1. How The Earth was formed
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How The Earth
was formed

2. Scientists believe that the Earth didn’t always exist
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Scientists believe that the Earth didn’t always exist
Our Earth was formed at some point in the distant past.
A time will come when it will no longer support life as we know it.

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The Earth in space

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Note how thin the atmosphere is

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Earthrise from lunar orbit

6. Our solar system is estimated to be about 4.6 billion years old.
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Our solar system is estimated to be about 4.6 billion years old.
That’s 4,600,000,000 years old.
That’s 4.6 x 109 years old in standard form or scientific notation.
How do we know this?
The age is estimated from a study of the radioactive decay of unstable isotopes.
Isotopes are different atoms of the same element having the same number of protons but different numbers of neutrons.

7. The simplest element, hydrogen, has three isotopes.
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The simplest element, hydrogen, has three isotopes.
Most hydrogen atoms have one proton only in their nucleus.
A few hydrogen atoms have one proton and one neutron in their nucleus.
Even fewer hydrogen atoms have one proton and two neutrons in their nucleus. These atoms are unstable and radioactive. The half life of this atom is 12.3 years but other elements have unstable isotopes that last far longer. The most common isotope of uranium has a half life of ×109 years, about the age of the Earth.

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The radioactivity never gets to zero but becomes insignificant with time
The half life ( t½ ) for 238U is ×109 years

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Minerals which were thought to contain pure, 100%, uranium 238 when formed now contain only about 50% of this type of uranium atoms. The rest has decayed into daughter isotopes. This tells us that the age of the Earth is about the same as the half-life of uranium, about ×109 years.
Careful measurements of uranium and other radioactive elements such as thorium gives the age of the Earth as about 4.6 x 109 years old.
There is a small margin of error in these measurements but this can be calculated.
Eg:- 4.6 x 109 ± 104 years

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Our solar system consists of the sun, the inner planets, the outer planets, all the moons, all the asteroids, the comets and debris such as the small fragments that form micro-meteorites.

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The formation and evolution of the Solar System began about 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. 

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Some of the molecular cloud was probably left over from the big bang but other material was ejected from the outer shells of novas and supernovas, the death throws of dying stars.

13. Those areas of higher density attract more material to them.
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The density of these gas clouds are not uniform. There are areas of slightly higher density and areas of slightly lower density.
Those areas of higher density attract more material to them.
As more matter slowly “falls” into the centre of higher density areas these areas start to spin.
The gas cloud evolves into a series of protoplanetary disks.

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Most of the collapsing mass of gas is pulled to the centre, by gravity, eventually forming a star.
As the gas cloud collapsed under gravity it starts to spin and form a disk with a bulge at the centre.
The flattened part of this protoplanetary disk would eventually become the planets, moons, asteroids, and other small bodies of a planetary system like our Solar System.

15. Modern research confirms this hypothesis.
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This is not a modern theory but was first put forward in the 18th century by Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace.
Modern research confirms this hypothesis.
Photographs from the Hubble Space Telescope actually show protoplanetary discs being formed.

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Protoplanetary disks forming in the constellation of Orion

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An artists impression of the protoplanetary disk that formed our early Solar System
The sun has just burst into life
There is still a great deal of dust and debris in the protoplanetary disk

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This was a slow process
Mathematical models show that denser material would accumulate towards the centre of the disk.
The inner planets, Mercury, Venus, Earth and Mars are much denser than the outer planets.
Less dense material tended to end up in the outer reaches of the protoplanetary disk.
Jupiter is a huge planet with a huge gravitational field but if one could find a sufficiently large ocean it would float. Jupiter is less dense than water!

19. The early solar system was chaotic.
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The early solar system was chaotic.
There was a period of intense bombardment as asteroid sized chunks of material would slam into the proto-planets.

20. This bombardment produced enough heat to melt most materials.
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This bombardment produced enough heat to melt most materials.
On each planet the denser material would sink to the centre which explains why the Earth’s core is almost certainly made up of mainly iron with some nickel.
The planet slowly cooled to form a crust.

21. The Earth cooled slowly
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The Earth cooled slowly

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Where all the water on our Earth came from is not completely understood.
Much came from comets and asteroids during a period of intense bombardment.
The early Earth was subjected to considerable bombardment until its gravitational field had “swept” the space of its orbit “clean”

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Early bombardment
Late heavy bombardment

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Early bombardment
The Hadean Aeon

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The Earth slowly cools over tens of millions of years and is under regular bombardment from comets and asteroids.

26. Late heavy bombardment
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Late heavy bombardment

27. Comets may not be the only source of water.
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Comets contain large quantities of water, as ice, and late bombardment by comets must have made a major contribution to the water in our oceans.
Comets may not be the only source of water.

28. One of the main gases given off by volcanoes is water vapour, (steam).
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Evidence suggests that some water was present at the time the Earth formed.
Today there is a considerable quantity of water in the mantle which is dissolved in molten magma under intense pressure.
One of the main gases given off by volcanoes is water vapour, (steam).
Clearly oceans could not form until the surface of the earth was cool enough to condense liquid water at the surface.

29. Our Moon – How did it form?
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Our Moon – How did it form?

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Most scientists now believe that our Moon formed after an enormous collision between the early Earth and a Mars sized planet which has now been called Theia
The cataclysmic collision would have vaporised most of the early Earth

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There is a great deal of technical evidence to support this theory.

32. Much of the evidence has been obtained from the study of moon rocks.
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Much of the evidence has been obtained from the study of moon rocks.
Most of the rocks and minerals on the Moon are like those on Earth suggesting a similar origin.
Even the isotopic composition is the same.

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The theory that the Moon was a separate entity orbiting the sun that was somehow “captured” by the Earth’s gravitational field has now been ruled out by scientists who work in the field.

34. To find out more we need to go back to the moon.
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To find out more we need to go back to the moon.

35. We could find out much more by exploring Mars and other planets.
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We could find out much more by exploring Mars and other planets.

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Any questions?
The End