The Origin and Evolution of the Earth — NCERT Solutions

The Big Bang Theory

Introduction:
The Big Bang Theory is the most widely accepted scientific explanation for the origin of the universe. It was proposed by Georges Lemaître and later developed by scientists like George Gamow.

Main Points:

1. Initial State: About 13.7 billion years ago, all the matter and energy of the universe was concentrated in an extremely small, hot, and dense point called a singularity.

2. The Big Bang: This singularity suddenly exploded in a massive event known as the Big Bang. This was not an explosion in space but an explosion of space itself.

3. Expansion: After the explosion, the universe began to expand rapidly. As it expanded, it cooled down. Within the first few minutes, protons, neutrons, and electrons formed. Over hundreds of thousands of years, these combined to form hydrogen and helium atoms.

4. Formation of Galaxies and Stars: Over billions of years, gravity pulled clouds of gas and dust together to form galaxies, stars, and planets. Our own Milky Way galaxy and the solar system formed about 4.6 billion years ago.

5. Continuing Expansion: The universe continues to expand even today, which is supported by the observation that galaxies are moving away from each other (Hubble's Law).

Conclusion:
The Big Bang Theory explains the origin of the universe from a single point of extremely high density and temperature, and accounts for the current large-scale structure and expansion of the universe.

Stages in the Evolution of the Earth

The evolution of the earth can be described in the following major stages:

Stage 1: Formation of the Earth (about 4.6 billion years ago)

The earth formed from the solar nebula — a cloud of gas and dust surrounding the early Sun. Solid particles collided and stuck together (accretion) to form planetesimals, which further combined to form the proto-earth. The earth was initially a homogeneous, hot, and molten body.

Stage 2: Differentiation (formation of layers)

As the earth was in a molten state, heavier materials (iron, nickel) sank to the centre forming the core, while lighter materials rose to form the mantle and crust. This process of separation based on density is called differentiation, giving the earth its layered internal structure.

Stage 3: Formation of the Atmosphere

• The primordial atmosphere (hydrogen and helium) was blown away by solar winds.
• A secondary atmosphere was formed through degassing (volcanic outgassing), releasing water vapour, CO₂, nitrogen, methane, and ammonia.
• Later, photosynthesis by early life forms added oxygen to the atmosphere, about 2,000 million years ago.

Stage 4: Formation of the Oceans (about 4,000 million years ago)

As the earth cooled, water vapour condensed to form clouds and rainfall. CO₂ dissolved in rainwater, further cooling the atmosphere. Water collected in depressions on the surface to form the oceans. The oceans formed within 500 million years of the earth's formation.

Stage 5: Origin of Life (about 3,800 million years ago)

Life began as a chemical reaction that produced complex organic molecules. These molecules could duplicate themselves, converting inanimate matter into living substance. The earliest life forms were microscopic and lived in the oceans. Around 2,500–3,000 million years ago, photosynthesis evolved, and life gradually moved from oceans to land.

Conclusion:
Thus, the earth evolved through a series of stages — from a hot molten body to a planet with a layered structure, atmosphere, oceans, and ultimately life — over a period of billions of years.

The Stardust project was launched by NASA (National Aeronautics and Space Administration), the United States space agency. It was a space mission designed to collect comet and interstellar dust particles and return them to earth for scientific analysis.

(Note: Students are advised to verify the latest details from the official NASA website as project information may be updated.)

Scientists are interested in collecting Stardust because:

1. Comets and interstellar dust particles are considered the building blocks of the solar system and have remained largely unchanged since its formation about 4.6 billion years ago.
2. Studying stardust helps scientists understand the chemical composition and physical conditions of the early solar system.
3. It may provide clues about the origin of life on earth, as comets are believed to have delivered organic molecules and water to the early earth.
4. It helps in understanding the origin and evolution of the universe by studying material from outside our solar system.

The Stardust is being collected from two main sources:

1. Comet Wild-2 (81P/Wild): The NASA Stardust spacecraft flew close to Comet Wild-2 and collected dust particles from its coma (the cloud of gas and dust surrounding the comet's nucleus).
2. Interstellar space: The spacecraft also collected interstellar dust particles — tiny grains of material drifting through space between the stars — using a special aerogel collector.

The collected samples were returned to earth in a sample return capsule for laboratory analysis.