ISP205 Lecture #10, Feburary 8, 2001

  1. Inverse Square Law
    1. intensity = number of photons / area
    2. the intensity decreases with the square of the distance
    3. Applies to radiation (including heat), sound, gravity ....
      (if not collimated)
    4. Bad news: it is harder to see distant objects (need larger telescopes)
    5. Good news: if the intrinsic brightness of a star is known one can determine
      its distance from the apparent brightness.
  2. Properties of Solar System
    1. All planets rotate in plane counterclockwise around the sun
    2. Their orbits are only slightly elliptical
      (Except: Mercury, Pluto)
    3. Most moons orbit in plane counterclockwise around planets
    4. The sun and most planets and moons spin counterclockwise
      (Except: Venus, Uranus, Pluto) 
      (picture from other book)
    5. The composition of the planets varies with distance to sun:
      1. Density tells you something about the composition

        density = mass / volume
      2. density of a mixture is the average of the density of the 
        individual components (weighted according to their proportions)
      3. typical densities: iron       : 7.9 g/cm3
                                 rock      : 2.5-3.5 g/cm3
                                 water     : 1 g/cm3
                                 water ice: 0.9 g/cm3
                                                  air         : depends on temperature and pressure
                                                 typical on earth: 0.0013 g/cm3
      4. Density of Planets: Mercury    5.4 g/cm3
                                     Venus       5.2 g/cm3
                                      Earth        5.5 g/cm3
                                      Mars        3.9 g/cm3
                                      Jupiter      1.3 g/cm3 (1400 earths !!!)
                                      Saturn       0.7 g/cm3 (!) Saturn would float !!!
                                      Uranus      1.3 g/cm3
                                      Neptune    1.6 g/cm3

      5. Conclusion ? 

        Planets fall in 3 groups:
        1) Terrestrial Planets: (Mercury, Venus, Earth, Mars) + Moon
        2) Giant Planets: (Jupiter, Saturn, Uranus, Neptune)
        3) Weird Planets: Pluto
        see images
  3. Other objects in the solar system than the Sun and Planets:
    1. Moons        satellites orbiting a planet
    2. Rings          dust and rocks orbit in disk around equator of a planet
      all giant planets have rings - saturns rings are most easily visible
    3. Asteroids    rocky objects that orbit the sun, mainly in asteroid belt
      most of them are in the Asteroid belt (between Mars and Jupiter)
      more than 10000 known (known orbits)
      size up to 1000km (Ceres)
      NEAR mission finished data taking of Eros - controlled crash next week
          Should hit ground Feb 12, 3:04 pm Eastern Time
          See Image of Eros; (size: 33km X 13km X 13km)
      Binary Asteroids !
       
    4. Comets       ice (water, carbon dioxide, carbon monoxide, methyl alcohol)
      orbit the sun on highly elliptical orbits
      come from the Ort cloud, ~50000AU away from sun (pluto: 40 AU !)
      more than 1000 known  (5-10/yr discovered)
                         
    5. Dust            tiny grains, for example rock

      we have special names for things that hit a planet:
    6. Meteors      dust grains, burning up in earths atmosphere
    7. Meteorites   any bigger piece hitting a planet or moon
  4. The solar system formed 4.5 billion years ago out of same material
    Where do we know that from ? Radioactive dating
    1. Unstable atomic nuclei decay into a "daughter" nucleus, which is
      a different element. It emits an electron or a helium nucleus (alpha particle)
    2. Example: alpha decay of 235U (pic)
    3. Decay happens randomly - there is no way to predict which nucleus
      decays next and when it decays
    4. BUT: The average time it takes for the next decay is a fixed number characteristic
      of the material.
    5. Usually given: Half-life - the time it takes for half the unstable nuclei in a given sample
      to decay. Exampe: 235U has a half-life of 700 million years

       

    6. Age determination: Compare number of unstable nuclei that did not yet decay to
      the number of daughter nuclei that decayed already
    7. Note: after    1 half life: 1/2survive
                          2             1/4 survive
                          3             1/8 survive
                          .... never zero !  (except if only a few left ...)
    8. This gives the age since the rock was last molten
      (only then the number of unstable nuclei + their daughter nuclei remains fixed)
      Therefore: rocks on earth can be much younger than the solar system or earth itself
      Age of solar system from radioactive dating of meteorites that solidified soon after the formation
          of the solar system..
  5. Other dating methods: crater counts
    1. Assuming a known,  constant rate of impacts, one can determine the age of a surface from the
      number of impact craters.

      Example: a 10km crater on the moon (depends on size of object) should occur every 1 million years.
      if we see 3000 that would mean the moon's surface would be 3 billion years old.
    2. BUT: only age of surface since last major geological activity
      Example: Mercury, Mars, and Jupiter have the same age, but have very different crater counts
  6. Formation of the solar system - brief overview
    1. Swirling gas clound collapsed into a rotating disk
    2. Sun and planetesimals formed - planetesimals stuck together forming planets
    3. Further away from sun its colder - more ice, gas - close to the sun only rock and iron
      can survive
    4. Overall composition of solar system (= composition of original cloud)