HOW WAS THE EARTH FORMED?

Part 2

Where the material from the second peak of supernova explosions was more concentrated, the outlying material became attracted and rotated around the concentrations. Due to the angular momentum, the gas and dust gradually formed an ellipsoid shape and as this ellipsoid rotated faster and faster, it flattened into a disc. Such discs have been discovered, e.g. around the bright star Vega in Lyra and also around Beta Pictoris. The material which spiraled in on the centre, formed the star (now the third generation of stars), whilst the outlying material gravitated into elliptical orbits around the newly forming star. Particles of material were constantly being overtaken by faster moving particles on the inside of the orbit and in so doing they adhered together to form larger and larger clumps spread about in all the orbits. G H Wetherill (in the Scientific American, 0ctober 1969) gave the results of his computer model. He started with 100 separate clumps (planetoids or planetesimals) and found that after 30 million years they had accreted to only 22 separate bodies; and after 79 million years they would have compacted to 11 separate bodies; and after 100 million years to only 4 separate bodies. The four inner terrestrial planets of the Sun could have accreted in this way - after 100 million years!

The newly formed Sun consisted of 78% hydrogen, 20% helium and 2% heavier elements, such as carbon, nitrogen, oxygen, neon, magnesium, aluminium, silicon and iron. The cores of the planets contained these heavy elements. The largest core formed was that of Jupiter, 28 800 km in diameter, Saturn a bit smaller, 20 000 km, the Earth 12 756 km and Mercury, 4878 km in diameter. When one considers the greater distances apart of Jupiter, Saturn and the other outer planets, one sees that the material must have been very evenly spread in space. Gases were occluded in the dusty material; the inner planets had most of their gases blown away by the solar wind which was very strong during the formative years of the Sun. The gravitational forces of the smaller planets were not able to hang on to their lighter gases, such as hydrogen and helium but the outer planets exercised sufficient force to retain very large gaseous envelopes. The pressure of the overlying layers of gas compressed the hydrogen and helium into liquids - Jupiter having a liquid hydrogen layer 25 000 km thick.

The separate clumps in the space between Mars and Jupiter were prevented from accreting into a single large planet by the gravitational force of Jupiter. The largest of these Minor Planets (by edict of the International Astronomical Union, they must not be called Asteroids) is Ceres, 1000 km in diameter. 7 are more than 300 km in diameter; 18 between 300 and 200 km; 41 between 200 and 100 km and the rest (thousands), less than 100 km. Some of them have been perturbed and now orbit the Sun in orbits within those of Mars and the Earth. Great numbers of small bodies have been left in the Kuiper Belt, outside the outer planets. Some of them have also been perturbed and now occupy orbits between the outer planets.

In the neighbourhood of the Earth the Moon was accreted as a separate body, as a companion planet of the Earth. (Theories which postulate that the Moon consists of matter that was blasted off the Earth (Chamberlin and Moulton) cannot explain why the Moon's orbit around the Earth is so very nearly a circle. If the Moon had been shot off from the Earth, its orbit would have been very eccentric).

When we look at the present surfaces of the planets, we see that they all have craters where clumps of matter crashed down. Mercury and the Moon never had an atmosphere nor any water and they bear silent testimony to the condition in which they were left at the end of the process of accretion when the last clumps had fallen onto the surfaces. These surfaces have been left unaltered because no water or wind erosion took place on the Moon and on Mercury. Many of the craters were formed by clumps of loosely aggregated matter which did not penetrate deeply into the surface and formed wide craters. Venus does have some craters but many must have been obliterated by volcanic action. The craters which were formed on the Earth's surface have all been worn away by water erosion. Those craters still extant were formed in recent times, during the lapse of the last 3,9 milliard years. Mars shows more craters than the Earth because Mars never had as much water as the Earth and its atmosphere is much more tenuous than that of the Earth. Many craters on Mars have, nevertheless been partly covered by dust. The surfaces of the Minor Planets that have, so far, been photographed, show hordes of craters, formed by solid bodies. Even the two tiny satellites of Mars are covered in craters. Of all the craters on the side of the Moon facing the Earth which are more than 1 km in size, the size which occurs most frequently, is 48 km. To form a crater of this size requires a loosely-aggregated clump 9 km in size.

