Lecture 11_First billion years

Early events leave no physical records due to plate tectonics which destroy geological records.
Importance of studying events through indirect evidence.
The Moon as a significant source of information about Earth's history.

Solar System Dynamics:
- All planets rotate around the sun in the same direction and plane.
- Formed simultaneously from a singular rotating disk of material.
Sun's Nature:
- A star, part of the galaxy, formed similarly to other stars.
Stellar Measurements:
- Stars' distance can be calculated by measuring their angle relative to Earth at different times of the year using triangulation.
- Speed of light is approximately 186,000 miles/second.
- Events in the universe are recorded; e.g., a dying star's light reflects its past state.

Doppler Shift:
- Indicates movement through changes in wavelength based on the object's velocity relative to the observer.
- Red Shift:
  - An increase in wavelength signifies a distant galaxy's movement away from Earth.
  - Astronomers measure the red shift by comparing spectral lines against a reference.

Initial state:
- A diffuse and roughly spherical nebula that slowly rotates.
- Gravity causes the nebula to contract, forming a flat, rapidly rotating disk.
- Proto-Sun forms in the center while gas and dust form planetesimals.
Planetary Formation:
- Terrestrial planets form from collisions of planetesimals.
- Giant outer planets primarily grow by accumulating gases.

Hydrogen cloud collapses under gravity, generating immense heat (millions of degrees).
Nuclear Fusion:
- Occurs under extreme conditions; hydrogen nuclei collide to form heavier isotopes.

Formation of helium from hydrogen results in loss of mass, converting to heat energy.
Increasing temperatures facilitate the formation of other elements on the periodic table.

Fusion up to iron releases energy; beyond iron, energy is consumed.
Neutron Capture:
- Absorption of neutrons forms heavier elements, typically occurring in supernova explosions due to high energy conditions.

Balance of Forces:
- Gravity and nuclear reaction forces compete within a star.
- When fusion ceases, gravity dominates, causing a collapse leading to a supernova.
Heavy elements are formed during this explosive process.

Types of Meteorites:
- Stony meteorites (rocky composition).
- Iron meteorites (metallic composition).
- Stony-iron meteorites blend rocky and metallic and serve as proxies for Earth's core.
Most meteorites date around 4.6 billion years, offering insights into primordial materials unaffected by Earth's erosion and tectonics.
Entry of meteoroids into the atmosphere creates identifiable streaks of light, representing ancient solar system materials.

Missions (1969-1972): Six lunar landings conducted by astronauts who collected samples and mapped the moon's surface.
The moon's surface is characterized by numerous craters of varying sizes.
Craters and Age:
- Crater counting reveals older surfaces have more craters, indicating significant impact history.
- Larger craters suggest a longer existence while overlapping craters indicate more recent impacts.
This evidence leads to the inference of a "Heavy Bombardment" period.