The Origin and Evolution of the Earth Notes

Nebular Hypothesis: Proposed by German philosopher Immanuel Kant and revised by mathematician Laplace in $1796$ . It suggested planets formed from a slowly rotating cloud of material associated with a youthful sun.
Revised Nebular Hypothesis ( $1950$ ): Otto Schmidt (Russia) and Carl Weizascar (Germany) proposed that the sun was surrounded by a solar nebula of hydrogen, helium, and dust. Friction and collisions led to a disk-shaped cloud, and planets formed through accretion.

Expanding Universe Hypothesis: Edwin Hubble provided evidence in $1920$ that the universe is expanding, with galaxies moving further apart.
The Big Bang Event: Occurred approximately $13.7$ billion years ago.
- Singularity: All matter existed as a "tiny ball" with infinite temperature and density.
- Rapid Expansion: Within fractions of a second of the explosion, the universe grew rapidly, though expansion later slowed.
- Atomic Formation: The first atoms formed within the first $3$ minutes. Within $300,000$ years, the temperature dropped to $4,500K$ , giving rise to atomic matter and making the universe transparent.
Steady State Concept: Proposed by Hoyle as an alternative, suggesting the universe remained roughly the same at any point in time; however, the expanding universe model is currently favored.

Star Formation: Occurred $5$ - $6$ billion years ago. Initial density differences led to gravitational attraction, forming galaxies. Large clouds of hydrogen gas (nebula) developed localized clumps that became dense gaseous bodies (stars).
Light Year: A measure of distance. Light travels at $300,000\,km/second$ . One light year equals $9.461 \times 10^{12}\,km$ . The mean distance between the sun and the earth is $149,598,000\,km$ , or $8.311$ minutes in terms of light years.
Planet Formation Stages:
- Stage 1: Stars are gas lumps in a nebula; gravity creates a core and a rotating disc of gas and dust.
- Stage 2: Matter condenses into small-rounded objects called planetesimals via cohesion.
- Stage 3: Planetesimals accrete through collision and gravitational attraction to form larger bodies (planets).

Initial State: The earth was a hot, barren, rocky object with a thin atmosphere of hydrogen and helium about $4,600$ million years ago.
Differentiation: As the earth heated and increased in density, materials separated based on density. Heavier materials (like iron) sank toward the center, while lighter materials moved toward the surface. This layered the earth into the crust, mantle, outer core, and inner core.

Three Stages:
1. Loss of Primordial Atmosphere: Solar winds stripped away the original hydrogen and helium atmosphere from terrestrial planets.
2. Degassing: As the earth cooled, gases (water vapour, nitrogen, carbon dioxide, methane, ammonia) were released from the interior through volcanic activity.
3. Photosynthesis: Living organisms modified the atmosphere's composition.
Formation of Oceans: Water vapour condensed as the earth cooled. Rainwater collected in depressions, forming oceans within $500$ million years of earth's formation (about $4,000$ million years ago).

Timeline: Life began to evolve around $3,800$ million years ago as complex organic molecules capable of self-duplication.
Photosynthesis and Oxygen: Photosynthesis evolved between $2,500$ - $3,000$ million years ago. Oceans became saturated with oxygen first, and about $2,000$ million years ago, oxygen began to flood the atmosphere.
Fossil Evidence: Microscopic structures of blue algae have been found in geological formations older than $3,000$ million years.