Accretion
Topic: Origin and Evolution of the Earth and the Solar System · NCERT: Class 11, Ch 2 "The Origin and Evolution of the Earth"
Meaning
Accretion is the process by which small bodies grow into large ones. Tiny bits of dust and rock in space collide and stick together. As a clump grows, its gravity gets stronger, so it pulls in even more material. In the revised nebular hypothesis, the planets grew this way. Planetesimals (small solid bodies) in the rotating disc of gas and dust around the young Sun joined together to make a few large planets. Accretion explains how the Earth was built from scattered material about 4.6 billion years ago.
Example
The Earth's Moon is a clear case. In the giant impact hypothesis, a Mars-sized body called Theia hit the young Earth about 4.5 billion years ago. The debris thrown into space then came together by accretion and formed the Moon.
Don't confuse with
- Differentiation: Accretion builds a planet by adding material from outside. Differentiation happens later, inside the planet. Heavy iron sinks to the centre and lighter material rises, which gives the crust, mantle and core.
- Planetesimal: A planetesimal is a small solid body, a thing. Accretion is the process that joins planetesimals into planets.