पृथ्वी (The Earth)

Early scientific theories regarding the Earth's formation fall into two broad philosophical schools:

A. Monistic (Single Star) Hypotheses:

• The Nebular Hypothesis (Immanuel Kant 1755, revised by Pierre-Simon Laplace in 1796): Asserted that the planets were formed out of a slowly rotating, hot, flattened primordial cloud of gas and dust (called a Nebula) associated with a youthful sun. As the nebula cooled, it contracted and spun faster, throwing off concentric rings of matter that condensed into planets.
• Modified Nebular Model (Otto Schmidt in Russia & Carl Weizsacker in Germany, 1950): Replaced the pure gas cloud with a solar nebula consisting of hydrogen, helium, and cosmic dust. Friction and collision of dust particles caused a disk-shaped cloud to form, and planets formed through accretion.

B. Dualistic (Binary / Two Star) Hypotheses:

• The Planetesimal Hypothesis (Chamberlin and Moulton, 1900): Proposed that a wandering companion star approached the proto-sun. The intense gravitational tidal pull of the passing star pulled out a cigar-shaped filament of solar matter. As the star sailed away, this separated material condensed into tiny solid bodies called Planetesimals, which accreted into planets.
• Sir James Jeans & Sir Harold Jeffreys: Refined this as the Tidal Hypothesis.

The Big Bang Theory (The Expanding Universe Hypothesis):

Formulated by Georges Lemaître and empirically confirmed by Edwin Hubble in 1920 (who observed that galaxies are moving away from each other, verified by cosmological redshift):

• Stage 1 (Singularity): In the beginning, all matter and energy forming the universe existed in one single unimaginably tiny point (a "Tiny Ball" or Singularity) of infinite temperature and infinite density.
• Stage 2 (The Explosion - 13.7 Billion Years Ago): The tiny ball exploded violently. Within a fraction of a second, an exponential cosmic inflation occurred, and the universe began expanding rapidly, converting pure energy into subatomic particles (protons, neutrons, electrons).
• Stage 3 (Atomic Synthesis - 300,000 Years After): The universe cooled down to 4,500 Kelvin, allowing electrons to bind with nuclei to form the first neutral atoms (predominantly Hydrogen and Helium). The universe became transparent to light (the Cosmic Microwave Background radiation).

Birth of Stars & Planetary Formation:

1. Irregularities in the distribution of primordial hydrogen/helium caused localized gravitational clumps to grow, developing into massive rotating clouds called Galaxies.
2. Within galaxies, localized localized gravitational gas clumps condensed into hot, dense nuclear fusion cores: Stars (approximately 5 to 6 billion years ago).
3. Within rotating disks of gas around proto-stars, matter condensed into microscopic grains. Grains collided through electrostatic and gravitational attraction, coalescing into millions of rocky bodies called Planetesimals.
4. Larger planetesimals collided and accreted under runaway gravitational pull, sweeping their orbital paths to form Planets (approx. 4.6 billion years ago).

A. Terrestrial vs Jovian Planets:

Our solar system consists of 8 major planets divided into two distinct groups by the Asteroid Belt:

Why Did Terrestrial Planets Lose Their Gases?

1. Terrestrial planets were too close to the Sun, where intense solar heat prevented volatile gases from condensing into solids.
2. The young Sun produced intense, violent Solar Winds that blew away the primordial hydrogen and helium envelopes from the inner planets.
3. Terrestrial planets have smaller mass and weaker gravity, unable to retain light gases.

B. Origin of the Moon: The Giant Impact ("The Big Splat"):

Approximately 4.44 billion years ago, a celestial body the size of Mars (named Theia) collided with the young, molten Earth in a catastrophic grazing impact:

• The collision blasted a colossal plume of vaporized rocky mantle into orbit around Earth.
• This orbital debris cloud cooled and accreted within weeks to form the Moon. This explains why the Moon possesses a tiny iron core and a composition identical to Earth's outer mantle.

A. Evolution of the Lithosphere (Differentiation):

Primordial Earth was a barren, hot, semi-molten sphere. As temperature increased due to gravitational compression and radioactive decay, the process of Differentiation took place:

• Heavy, dense metallic elements (especially Iron and Nickel) sank toward the center of the planet to form the dense metallic Core.
• Lighter, less dense silicate materials floated upward to the surface, cooling and solidifying to form the outer rocky Crust.
• Intermediate materials formed the Mantle. Earth became a layered sphere!

B. Evolution of Atmosphere & Hydrosphere (Degassing):

1. Stage 1 (Loss of Primordial Atmosphere): Solar winds stripped away the original hydrogen-helium envelope.
2. Stage 2 (Degassing): Intense internal volcanic eruptions expelled colossal volumes of trapped interior gases: water vapor, carbon dioxide, nitrogen, methane, and ammonia (with virtually zero free oxygen).
3. Stage 3 (Rainfall & Oceans): As Earth cooled, water vapor condensed, falling as torrential rain for millions of years. Rainwater filled massive depressions to form the Oceans (c. 4,000 million years ago). Ocean waters absorbed atmospheric $CO_2$, further cooling the planet.

C. Origin of Life & Oxygenation:

Around 3.8 billion years ago, life began in the oceans as complex organic macromolecules and blue-green algae (cyanobacteria). Through photosynthesis, algae converted solar energy, producing free oxygen. By 2.0 billion years ago, oceans became saturated with oxygen, which escaped into the atmosphere, creating the life-shielding Ozone Layer ($O_3$)!