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• There is only one place in the universe, as far as we know, that can support life-a modest-sized planet called Earth that orbits an average-sized star, the Sun
• Life on Earth is ubiquitous; it is found in boiling mud-pots and hot springs, in the deep abyss of the ocean, and even under the Antarctic ice sheet
• Living space on our planet, however, is significantly limited when we consider the needs of individual organisms, particularly humans
• Earth is still, by far, the most accommodating planet based on what we know about other bodies in the solar system and the hundreds of planets recently discovered orbiting around other stars
• Earth


  • It became hot enough to support a magma ocean
  • It survived a several-hundred-million-year period of extreme bombardment
• The oxygen-rich atmosphere that makes higher life-forms possible developed relatively recently
• Earth


  • If it were considerably larger, it would have retained a thick, hostile atmosphere consisting of ammonia and methane, and possibly hydrogen and helium
  • If it were much smaller, oxygen, water vapor, and other volatiles would escape into space and be lost forever
• Earth


  • If it did not have a rigid lithosphere overlaying a weak asthenosphere, plate tectonics would not operate
  • If it did not have a molten metallic core, it could not support a magnetic field which prevents lethal cosmic rays from showering Earth's surface
• If Earth were about 10 percent closer to the Sun, like Venus, our atmosphere would consist mainly of the greenhouse gas carbon dioxide and the surface temperature would be too hot to support higher life-forms
• If Earth were about 10 percent farther from the Sun, the oceans would freeze over and Earth's active water cycle would not exist, and without liquid water all life would perish
• Earth's proximity to a modest-sized star allowed enough time for the evolution of humans, who first appeared on this planet only a few million years ago
• The first organisms to inhabit Earth were extremely primitive and came into existence roughly 3.8 billion years ago
• About 2.2 billion years ago an atmosphere with free oxygen came into existence, allowing the evolution of the forebearers of the vast array of organisms that occupy Earth today
• About 65 million years ago an asteroid 10 kilometers in diameter struck Earth, causing a mass extinction during which nearly three-quarters of all plant and animal species were obliterated, including dinosaurs
• The extinction of dinosaurs opened new habitats for small mammals that survived the impact, leading to the development of many large mammals that occupy our modern world
• According to the Big Bang theory, the formation of our planet began about 13.7 billion years ago with a cataclysmic explosion that created all matter and space
• Within stars' interiors, hydrogen atoms convert to helium atoms, releasing enormous amounts of radiant energy
• The heaviest elements (beyond number 26) are only created at extreme temperatures during the explosive death of a star perhaps 10 to 20 times more massive than the Sun
• The solar system, including Earth, formed about 4.6 billion years ago from the solar nebula, a large rotating cloud of interstellar dust and gas
• Formation of the planets

  1. Matter gradually coalesced into clumps that collided and stuck together to become asteroid-size objects called planetesimals

  2. Repeated collisions and accretion (sticking together) grew the planetesimals into eight protoplanets and their moons
• A giant impact occurred between a Mars-sized planetesimal and a young, semi-molten Earth, ejecting huge amounts of debris into space, some of which coalesced to form the Moon
• Early evolution of Earth


  1. High-velocity impact of interplanetary debris (planetesimals) and the decay of radioactive elements caused the temperature of our planet to steadily increase

  2. Melting produced liquid blobs of heavy metal that sank under their own weight, forming Earth's dense iron-rich core

  3. This early period of heating also resulted in a magma ocean, perhaps several hundred kilometers deep
• Within the magma ocean, buoyant masses of molten rock rose toward the surface and eventually solidified to produce a thin, primitive crust
• The lightest materials, including water vapor, carbon dioxide, and other gases, escaped to form a primitive atmosphere and shortly thereafter the oceans
• Planets


  • Earth
• Formation of Earth


  1. Hydrogen and helium atoms created

  2. Our galaxy forms 10 Ga

  3. Heavy elements synthesized by supernova explosions

  4. As material collects to form the protosun rotation flattens nebula

  5. Solar nebula begins to contract 4.7 Ga

  6. Continual bombardment and the decay of radioactive elements produces magma ocean

  7. Chemical differentation produces Earth's layered structure

  8. Formation of Earth Moon system

  9. Outgassing produces Earth's primitive atmosphere and ocean
• Mars-size object impacts young Earth

  4.5 Ga
• Debris orbits Earth and accretes

  4.6 Ga
• We can be thankful for our atmosphere; without it there would be no greenhouse effect and Earth would be nearly 60 °F colder. Earth's water bodies would be frozen and the hydrologic cycle would be nonexistent.
• Earth's original atmosphere 4.6 billion years ago


  Consisted of gases most common in the early solar system-hydrogen, helium, methane, ammonia, carbon dioxide, and water vapor
• The lightest of these gases, hydrogen and helium, apparently escaped into space because Earth's gravity was too weak to hold them. Most of the remaining gases were probably scattered into space by strong solar winds from a young, active Sun.
• Outgassing


  Process through which gases trapped in the planet's interior are released
• Early in Earth's history, when massive heating and fluid-like motion occurred in the mantle, the gas output must have been immense.
• Earth's primitive atmosphere probably consisted of mostly water vapor, carbon dioxide, and sulfur dioxide with minor amounts of other gases, and minimal nitrogen. Most important, free oxygen was not present.
• Formation of oceans


  1. Water vapor condensed to form clouds

  2. Torrential rains began to fill low-lying areas, which became the oceans
• Nearly 3.5 billion years ago, photosynthesizing bacteria began to release oxygen into the water.
• During photosynthesis, the Sun's energy is used by organisms to produce organic material (energetic molecules of sugar containing hydrogen and carbon) from carbon dioxide (CO2) and water (H2O).
• The first bacteria probably used hydrogen sulfide (H2S) as the source of hydrogen rather than water.
• Cyanobacteria (once called blue-green algae) began to produce oxygen as a by-product of photosynthesis.
• Initially, the newly released oxygen was readily consumed by chemical reactions with other atoms and molecules (particularly iron) in the ocean.