carbon dioxide. When the hot surface cooled below 100 degrees C, the boiling point of water, the first rains fell and most of the water eventually condensed to form oceans. The water played two crucial roles in the subsequent development of the Earth’s atmosphere. First, it dissolved some of the carbon dioxide, leaving an atmosphere much thinner than it would otherwise have been. Secondly, pools of liquid water provided an environment for life to get a grip. Then the plant life formed in the seas, by breaking down carbon dioxide and releasing oxygen in photosynthesis, transformed the atmosphere of the planet.

All this took time. Early life forms found oxygen poisonous, but by 2,000 million years ago the presence of oxides in rocks reveals that some forms of life had learned to live with oxygen, which was building up in the atmosphere.

Once the atmosphere became rich in oxygen, it began to interact with energy in sunlight – ultra-violet radiation – to produce a layer of ozone high above the ground. Ozone is a form of oxygen with three atoms joined together in each molecule (O3), instead of the customary diatomic oxygen (O2); it absorbs ultra-violet energy, which kills living organisms.

Before the Earth’s atmosphere had an ozone layer, life could exist only in the sea and ultra-violet radiation from the Sun sterilised the land. After the ozone layer formed, life could leave the sea and colonise the land. The present-day pattern was established at last, and oxygen-breathing animals could develop to take advantage of the new atmosphere, using the oxygen released by the carbon dioxide-breathing plants.

Suppose our planet warmed up a little, as it might do if the Sun got a little hotter. One of the first things that would happen would be that the extra heat would make more water evaporate from the sea, and the water vapour would produce more clouds. White clouds are very good at reflecting away heat from the Sun – they act like mirrors to bounce the incoming heat radiation back into space. So the Earth responds to increased solar heating by becoming shinier and reflecting more heat away. The result is that there is little or no change in the temperature at the surface. On the other hand, if the Sun were to cool by a small amount there would be less evaporation and fewer clouds, so a greater proportion of the incoming heat would reach the ground, and the Earth itself might not cool at all.

All this means that the atmosphere maintains very stable conditions on the surface of the Earth, where the average temperature has stayed around 15 degrees  Centigrade for thousands of millions of years. Such stable conditions are vital for the development of life.

One comment on “From dust to life – the birth of the Earth”

  1. 1. Esta says:

    how scientifically beautiful

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