(a) What is meant by accretion? (b) Why are the terrestrial planets denser at their centers than at their surfaces?

Accretion is the process by which celestial bodies, like planets and stars, grow in mass by attracting and gathering cosmic materials, such as dust, gas, and other celestial bodies through gravitational forces. Over a long period, these accumulated materials form a larger body, like a planet or a star.

The terrestrial planets, which include Mercury, Venus, Earth, and Mars, have denser cores compared to their surfaces due to a combination of factors: 1. Gravitational Compression: The intense gravitational force exerted at the core of a planet compresses the materials closer together, leading to a denser core compared to the surface. 2. Differentiation: Planets are believed to have undergone a process called differentiation early in their formation. This process involves the denser and heavier elements, such as metals and metallic compounds, sinking towards the center of the planet due to gravity, while lighter elements and materials, like silicates and gases, rise towards the surface. This process creates a denser core and a less dense surface. 3. Thermal Effects: As planets form, heat generated by various processes like accretion, radioactive decay, and the sinking of denser materials to the core causes the temperature to be higher in the core than at the surface. This temperature gradient leads to a more fluid state of materials in the core, with the increased pressure due to gravitational compression causing the core to remain denser than the surface. In conclusion, the density gradient observed in terrestrial planets can be attributed to factors such as gravitational compression, differentiation, and thermal effects, making the cores denser than their surfaces.