Question

The cores of terrestrial planets are mostly made of metals such as iron and nickel, while their crusts tend to be made of lighter elements, such as silicon and oxygen. Why?

(a) Over billions of years, convection carries metals to the cores of terrestrial planets.
(b) Terrestrial worlds are made almost entirely of these metals, so it is natural to find them in large quantities in their cores.
(c) The cores of terrestrial planets contain radioactive elements that produce these metals.
(d) Collisions of asteroids with terrestrial planets have enriched their surfaces with lighter elements.
(e) Heavier elements tended to sink to the centers of terrestrial planets, back when they were molten.

Answer 1

Terrestrial planets have cores made of iron and nickel because during their molten phase, gravity caused denser materials to sink and form a core, while lighter materials formed the crust.

Step-by-step explanation:

The cores of terrestrial planets such as Earth, Venus, and Mars are mostly made of heavy metals like iron and nickel because, during the early formation of these planets, they were once molten. During this molten phase, the process known as differentiation occurred, whereby gravity caused the denser materials to sink toward the center of the planet, forming a metallic core, while the lighter silicates floated to the top, forming the crust. This means that the correct answer to why terrestrial planets have metallic cores and silicate crusts is that heavier elements tended to sink to the centers of terrestrial planets when they were molten (option e).