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Planet Earth

Planet Earth is the densest major body in the solar system. It is made up of three shells:a very thin crust, "the mantle" and the core. The mantle and core are both divided into two parts.The earth's crust varies in thickness, being thinner under the oceans, thicker under the continents. It is divided into separate solid plates which independently float around on top of the hot mantle below. The crust is primarily quartz (silicon dioxide) and other silicates like feldspar The upper mantle is mostly olivene and pyroxene (iron/magnesium silicates), calcium and aluminum. . The lower mantle is probably mainly composed of silicon, magnesium and oxygen. The core is believed to be mostly iron or nickel/iron. The Earth's composition (by mass) is:

35% Iron
29% Oxygen
15% Silicon
13% Magnesium
2.5% Nickel
2% Sulfur
0.05% Titanium

The Earth's atmosphere is 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide and water.
71% of the Earth's surface is covered with water. Heat capacity of the oceans helps to regulate the Earth's climate keeping the temperature relatively stable.The oxygen in Earth's atmosphere is produced and maintained by biological processes as Oxygen is very reactive and can quickly combine with other elements. The Earth has a modest magnetic field produced by electric currents in the outer core.