THE EARTH – OUR HOME PLANET

The Earth is the largest and most massive of the four terrestrial planets. Although its diameter of 12,756 km is only a few hundred kilometers greater than that of Venus, it is marginally more massive than the other three terrestrial planets (Mercury, Venus, and Mars) put together. The Earth is unique among the known planets in having liquid water on its surface (just over 70 percent of its surface is covered by water), in having an atmosphere that contains substantial quantities of oxygen (about 21 percent by volume), and in supporting a diverse variety of lifeforms.

Composition and Structure

The fact that the densities of the surface rocks – about 2,500–3,000 kg/m3 – are significantly less than the mean density of the planet (5,520 kg/m3) indicates that the composition and density of the Earth's deep interior differs from that of its surface layer and implies the presence of a large central core consisting mainly of iron. Immediately after its formation (see Chapter 10), the Earth is believed to have been molten throughout. This enabled differentiation to take place – the heavier elements (e.g., iron) sank to the center, and the lighter rocks floated to the surface. Even today, 4,600 million years after the birth of our planet, energy released by the radioactive decay of elements such as uranium and thorium ensure that the central temperature of the Earth is maintained at about 5,000 K or more.