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Geology 130F

 Lecture Ten


Reading:  pp127-152 in Moons & Planets (Hartmann)
(also, chapter 19 in Beatty and Chaikin; chapter 3, pp52-57 in Christiansen and Hamblin)
  1. Stony (95% of falls, 52% of finds)
    1. Chondrites (51%, 88%)
      1. Carbonaceous
        1. The most primitive, undifferentiated material
        2. Formed from low T condensates
        3. Similar to Solar composition
        4. Contain ``chondrules'' ~1mm spherical beads.
        5. Olivine, pyroxene, glass, metals
      2. Ordinary (most Chondrites, 85%)
        1. typically metamorphic material
    2. Achondrites (1%, 8%)
      1. Calcium poor
      2. Calcium rich (shergottites)
        1. Similar to lunar and terrestrial basalts
        2. No metal grains
        3. highly differentiated---magmatic or impact origin
  2. Irons (3% of falls, 42% of finds)
    1. Clearly differentiated
    2. Granular structure---grain sizes indicates (slow) cooling rate
  3. Stony-Irons (1% of falls 5% of finds)
    1. Possibly examples of a mantle-core transition region.

More on Metorites

(Ph.D. Thesis of Brett Gladman, a postscript file) 


Reading: pp 155-162 in Moons & Planets (Hartmann)
(Chapter 18 in Beatty and Chaikin; Chapter 3, pp57-69 in Christiansen and Hamblin)

Main Belt Asteroids

The bulk of known asteroids are found between Mars and Jupiter (having semi-major axes between 2 and 4 Au, or 3x108<6x108km). They have typical inclinations of 0.1, so they fill a region about 30,000,000-60,000,000 km high. The orbits are all prograde. Inclinations and eccentricities are typically 0.05-0.1; asteroids with larger eccentricities are removed by collisions with Earth, Mars, or Jupiter. Roughly 10,000 objects have been observed in this region, but we know that there are many more that have not been seen; the number of asteroids of radius r is proportional to 1/r3, or

n(r)=100(100 km/r)3,

for radii between centimeters and a few hundred kilometers. There is very little dust, since drag forces acting on the dust remove it from the belt and deposit it on the sun. However, IRAS did detect dust in the belt, indicating that dust is being generated, probably by collisions. It has been speculated that interplanetary dust plays a role in the Earth's climate, producing the ice ages (see, for example, the paper by  Kortenkamp and Dermott .

General Properties of Main Belt Asteroids

  1. Low eccentricities (0.1)
  2. Low inclinations (0.1)
  3. Low albedos (dark), typically either 0.04 (C or carbonaceous type) or 0.2 (S or silicaceous=stoney and M or metalic types).
  4. Highly cratered, old surfaces
  5. Aysmmetric shapes
  6. Many consist of differentiated material (Vesta, for example)
  7. Metallic and stoney asteroids are found closer to the sun, while less differentiated C and D asteroids are found farther out.
  8. From the scaling of n(r) given above, the total number of belt asteroids is of order 1016 (with a minimum size of ~1 cm, since smaller bodies are subject to drag and hence fall into the sun). The volume of the belt is ~1040 cm3, so the average distance between objects is ~1,000 km. The distance between objects larger than 1 kilometer is ~1,000,000km.

Asteroids are believed to have suffered numerous collisions since their formation, a belief bolstered by images of Ida (above, with its moon Dactyl) and Gaspara obtained by the Galileo spacecraft on its way to Jupiter. Note the irregular shape of Ida, and the numerous impact craters. Gaspara also has numerous craters; even tiny Dactyl has been pounded.

Planet Crossing Asteroids

Chiron and Trans-Neptunian (or Kuiper Belt) Objects

Chiron is a large (10km) body with a semi-major axis of 13.7 Au and an eccentricity of e=0.4. Its orbit takes it from 13.7(1-e)=8 Au to 13.7(1+e)= 19.4 AU. The semi-major axis of Saturn is 9.54Au, while that of Uranus is 19.2 AU. Thus Chiron crosses the orbits of both these giant planets, and it is dynamically unstable. There are now six objects known to cross the orbits of the giant planets. Since all have unstable orbits, with lifetimes much shorter than the lifetime of the solar system, there must be some source of Chiron-like objects. In 1992  a possible source was found.

We now know of several hundred objects with semi-major axes greater than 39AU (the semi-major axis of Pluto). These are known as TNO's or Kuiper belt objects. TNOs are currently classified, on the basis of orbital properties, into three classes.

Objects in the first class are called ``classical Kuiper belt objects''. They have semimajor axes between 42 and 50 AU and eccentricities of order e=0.1; even at periapse (closest approach to the sun) they are more than 38 AU from the sun, leaving them well outside the location of Neptune (at 30 AU).

Plutinos are objects trapped in orbital mean motion resonances with Neptune, including the 3/2 at 39.4AU, and a few in the 2/1 at ~37.7AU. The first such object to be discovered was Pluto, hence the name. Plutinos have eccentricities ranging up to e=0.34.  They cross the orbit of Neptune, but the resonant forces exerted by Neptune ensure that the Plutinos never actually suffer close encounters with the planet.

The third class of objects is called the ``scattered belt''. These objects have large semimajor axes, eccentricities, and inclinations. They have perihelia near the orbit of Neptune, and are believed to have evolved from objects that crossed the orbit of Neptune in the past. Here is a plot (from Dave Jewitt) of orbits in the Kuiper Belt.   The red orbits correspond to Plutinos, the blue to classical objects, and the black to scattered belt objects.

The total amount of mass in the  Kuiper belt is estimated to be of order 0.1-1 Earth Mass.

The origin and evolution of TNO's is currently a hot research topic. A good source is Dave Jewitt's Kuiper Belt page.

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