Lecture 2

• the earth was photographed by Voyager 1 in 1990
• curiosity rover landed on mars in 2012
• there is evidence of past water on mars
• Jupiter has 80 moons - Io is the most volcanically active in the solar system, Europa is covered by ice with an ocean beneath, Ganymede possibly has underground saltwater ocean, Callisto is the most cratered surface in the solar system
• Saturn has 83 moons - Titan has oceans of methane and ethane, Enceladus sprays water into space
• on Titan there are oceans of methane and ethane (mean temp 180 degrees C) - lack of waves suggests the oceans are highly viscous
• age of the Earth is 4.45 x $10^9$a
• Age of the Solar System is 4.57 x $10^9$ a
• Age of the Universe is 13.8 x $10^9$ a
• Diameter of the observable Universe: ~93 x $10^9$ light years or 4.3 x 10^23 km
• Number of stars in the Milky Way galaxy ~4 x 10^11
• Number of galaxies in the observable Universe ~5 x10^11
• Number of atoms in the observable Universe ~10^78 –10^82
• The Sun is a G-type main-sequence star with a surface temperature of 5500°C
• Most stars in the Milky Way galaxy are smaller than the Sun and significantly less luminous
• Lifetime of a star is inversely related to its mass

  • Stars much smaller than the Sun can exist for up to a trillion years
  • Most massive stars may only last for a few million years
• The longevity of a star is highly relevant to the evolution of life on its planets
• A 70 kg human contains:

  • Hydrogen 6.5 kg
  • Oxygen 43.5 kg
  • Carbon 13.5 kg
  • Nitrogen 3.6 kg
  • Calcium 1 kg
  • Magnesium 20 g
  • Iron 5 g
  • Silicon 1.4 g
  • Nickel 0.01 g
  • Selenium 0.01 g
• hydrogen was made in the big bang while heavier elements are made in stellar nucleosynthesis - collapse of small and large stars. The heaviest elements are made in supernova nucleosynthesis
• key properties of a habitable planet

  • Presence of liquid water overextended (evolutionary) period of time
  • Continuous protection from cosmic radiation
  • Sufficiently low rate of highly disruptive impacting bodies(comets, asteroids etc.)
• there are specific conditions for water to be at a solid, liquid and gaseous phase all of which are met on Earth
• pressure controls the temperature of the phase change of water
• Prokaryotes


  single-celled organisms that lack a membrane-bound nucleus (archaea and bacteria)* - appeared ~3.8 Ga ago
• Eukaryotes


  organisms with cells that contain a nucleus and other structures enclosed within membranes (animals, plants,fungi) - evolved 1.6 to 2.1 Ga ago
• Abiogenesis Habitable Zone

  conditions for origin of life are narrower than for subsequent adaptation of life
• galactic habitable zone

  • Frequency of supernovae explosions and black holes generating intense cosmic radiation increases towards galactic centre
  • Earth’s magnetic field protects surface from lower-level cosmic radiation
  • Moderate metallicity: stellar system must have appropriate concentration of various elements to form rocky ‘terrestrial’ planets: metallicity decreases toward galactic halo
  • need enough time
  • don't want to be too near supernova explosions or black holes
  • need just enough metal to form planets like the Earth
• Jupiter both sucks in asteroids/comets due to its huge gravitational pull and therefore reduces the risk of them hitting the Earth
• Jupiter causes asteroids to be pulled out of the asteroid belt, creating a potential hazard for Earth
• The surface temperature on Earth is ‘just right’ for life and has been so since early in Earth’s history
• Habitable zone will vary around other stars depending on the temperature of the star
• Kepler mission was to detect exoplanets
• There are probably billions of Earth-sized planets orbiting in the habitable zone of sun-like stars and red dwarfs in the Milky Way galaxy
• • Ground-based telescopes are now detecting light directly from exoplanets
  • Most common type are between Earth and Neptune in size“super Earths”
  • Theories on solar system formation are being re-evaluated
  • • Migration of planets into different orbits after formation
  • • Different densities of gas and rock/dust in protoplanetary disks
  • New Earth-based telescopes and space telescopes will provide exciting new data
• Keeping Earth in the Goldilocks' Zone

  • The global heat budget
  • Atmospheric density and composition and the ‘greenhouse effect’ (+15°C compared to –18°C)
  • Seasonality caused by the Earth’s axial tilt
  • Land-sea distribution (continental drift) and out gassing of volcanoes
• The Moon is tidally locked to the Earth, but the Moon also affects the Earth not just through tides, but by progressively slowing its rotation (there were about 500days in a year 1 Ga years ago) and by helping to limit the variation in the Earth’s obliquity. This is important for climate as a high obliquity (near 90°) would result in ice around the equator and ice-free poles.