Question

The cores of terrestrial planets are composed of mostly heavy elements (e.g. nickel, iron) while their crusts are made of lighter elements (e.g. silicon, oxygen). Why is this?

(a) The light elements fell onto the surface later in the formation of the planets.
(b) Heavy elements decay into lighter elements on the surface of the planets.
(c) Heavy elements tended to sink to the centres of the planets early in their formation.
(d) The cores contain radioactive elements which decay into heavy elements.
(e) Convection has caused heavier elements to sink faster than light elements.

Answer 1

The terrestrial planets have dense cores made of heavy elements because, during their hot, molten phase, gravitational separation caused these elements to sink to the center. This process of differentiation resulted in lighter elements forming the crust.

Step-by-step explanation:

The composition of terrestrial planets, with heavier elements such as nickel and iron in their cores and lighter elements like silicon and oxygen in their crusts, is due to the process called planetary differentiation. This process occurred when the planets were hot and molten, allowing denser materials to sink to the center under the force of gravity, a phenomenon known as gravitational separation. The correct answer to the student's question is (c) Heavy elements tended to sink to the centres of the planets early in their formation.

Terrestrial planets like Earth, Venus, Mars, and Mercury were once hot enough to make their materials molten, which facilitated the segregation of elements by density. Lighter silicates floated towards the surface forming the crust, while heavier metals like iron and nickel sank to form the core. The giant planets, although composed differently, also show evidence of dense cores beneath their gaseous atmospheres, supporting the inference of gravitational separation during formation.