Astronomy paper 2 topic 9

• Moon


  • Has a crust, mantle and core (inner and outer) like the Earth
  • Mean thickness of the lunar crust is 50-60 km, typically about 3 times that of the Earth's crust
  • Radius of the Moon's core is less than 25% of the Moon's radius (on Earth the core extends to more than 50% of its radius)
  • Moon's core is not at the physical centre of the Moon but offset by about 2 km towards the near side, i.e. towards Earth
• Luna 3 was the first spacecraft to photograph the Moon's far side


  1959
• The Moon's far side is almost completely covered by highlands
• US President John F. Kennedy announced the Apollo space programme of landing men on the Moon and returning them safely back to Earth 'before this decade (the 1960s] is out'


  May 1961
• Objectives of the Apollo space programme

  • Collection of lunar soil and rocks for analysis on return to Earth
  • Deployment of scientific experiments on the lunar surface (Laser Ranging Retro Reflectors, passive seismometers, lunar dust collectors and solar wind composition experiments)
• Neil Armstrong was the first of a total of 12 astronauts to step onto the Moon's surface


  July 21, 1969
• Giant Impact Hypothesis


  A large body about the same size as Mars (Theia) struck a glancing blow with a nascent Earth, vaporising Theia and part of the Earth, with the debris slowly cooling and condensing to form the Moon
• Alternative theories for the Moon's origin

  • Fission Theory: The Earth was spinning so rapidly that part of it (now filled by the Pacific Ocean) spun off and formed the Moon
  • Capture Theory: The Earth and the Moon were formed at different places in the Solar System, but the Moon became 'captured' by Earth's gravitational force
  • Condensation (Co-accretion) Theory: The Earth and Moon formed together at the same time out of material from the solar nebula
• The Giant Impact Hypothesis is supported by the Moon's lack of substances that evaporate easily ('volatiles' such as water) and its small iron core, but not all astronomers agree with this model and it remains a hypothesis