Answer: (c) 4.6 billion years.

Explanation: The age of the universe is approximately 13.7 billion years (the time of the Big Bang). The Earth and our solar system formed much later, about 4.6 billion years ago.

Answer: (b) Differentiation.

Explanation: Differentiation is the process of the Earth separating into layers like the core, mantle, and crust. Solar winds, degassing (volcanic eruptions), and photosynthesis were all crucial processes in the formation and modification of our atmosphere.

Answer: (b) Planets between the sun and the belt of asteroids.

Explanation: The inner planets (Mercury, Venus, Earth, Mars) are also called Terrestrial planets. They are located between the Sun and the asteroid belt. The outer planets (Jupiter, Saturn, Uranus, Neptune) are beyond the asteroid belt.

Answer: (d) 3.8 billion.

Explanation: The Earth formed around 4.6 billion years ago. The oceans formed around 4 billion years ago, and the earliest evidence of life (like microscopic bacteria) dates back to approximately 3.8 billion years ago.

Answer: (c) Big Bang theory.

Explanation: The Big Bang Theory explains the origin of the universe, not just the Earth. The Nebular hypothesis, Binary theories, and Planetesimal hypothesis were all early theories specifically trying to explain the formation of the planets and the solar system.

Answer: Terrestrial planets (Mercury, Venus, Earth, Mars) are rocky because they formed close to the Sun where it was too warm for gases to condense into solid particles. The intense solar winds blew away most of the lighter gases like hydrogen and helium, leaving behind heavier, rocky materials and metals that accreted to form dense, smaller planets.

Answer:

• (a) Kant and Laplace: Their Nebular Hypothesis was a 'monistic' theory, suggesting that the Sun and all the planets formed from a single, slowly rotating cloud of gas and dust (nebula).
• (b) Chamberlin and Moulton: Their Planetesimal Hypothesis was a 'dualistic' theory. They argued that a wandering star passed close to our Sun, pulling a cigar-shaped filament of material from it. This material later cooled and condensed to form the planets.

Answer: Differentiation is the process through which the early, molten Earth separated into layers of different densities. During this process, heavier materials like iron and nickel sank towards the center to form the core, while lighter materials like silicates and aluminum rose towards the surface to form the crust and mantle.

Answer: Initially, the Earth's surface was a barren, rocky, and hot landscape. It had a thin atmosphere primarily composed of hydrogen and helium. The surface was frequently bombarded by asteroids and other celestial bodies, and there was intense volcanic activity, which contributed to its molten state.

Answer: The Earth's initial or primordial atmosphere consisted mainly of the lightest gases, hydrogen and helium. This early atmosphere was, however, stripped off by the strong solar winds emanating from the young Sun. The atmosphere we know today formed later through processes like degassing and photosynthesis.

Answer: The Big Bang Theory is the leading scientific explanation for the origin of the universe. It proposes that all matter and energy in the universe were once concentrated in an unimaginably small, hot, and dense point called a singularity.

About 13.7 billion years ago, this singularity underwent a massive expansion, not an explosion in the conventional sense, but an expansion of space itself.

• Key Stages:
  1. Expansion and Cooling: Within the first few seconds of the Big Bang, the universe expanded rapidly and began to cool down. This allowed energy to be converted into matter, like protons, neutrons, and electrons.
  2. Formation of Atoms: After about 380,000 years, the universe had cooled enough for the first atoms (primarily hydrogen and helium) to form. The universe became transparent to light at this stage.
  3. Galaxy Formation: Over billions of years, gravity caused these atoms to clump together, forming vast clouds of gas and dust (nebulae). These nebulae eventually collapsed under their own gravity to form the first stars and galaxies.

The evidence supporting the Big Bang Theory includes the observed expansion of the universe (galaxies moving away from each other) and the presence of Cosmic Microwave Background (CMB) radiation, which is the faint afterglow of the Big Bang.

Answer: The evolution of Earth's present atmosphere occurred over billions of years in three distinct stages.

• Stage 1: Loss of Primordial Atmosphere: The earliest atmosphere consisted of hydrogen and helium. However, due to the Earth's hot initial state and the powerful solar winds from the young Sun, these light gases were stripped away and lost to space.
• Stage 2: Formation of the Secondary Atmosphere (Degassing): As the Earth cooled, a new atmosphere was formed from gases released from the planet's interior through intense volcanic activity. This process is called degassing. This new atmosphere was rich in water vapour, carbon dioxide, nitrogen, and methane, but had very little free oxygen.
• Stage 3: Modification by Life (The Living World):
  1. Condensation and Oceans: As the Earth cooled further, the water vapour in the atmosphere condensed and fell as rain, leading to the formation of oceans around 4 billion years ago. The oceans absorbed a large amount of atmospheric carbon dioxide.
  2. Photosynthesis: Around 2.5 to 3 billion years ago, early life forms like blue-green algae (cyanobacteria) evolved. Through the process of photosynthesis, they began to convert carbon dioxide and water into food, releasing free oxygen as a byproduct. Over millions of years, this process saturated the oceans with oxygen and then filled the atmosphere, leading to the oxygen-rich air we breathe today.