Craters could not form on the liquid surfaces of the gas giants but all their satellites are peppered with craters.

The testimony borne by the cratered surfaces, shows the condition reached when the greatest part of the bombardment had come to an end at about 3,9 milliard years ago. The dated ages of Moon rocks, show that the process of accretion had started about 4,6 milliard years ago. If we draw a straight line 46 centimetres long to represent 4,6 milliard years, then every centimetre represents one hundred million years. The bombardment would have come to an end 7 centimetres from the left hand end of the line. To the present time, 39 centimetres were left.

The Moon has several large plains, the Maria, where heavy solid bodies crashed. The gravity above these plains is stronger than elsewhere above the Moon's surface showing that dense objects lie buried under the lava which covered the plains. The fact that there are relatively few craters in the maria, shows that the maria must have been formed at the end of the period of accretion, because the date of the lava is 4,6 milliard years.

The great lava flows on the Moon show that immense heat was set free when the objects crashed. At a point of impact, the rocks would have been, not only liquefied, but also vaporised and spread over great areas, e.g. the 1000 kilometer long rays emanating from crater Tycho near the Moon's south Pole. The date of 4,6 milliard years corresponds very well with Schramm's finding of 5 milliard years for the time of the last peak of supernova explosions. The Sun and the stars in its neighbourhood, must have formed from this peak of supernovae and, together with these stars, their retinues of planets.

As the material which went to form the Earth, crashed down, the violence of the impacts, as well as the pressure exerted by the overlying layers, caused the metals to melt and, being denser, they sank to the nucleus of the Earth. Today the Earth is credited with an inner core 2500 km in diameter of which the density has been raised from 7,7 to about 12 grams per cubic centimetre, as a result of the pressure. The temperature at the core is 6800 degrees and although the melting point of iron is only l100°, this inner core is solid. Above the inner core there is a layer of molten iron-nickel 2230 km thick. This is where the Earth's magnetism is seated. It became possible to calculate the depths of the various layers from the study of waves which emanate from earthquake centres. Above the liquid nickel-iron layer, there is the semi-liquid mantle 2860 km thick. On the mantle floats the rocky crust making up the continents, on the average 38km thick, but at some points under the deepest oceans, only 27 km thick.

How did the Earth obtain its plentiful supply of water? Some theorists hold that the water came from comets, it being known that the nuclei of comets consist of fine particles, occluded in ices of methane and ammonia and water. However, comet nuclei are very small, not more than 50 km in size, and it is difficult to see how, even millions of comets could have supplied the Earth with its trillion (10¹³) cubic kilometres of water. One should rather look to the action of volcanoes to explain the source of the Earth's water. Water was occluded in the primeval nebula and as this matter accreted, water was also bound within the inner layers of the Earth. Hydrogen and oxygen were in plentiful supply in the matter of the supernova explosions and the two elements would readily have combined to form water. The plummeting material would certainly have carried water with it. Later, volcanoes, of which the Earth had thousands, belched out gases, 95% of which consist of steam. When the steam came into the atmosphere, it readily condensed into boiling water which poured down in torrents. These torrents eroded the craters on the surface so that only traces of them are left, e.g. the Vredefort dome which is the oldest and largest impact crater remaining on Earth's surface. The thousands of metres of sedimentary rock in which the gold reefs of the Southern Transvaal and Northern Free State occur, required torrents gushing for many hundreds of millions of years for their deposition.

Why hasn't Venus got large masses of water? This is due to the fact that the steam which was belched out on Venus, could not condense in Venus' atmosphere, the temperature of which was (and still is) over 450°. The steam was thus blown away by the Solar wind and Venus was left high and dry.

In the same way as steam was blown away from Venus, hydrogen and helium was blown away from the Earth and Earth was left with an atmosphere of gases which were readily formed in the remnants of the supernova explosions; gases such as methane, CH₄, ammonia, NH₃ and carbon dioxide, C0₂ and of course steam. Why not oxygen? Because all the oxygen would have readily formed oxides, sulphates and carbonates of all and every substance in the neighbourhood - the stuff of which the rocks of Earth's crust is composed and, of course, oxygen would have combined with hydrogen to form water.

The last craters that were formed, were easily eroded away by the torrents of hot water, very hot water.

The atmospheric gases, methane, ammonia, carbon dioxide played a critical role in the development of life on Earth. How did life originate in the first place?

