The Early Sun and Evolution of Terrestrial Planets

Before discussing the occurrence of water on surfaces of the terrestrial planets at present and in the past we shortly describe the early Sun evolution since this had important influence on the water history of the planets. 

The Sun formed by a collapse of an interstellar dust and gas cloud about 4.6 billion years ago. The formation of the Sun was a fast process, it took only several 10 years to reach the main sequence in the Hertzsprung-Russell diagram (see Chap. 9) where the temperature of a star is plotted versus its radiation. A main sequence star is characterized by hydrostatic equilibrium throughout its interior. Hydrostatic equilibrium is reached when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. Depending on its mass, a star may remain on this main sequence (see Chap. 9) for several billion years (for the Sun about 9 billion years). 

However, the early sun was quite different from the sun we know. Its luminosity was only about 70% of its present value. It rotated much faster. SteUar rotation is an important parameter to trigger stellar activity. The present day sun shows an activity cycle with a period of about 11 years. The number of sunspots varies but also the number of flare occurrence (these are energetic outbursts caused by a reconnection 

The early sun was less luminous but the energetic outbursts were of higher amplitude and occurred more frequent than presently. The UV emission was about 10, the extreme UV 100 and the X-ray emission 1000 times the present values. Interactions of high-energy radiation and the solar wind with upper planetary atmospheres may have led to the escape of important amounts of atmospheric constituents (GUdel, 2007 [148]). 

The lower luminosity of the early Sun would imply lower surface temperatures on the terrestrial planets and e.g. the Earth would have been frozen totally. From geology we know that Earth was never frozen totally. This paradox is called the 

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