Answer: (c) Pierre-Simon Laplace

Answer: (c) Volcanic eruptions

Answer: (b) Thick atmospheres of helium and hydrogen

Answer: (b) The Giant Impact Hypothesis or "The Big Splat"

Answer: (d) Heavier materials sank, lighter materials rose

Answer: Planetesimals are small, solid bodies of rock, dust, and ice that formed in the early solar system. They are considered the building blocks of planets. Through a process called accretion, these planetesimals collided and stuck together due to gravitational attraction, gradually growing larger and larger to eventually form protoplanets and then planets.

Answer:

Feature	Terrestrial Planets (Inner)	Jovian Planets (Outer)
Composition	Made of rock and metal, have a solid surface.	Gas giants, made mostly of hydrogen, helium, and ice.
Size & Density	Smaller in size and have higher density.	Much larger in size and have lower density.

Answer: Accretion is the process where smaller particles (like dust and gas) and larger bodies (like planetesimals) clump together under the influence of gravity to form larger objects. In the context of planet formation, it describes how planets grew in size by sweeping up and accumulating material in their orbital path.

Answer: The formation of oceans had a huge impact on the atmosphere. As water vapour condensed and fell as rain, it formed vast oceans. These oceans acted as a giant sink, dissolving a very large amount of the atmospheric carbon dioxide (CO₂). This significantly reduced the CO₂ levels in the atmosphere, paving the way for a climate suitable for life.

Answer: The Geological Time Scale is a chronological system that relates geological strata (rock layers) to time. Its significance lies in its ability to organize Earth's history into different units—Eons, Eras, Periods, and Epochs. It helps scientists and geographers to understand the sequence of major events, such as the evolution of life, the formation of continents, and past climate changes.

Answer: The formation of stars and planets is explained by the Nebular Hypothesis. The stages are:

1. Gravitational Collapse: A vast, slowly rotating cloud of gas and dust, called a nebula, begins to collapse under its own gravity.
2. Formation of a Core (Protostar): As the nebula collapses, it spins faster and flattens into a disk. Most of the material gets concentrated at the center, forming a hot, dense core known as a protostar. This core is the future star.
3. Formation of a Protoplanetary Disk: The surrounding flattened, rotating material forms a disk around the protostar. This is called the protoplanetary disk.
4. Accretion and Planetesimal Formation: Within this disk, dust and ice particles start sticking together through cohesion and electrostatic forces. These clumps grow into larger bodies called planetesimals.
5. Formation of Planets: Through the process of accretion, these planetesimals collide and merge due to gravity, gradually sweeping up material in their path to form larger protoplanets. Over millions of years, these protoplanets evolve into the stable planets we see today. The central protostar eventually becomes hot enough to ignite nuclear fusion, becoming a star (like our Sun).

Answer: The evolution of the Earth's lithosphere (the rigid outer part of the earth, consisting of the crust and upper mantle) involved several key processes following the planet's formation:

1. Initial Molten State: Initially, due to frequent collisions and radioactive decay, the Earth was in a hot, molten state.
2. The Process of Differentiation: During this phase, the Earth's materials began to separate based on density. Heavy, dense materials like iron and nickel sank to form the core. Lighter materials, such as silicates, rose to the surface.
3. Formation of the Crust: As the planet began to cool, this lighter material on the surface solidified to form the Earth's first crust. This early crust was likely thin, unstable, and constantly recycled back into the mantle through intense volcanic and tectonic activity.
4. Continental and Oceanic Crust: Over time, processes like volcanic activity and partial melting of the mantle led to the formation of two types of crust: the less dense, thicker continental crust (rich in granite) and the denser, thinner oceanic crust (rich in basalt).
5. Plate Tectonics: The lithosphere broke into several large and small plates that float on the semi-molten asthenosphere below. The movement, collision, and separation of these plates (plate tectonics) have continued to shape and reshape the Earth's surface, forming continents, mountains, and ocean basins over billions of years.

Answer: Edwin Hubble provided the evidence that the universe is expanding.

Answer: According to the passage, a 'singularity' is a tiny, hot, and dense point from which the universe began.

Answer: The formation of atoms became possible because the expansion of the universe led to its cooling, which allowed energy to convert into matter and particles to combine.

Answer: This diagram illustrates the Nebular Hypothesis for the formation of a solar system.

Answer: The central body labeled 'Protostar' is a very young, developing star. It will eventually become a full-fledged star (like our Sun) once its core becomes hot and dense enough for nuclear fusion to begin.

Answer: The smaller bodies within the disk are called planetesimals. Their role is to act as the building blocks for planets. They collide and accrete to form larger protoplanets.

Answer: This statement is entirely justified. Before photosynthesis, the atmosphere was rich in carbon dioxide, methane, and water vapour but lacked free oxygen. The evolution of photosynthetic organisms (like cyanobacteria) fundamentally changed this composition.

• Oxygen Production: Photosynthesis uses carbon dioxide and sunlight to produce energy, releasing free oxygen (O₂) as a waste product.
• Atmospheric Change: Over millions of years, this process first saturated the oceans with oxygen and then began to fill the atmosphere.
• Paving the Way for Complex Life: This rise of atmospheric oxygen was crucial for the evolution of more complex, aerobic (oxygen-breathing) life forms, including animals. It also led to the formation of the ozone layer, which protects the Earth's surface from harmful ultraviolet radiation.

Therefore, photosynthesis was indeed the critical turning point that transformed a toxic early atmosphere into the life-sustaining one we have today.