Eight & Nine
Reading: pp127-152 in Moons & Planets (Hartmann)
(also, chapter 19 in Beatty and Chaikin; chapter 3, pp52-57 in Christiansen
(Ph.D. Thesis of Brett Gladman, a postscript file)
Stony (95% of falls, 52% of finds)
Chondrites (51%, 88%)
The most primitive, undifferentiated material
Formed from low T condensates
Similar to Solar composition
Contain ``chondrules'' ~1mm spherical beads.
Olivine, pyroxene, glass, metals
Ordinary (most Chondrites, 85%)
typically metamorphic material
Achondrites (1%, 8%)
Calcium rich (shergottites)
Similar to lunar and terrestrial basalts
No metal grains
highly differentiated---magmatic or impact origin
Irons (3% of falls, 42% of finds)
Granular structure---grain sizes indicates (slow) cooling rate
Stony-Irons (1% of falls 5% of finds)
Possibly examples of a mantle-core transition region.
Reading: pp 155-162 in Moons & Planets (Hartmann)
(Chapter 18 in Beatty and Chaikin; Chapter 3, pp57-69 in Christiansen
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,
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
Low eccentricities (0.1)
Low inclinations (0.1)
Low albedos (dark), typically either 0.04 (C or carbonaceous type) or 0.2
(S or silicaceous=stoney and M or metalic types).
Highly cratered, old surfaces
Many consist of differentiated material (Vesta, for example)
Metallic and stoney asteroids are found closer to the sun, while less differentiated
C and D asteroids are found farther out.
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
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
Planet Crossing Asteroids
Aten: Crosses earth's orbit. Semi major axis less than one AU.
Apollo: Earth and Mars crossing orbit, semi major axis greater than one
Amor: Mars crossing orbit.
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
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
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
Eight & Nine
to the list of lectures.
to Geology 130 Home page.