Fred Hoyle supports the panspermia theory. He says that the universe is pervaded by micro-organisms in the molecular clouds in space and even with bacteria. Hoyle reckons that the conditions on the primeval Earth were too harsh for life to develop. Peter D Ward and Donald Brownlee have written an excellent book "Rare Earth" in which they also support the panspermia idea. The harsh conditions that Hoyle mentions, did come to an end. Be that as it may, everyone agrees that the first living forms on Earth were unicellular blobs that developed in water, containing nutrients that were conducive to the processes by which cells split and multiplied into millions and formed multicellular organisms. The gases in the primeval atmosphere played a critical role in this development. To find out what happened, Stanley Miller and Harold Urey in 1952 passed electric sparks through a mixture of the gases methane, ammonia, hydrogen and water vapour in a sealed flask. After a week, they found that the collecting tube below the flask contained amino acids and that the pressure in the flask had been reduced. The amino acids must therefore have been formed from the gases in the flask.

Amino acids are the cross-rungs between two helical strands of phosphates and sugars in the molecule of deoxyribonucleic acid which F H C Crick and J D Watson had developed in 1960 (for this they won the Nobel prize). This helical molecule has the ability to "tear apart" and then to build up two separate DNA molecules - it enables the cell to divide into two to form two cells.

On the primeval Earth, somewhere between 3,8 and 3,5 milliard years ago (a time-span of three hundred million years) the self-replicating cell became established fact. The oldest signs of fossils from living organisms date back to 3,5 milliard years ago. Lightning had played the role of the electric sparks used by Crick and Watson.

For about two milliard years, the waters on earth contained a green slime, the ancestors of all later life forms. The green of the algae was due to the chlorophyll molecule which has an atom of magnesium in its centre and Earth was the green planet. The chlorophyll acted as catalyst which enabled the living cells to combine with water to form H₂CO, formaldehyde, which, by the way does occur in interstellar nebulae.

Water combines with carbon dioxide to form carbonic acid:

H₂O + CO₂ -> H₂CO₃

but in the presence of the catalyst chlorophyll and sunlight, they form formaldehyde and oxygen is set free:

H₂O + CO₂ -> H₂CO + O₂.

More involved reactions led to the formation of nutrients, such as glucose but still accompanied by the liberation of oxygen:

12H₂O + 6CO₂ + light -> C₆H₁₂O₆ + 6H₂O + 6O₂
energy                 

This process of photosynthesis occurred in all the cells and the amount of oxygen in the atmosphere steadily increased. Photosynthesis is the process whereby cells manufacture their food using water, carbon dioxide and sunlight. The atmosphere of methane, ammonia, nitrogen, carbon dioxide and water vapour thus gave way to an atmosphere which today consists of oxygen 20%, nitrogen 79% and l% of traces of carbon dioxide, argon, neon.

The Earth has now become the blue planet, the atoms in the atmosphere dispersing the short wave lengths of sunlight. The oxygen that was set free by living cells as a waste product, became the energy storehouse for the animals which later populated the Earth. The existence of life on Earth depends on many factors that are finely balanced. The total numbers of carbon and oxygen atoms are approximately equal. If there was a preponderance of carbon atoms, the carbon dioxide would have suffocated life; if there was a preponderance of oxygen, living cells would have been oxidised out of existence. Water, the most important substance in the life chain, is at its densest at 4° above its freezing point, so that ice floats and life can go on below the ice. If water was at its densest at, or below, freezing point, lakes and seas and oceans would have been solid blocks of ice and life could not exist.

The Earth's orbit lies within the Sun's ecosphere where water can exist in its three states. Venus is too hot; Mars too cold. The average height of the continents above sea level, is only 840 metres, whereas the average depth of the oceans is 3,8 km. 29,2% of Earth's surface is dry land and 70,8% is under water. If Earth had 10% more water, all the continents would have been under water and no land animals could have evolved. Only one third of the dry land is arable! The Earth's magnetic field wards off inimical rays such as gamma rays and the atmosphere absorbs X-rays and most of the ultraviolet in sunlight. And so we can go on, enumerating the fine balances on which life on Earth depends. No wonder Hoyle says, it is done by design of the Intelligent Universe.

Jan Eben van Zyl