Lecture 5

Formation of the Earth (Bennett Chapter 4.2)

• evolution of Solar System

• Formation of the Sun:  cloud of gas and dust collapsed into an accretion disk rotating due to conservation of angular momentum
• Central portion of this disk becomes the Sun, some small particles began to condense into planetesimals in the disk

Here is a frame from a computer simulation of the process:

and here are actual images from the Hubble Space Telescope of  protoplanetary disks around protostars:

• once density of gas in Protosun was sufficient for nuclear burning, Sun became a luminous object and cleared nebula as pressure from its light and solar wind pushed material out of Solar System
• planets helped to clean up by absorbing some planetesimals and ejecting others
• once Solar System was mostly clear of debris, planet building ended
• eventually, most planetesimals were destroyed in collisions and or ejected to remote parts of Solar System (movie)
• some of these smaller bodies, however, settled in asteroid belt between Mars and Jupiter, while a few were captured as moons of planets
• OR, some moons may have formed from collisions of planets (movie)
• Solar System is still filled with smaller bodies, such as satellites (moons), asteroids, and comets

  

  

• rings around giant planets, such as Saturn's, are probably result of stray planetesimals being torn apart by gravity when they ventured too close to planet (movie)

  

  

  Jupiter's Rings

  Uranus's Rings

  For an animation of Voyager's flight by Saturn, click here.

  For a movie of Saturn's rings rotating, click here.

• tidal forces (differential gravitational forces) that created rings also evident in Jupiter's effects on its moon Io

  

• The gas giant planets (Jupiter, Saturn, Uranus, and Neptune) formed far enough from the Sun that ices could condense, producing more massive protoplanets that became massive enough to capture hydrogen and helium gases
  
• Venus, Earth and Mars acquired their atmospheres at later stage in formation of Solar System
  • most likely scenario is from outgassing (from gas blown out of volcanos)
  • on Earth, oxygen, essential to animals, was produced by plants breaking down CO₂

• today, many solid surfaces scarred by craters from meteorite impacts;  (movie); places without craters tell us something interesting is going on!

  

  

  

• even crazy tilted rotational axis of Uranus thought to have resulted from impact

Formation of Earth's Moon

• Earth's moon is very large relative to Earth, and is the only large moon of a terrestrial planet
  
• what does a successful theory need to get right?

  • composition of Moon (and of Earth)
    • iron core between 220 and 450 km across, 1-4% of Moon's mass, iron deficit compared with Earth
    • Earth includes solar-type He/Ne (identified in comparison with Solar Wind)
    • Moon composition is similar to Earth's mantle
    • Moon deficient in water and other volatile compounds compared with Earth
    • proportions of different oxygen isotopes on the Moon (O₁₆, O₁₈, etc) are similar to those in Earth's minerals

  • mass of Moon (and of Earth) -- Moon's mass is 7.35 x 10²² kg, Earth is about 82 times more massive

    

  • age of Moon (and that of the Earth)

    • hafnium (half-life = nine million years)/tungsten samples show Moon formed only 50 million years after Solar System was born 4.6 billion years ago
    • rock samples from northern parts of Greenland (why do you need to look there?) suggest that Earth of similar age

  • distance between Moon and Earth -- 239,000 miles on average
  • angular momentum (total spin) of Earth-Moon system, including rotation of Earth and Moon and revolution of Moon around Earth

• theories of Moon's origin
  • spun off Earth (``fission hypothesis'')

    

    • in 1878, George Darwin (son of Charles) proposes that newly-born, still molten Earth started spinning faster and faster

    • Earth then threw off piece from its surface that became Moon
    • explains why Earth and Moon rocks are similar (they were same body at one point)
    • explains missing Moon iron by claiming that rock which formed Moon came from parts of Earth that are iron-poor
    • BUT can't explain total spin rate of Earth-Moon system (Earth would have to rotate 4 times faster than total angular momentum of system)
    • AND why does Moon have iron in core at all?

  • assembled independently at same time as Earth out of primitive rocks and dust (``double planet hypothesis''), explains why Earth and Moon rocks are so similar, BUT then iron core content should be the same as for Earth!

    

  • formed elsewhere and then captured (``capture hypothesis'')

    

    • other examples of likely captures -- Trition? (Pluto and Charon -- shown below actually formed from an impact), perhaps some of the Jovian moons and moonlets

      

    • explains why the Moon has little iron if formation was in part of Solar System where iron content lower
    • BUT it is much more likely that Moon would crash into Earth or receive gravity kick that would send it out into space
    • AND if captured, how was kinetic energy dissipated?
    • AND what about iron deficit? body as large as Moon -- if formed from same material as other planets -- should have iron core like Earth, Venus, and Mars
    • AND cannot explain why Earth and Moon rocks are so similar except for the missing iron

  • giant impact of large body with young Earth might explain both Moon's composition and total spin rate of Earth-Moon system

Giant impact (``Big Whack'' or ``The Big Splat'') theory of Moon's origin

• proposed independently by two groups in early 1970's
• what might have happened...
  • 4.5 billion years ago (about 50 million years after Sun formed), large chunk of rock (1-3 times size of Mars) slams into Earth
  • Earth's primordial atmosphere boils off into space
  • Earth's mantle melts
  • ejecta thrown into space forming ring about young Earth
  • debris from ring starts clumping together (can't happen in same way to ringed Jovian planets)
  • impact could explain tilt of Moon's orbit and Earth's rotational axis

    

• first reaction: general resistance to catastrophe explanation (big surprise!)
• fits in nicely with theory that collisions of planetesimals build up planets
• theory tested by computer simulations (movie)
  • initial simulations require impactor size of Mars (with roughly 1/10th of Earth mass) to leave Earth-Moon system with correct amount of spin
  • impactor is destroyed, its iron sinks to deformed Earth's core, and plume of rock, magma, and vapor is boosted into Earth's orbit
  • occasionally in these simulations, fairly large, rocky moon forms
  • resulting moon has little iron, consists mostly of rocky mantle material of impactor and Earth
  • resulting moon has little water and other volatile compounds, because ejecta so hot that gases escape into space
  • newer simulations have looked into details of Moon's coalescence from ring of debris

    • over 100 years after impact, debris cools down and forms swarms of particles of various sizes
    • less than one year later, one or two moons form within 14,000 miles from Earth
    • particles in outer part of disk clump easily, but those in inner disk are kept apart by Earth's gravity (like Jovian rings)
    • gravitational forces probably scattered inner disk material back on to Earth

To read some more about the origin of the Moon, go to the Hand-Outs and Reference Materials page.

If you want to read more technical details about the impact theory of the Moon's origin, click here or here.

Next Lecture