Earth is the third planet from the Sun and (as you may know…) has one natural satellite: the Moon. Earth’s Moon is larger than that of any other satellite, apart from the moon of Pluto: usually satellites are much smaller than their parent planets.
A lot of characteristics make our planet unique, one of the most important is free liquid water on the surface. This is possible only because the temperature is above the freezing point and below the boiling point of water accompanied with an atmosphere which is thick enough and composed by the very right mix of gases needed for life to exist.
- Earth from Space, image by NASA.
The diameter of the Earth is 12,000 km. At the centre, there is an iron–nickel core where the temperature is 5000 K (which is almost 5000 degrees Celsius, which are about 9000 degrees fahrenheit!), the pressure is 3×1011 Pa and the density 12,000 kg/m3. As a comparative measure, the pressure of the atmosphere at sea level is about 1,01×105 Pa, so that pressure is about one million times bigger! Also the mean density for the whole planet is around 5520 kg/m3. Below the surface, the core is divided into two layers, inner and outer core. The inner core, below 5150 km comprises only of 1.7% of the mass of the Earth! It is solid because of high pressure. The outer core is molten. Also, the solid inner core rotates with respect to the outer core and mantle.
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