NASA / JPL-CALTECH / SCIENCE PHOTO LIBRARY NASA / JPL-CALTECH / SCIENCE PHOTO LIBRARY
Planetary formation. Illustration of the process by which a rocky planet forms (down left), resulting in a planet (right) with a crust, mantle and core. The initial stage is the accretion (top left) of material from a protoplanetary disc around a star. As the newly forming body increases in size, pressure and temperatures rise in its core (far left) and some elements melt and sink to the centre (bottom left). Volcanic activity and bombardment of the surface occurs in the early history of a planet (bottom left), with a more stable environment as the planet cools (main illustration).
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