In the beginning there was a Bang, and boy it was big. Or so the current theory has it. By dead reckoning, extrapolation from observed galaxy speeds, the universe came into its present being around 13.5 billion years ago.
So a little less than half of this time ago a supernova blew out an extensive cloud of gas and dust. From this a disk formed because of the 'original spin', or rotational momentum. This developed into the Solar Nebula around 5 billion years ago.
The Solar Nebula was a cloud of dust and gas, mainly hydrogen and helium, around 5 billion years ago, which developed in to the solar disk. This disk allowed the formation of the Sun, planets, and other members of our solar system, taking the forms we know by about 4.3 billion years ago.
The angular momentum of the solar disk caused the bulk of the mass to accumulate in the centre to form the Sun, while the lighter elements and molecules drifted further out. The atoms and molecules gradually formed particles, which were drawn together by gravity to form ever larger planetesimals, till eventually they had accumulated enough mass to form planets, and clean out their orbital zone of the remaining debris.
During the planet formation phase, there would have been many impacts. As the bodies grew larger, some of these impacts would have had significant long-term effects. For example, Venus has a rotation which is opposite in direction to its orbital direction, and Uranus has a magnetic field perpendicular to its pole of rotation.
The Earth's Moon may have been formed by the impact of Theia, a planetoid the size of Mars, which had been forming in the same orbit as Earth, around 4.3 billion years ago. Being ripped off the surface region of the Earth explains the lower average density of the Moon, which wouldn't have been the case if the Moon had formed from the same debris which formed the Earth.
Magnetic fields of planets are created by the alignment of electric currents in the liquid core. Magnetism may also be present in solid rock which cooled while in a strong magnetic field.
Mars lost its magnetophere early in its existence, allowing the solar radiation to interact with the ionosphere directly.
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1821 - 1902
Rudolf Virchow was a German physician, pathologist and prehistorian, as well as a social and political writer and reformer.
She looked as if she had been poured into her clothes and had forgotten to say 'when'.
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