EL SCI 2

• The Earth, the planet we live on, is part of the Solar System
• If we want to know how the Earth formed, we need to understand the origin and evolution of the Solar System
• The age of the Solar System is 4.6 billion years old based on radioactive dating of meteorites
• Milky Way galaxy


  A huge disc- and spiral-shaped aggregation of about at least 100 billion stars and other bodies
• Milky Way galaxy


  • Its spiral arms rotate around a globular cluster or bulge of many, many stars, at the center of which lies a supermassive blackhole
  • It is about 100 million light years across
  • The solar system revolves around the galactic center once in about 240 million years
  • It is part of the Local Group of galaxies, which in turn is part of the Virgo supercluster of galaxies
• Based on the assumption that they are remnants of the materials from which they were formed, radioactive dating of meteorites suggests that the Earth and solar system are 4.6 billion years old
• Large scale features of the Solar System

  • Much of the mass of the Solar System is concentrated at the center (Sun) while angular momentum is held by the outer planets
  • Orbits of the planets are elliptical and are on the same plane
  • All planets revolve around the sun
  • The periods of revolution of the planets increase with increasing distance from the Sun; the innermost planet moves fastest, the outermost, the slowest
  • All planets are located at regular intervals from the Sun
• Small scale features of the Solar System

  • Most planets rotate prograde
  • Inner terrestrial planets are made of materials with high melting points such as silicates, iron, and nickel. They rotate slower, have thin or no atmosphere, higher densities, and lower contents of volatiles - hydrogen, helium, and noble gases
  • The outer four planets - Jupiter, Saturn, Uranus and Neptune are called "gas giants" because of the dominance of gases and their larger size. They rotate faster, have thick atmosphere, lower densities, and fluid interiors rich in hydrogen, helium and ices (water, ammonia, methane)
• Abundance of elements


  • Except for hydrogen, helium, inert gases, and volatiles, the universe and Earth have similar abundance especially for rock and metal elements
  • The sun and the large planets have enough gravity to retain hydrogen and helium. Rare inert gases are too light for the Earth's gravity to retain, thus the low abundance
  • Retention of volatile elements by the Earth is consistent with the idea that some materials that formed the Earth and the solar system were "cold" and solid; otherwise, the volatiles would have been lost
  • The presence of heavy elements such as lead, silver, and uranium on Earth suggests that it was derived from remnants of a supernova and that the Sun is a second-generation star made by recycling materials
• Nebular Hypothesis


  1. A rotating gaseous cloud that cools and contracts in the middle to form the sun and the rest into a disc that become the planets

  2. Failed to account for the distribution of angular momentum in the solar system
• Many theories have been proposed since about four centuries ago. Each has weaknesses in explaining all characteristics of the solar system
• Hypotheses on the origin of the Solar System

  • Buffon's Sun-comet encounter
  • James Jeans' sun-star encounter
  • T.C. Chamberlain and F. R. Moulton's planetesimal hypothesis
  • Ray Lyttleton's sun's companion star colliding
  • Otto Schmidt's accretion theory
  • M.M. Woolfson's capture theory
• Flaws/drawbacks of the hypotheses

  • Fails to explain how planets are formed (hot gas from the sun/star expands and will not form planets)
  • This type of encounters are extremely rare
• Nebular Hypothesis


  In the 1700s Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace independently thought of a rotating gaseous cloud that cools and contracts in the middle to form the sun and the rest into a disc that become the planets
• The nebular theory failed to account for the distribution of angular momentum in the solar system
• The currently accepted theory on the origin of the solar system relies much on information from meteorites
• Protoplanet Hypothesis - Current Hypothesis

  1. About 4.6 billion years ago, in the Orion arm of the Milky Way galaxy, a slowly-rotating gas and dust cloud dominated by hydrogen and helium starts to contract due to gravity

  2. As most of the mass move to the center to eventually become a proto-Sun, the remaining materials form a disc that will eventually become the planets and momentum is transferred outwards

  3. Due to collisions, fragments of dust and solid matter begin sticking to each other to form larger and larger bodies from meter to kilometer in size. These proto-planets are accretions of frozen water, ammonia, methane, silicon, aluminum, iron, and other metals in rock and mineral grains enveloped in hydrogen and helium

  4. High-speed collisions with large objects destroys much of the mantle of Mercury, puts Venus in retrograde rotation

  5. Collision of the Earth with large object produces the moon. This is supported by the composition of the moon very similar to the Earth's Mantle

  6. When the proto-Sun is established as a star, its solar wind blasts hydrogen, helium, and volatiles from the inner planets to beyond Mars to form the gas giants leaving behind a system we know today
• Exploration of Mars has provided evidence of seasonal flow of liquid water (in the form of brine salty water) on the surface of Mars
• Rosetta's mission to rendezvous with and land a probe on a comet in the Kuiper Belt has provided analysis of the water (ice) from the comet suggesting its isotopic composition is different from water from Earth
• NASA's New Horizon spacecraft's flyby of Pluto revealed a complex terrain with ice mountains and vast crater free plains, suggesting recent (last 100 millions of years) of geologic activity
• The presence of craters on planets and moons is proof of the "violent past" of the solar system, with objects subject to bombardment and collision early in its evolution
• On Earth, geologic processes have shaped and reshaped the surface removing evidence of cratering
• The importance of water to a planet's habitability will be discussed in the next lesson
• Some scientists speculate that part of the water on the Earth's surface were brought by comets, but the difference in isotopic composition of water suggest this hypothesis is unlikely
• Recent advancements have reported the presence of a solar system in other parts of the galaxy, raising the possibility of finding a similar system within the Milky Way Galaxy and potentially an Earth-like planet