*Why care about meteorites? Objects that fall from the sky have been
worshipped or feared for centuries.
*Poor man's space probe- samples of the asteroid belt, the moon, Mars, ??
*Largest input of mass to the Earth today; 50 million tons of new stuff a
year.
*Definitions of meteorites, meteors, meteoroids
*coarse division of meteorites into chondrites,and achondrites, not into
three divisions like book. Another way to separate them is to call them
differentiated and undifferentiated.
Undifferentiated meteorites (chondrites)
*defined by chondrules, little droplets of formerly molten
silicates
*chondrites are all old- 4.556 billion years
*carbonaceous chondrites contain low temperature "glue" and have a bulk
composition very similar to that of the sun
*the age and composition of chondrites suggest they are leftovers from the
solar nebula, and may represent all the solid stuff of the solar system.
*ordinary chondrites are like the carbonaceous but without the low
temperature glue. They are much more common than the other classes of
meteorites and thus we get to see more of the variation within the group that
exists in nature.
*most OC's are equilibrated; they have been exposed to elevated
temperatures and pressures, their minerals have undergone chemical exchange to
meet a new set of equilibria- they are metamorphic.
*non-equilibrated OC's are similar to CC's, with one exception- many of the
CC's have apparently undergone aqueous alteration at low temperatures.
*Many chondrites are breccias- some say all of them are. They are
certainly sedimentary, but we don't always know how the disparate parts came
together. In some, however, it is clear that they were subjected to impact,
and are in fact samples of regolith.
Differentiated meteorites (achondrites)
*More than just the lack of chondrules, they are the product of
planetary processes, such as planetary differentiation or volcanism.
*the most common achondrites are basalts, and probably formed in volcanic
environments just like they do on Earth and other planets, through partial
melting. Eucrites are the most common kind. More on that in a minute.
*Other achondrites are cumulates, still volcanic but this time made up of
the settled crystals from magma chambers deep beneath a volcano. Diogenites
are the most common kind- again, more on them in a minute.
* Unlike the more common achondrites, irons do not resemble any volcanic
rocks from the surface of the Earth. What they resemble is the core of
planets, the metal that has drained to the center of a body during
differentiation.
*Irons are the most common specimens seen in museums just because they are
more durable, and are found more often in farmers fields -they don't weather
away to look just like earth rocks like ordinary stony meteorites do.
*irons show cooling rates consistent with their being the cores of small
bodies, but not too small.
*Stony-iron meteorites are mixtures of silicate minerals and metal, and
come in two flavors-
*mesosiderites, which are always impact breccias
*pallasites, which are equilibrated mixes of iron and either olivine or
pyroxene. these probably represent a core-mantle boundary in a differentiating
asteroid.
*almost all of the achondrites are nearly as old as the chondrites, but not
quite- 4.44 Ba instead of 4.556.
Asteroids
*More than 10,000 small bodies (smaller than about 1000 km) circle the sun
and act as minor planets. These asteroids are usually too small to look at with
a telescope.
*the vast majority of the asteroids have orbits that lie between that of
Mars and Jupiter, the region called the asteroid belt.
*Other asteroids can have orbits that cross mar's (amors) or Earth's
(apollo), or other less stable locations in the solar system.
*It's pretty clear that the asteroid belt is the source of most meteorites,
since we've back-calculated the orbits of several and that's where they seem to
have come from (see figure 3.6). So meteorites are samples of asteroids.
*there's NO evidence that meteorites are samples of comets. maybe samples
of comets that have become asteroids through orbital evolution, but that's
stretching the definition.
*Within the asteroid belt there are some interesting gaps that correspond
to places where there are 2:1 or 3:1 or such resonances with the orbit of
Jupiter.
*only 33 asteroids are bigger than 200 km, but more than 3000 are bigger
than 20 km and more than 300,000 are thought to exist bigger than 2 km in
size.
*We have photographed two main belt asteroids, Gaspra and Ida. Both are
cratered, have some sort of smooth regolith (not expected), are irregular in
shape, and seem to be made of a mixture of silicate minerals and metal, and had
fairly strong magnetic fields. Ida is bigger, seems more heavily cratered, and
has it's own little moon (Dactyl).
*Phobos and Deimos , the two small and irregular moons of Mars, are similar
and probably represent captured asteroids. They have low densities, Are
heavily cratered, and show most of the same features as Gaspra and Ida.
*irregular shape of these bodies strongly suggests that they are
collisional fragments. Sometimes two fragments may orbit together as a
contact binary, such as Toutatis (fig. 3.12)
*Other asteroids have been studied spectrographically. Sunlight bouncing
off a mineral surface says someting about those minerals, because they absorb
certain frequencies of light- the light bouncing off isn't the same as the
light going in. Various classes of asteroid spectra have been developed, and
they seem to echo the classification of meteorites. There are asteroids that
look carbonaceous, stoney-iron, basaltic, and "ordinary chondritic".
*the three largest asteroids: Ceres looks carbonaceous, Pallas looks
ordinary chondritic, and Vesta looks like a mixture of basalt and pure olivne
and pyroxene.
*recent studies of Vesta and small fragments like it, and images of Vesta
from Hubble, make it almost certain that the eucrites and diogenites come from
there.
*These compositional classes of asteroids also show some variation with
location in the belt, with more primitive types in the outer region and more
differentiated types in the inner region (see fig. 3.13)
Asteroid evolution
*meteorites and asteroid spectra show that these bodies have
undergone various stages of differentiation, from the completely primitive to
the completely differentiated.
*basalts, cumulates, stoney-irons and irons make a complete picture of a
differentiated body. Heat source could have been just about anything, but
short-lived radionuclides are my favorite.
*aqueous alteration and metamorphism of chondrites speak of only moderate
thermal activity, perhaps in an onion-skin style model
*regolith breccias and mesosiderites show that these bodies may have been
disrupted and re-mixed to form rubble-pile asteroids.
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