For nearly two centuries, it has seemed reasonable to the scientific community that meteorites might come from asteroids. But as late as the 1970s, there were no known physical mechanisms for transporting asteroid fragments to Earth. For instance, collisions sufficient to produce such drastic orbital changes would instead vaporize the asteroidal material. The problem is now solved, except for the details (Wisdom, 1985). Throughout the asteroid belt, there are zones where resonant gravitational perturbations by planets, primarily Jupiter and Saturn, form dynamically chaotic zones. When inter-asteroidal collisions near the boundaries of these zones send fragments into them, orbital eccentricities are rapidly increased and the fragments cross the orbits of the other planets—including Earth's. Fragments reach Earth primarily from the inner parts of the asteroid belt, especially near the 3:1 commensurability with Jupiter (those in the outer belt reach Jupiter first and are generally ejected from the solar system). Earth-crossing asteroids often continue to have aphelia in the asteroid belt, and thus they continue to suffer collisions with main-belt asteroids. The smaller fragments that encounter Earth as meteorites result from a multi-generational collisional cascade and come both directly from the resonances in the main belt and from cratering and collisions involving Earth-crossing asteroids.

A few meteorites are from the Moon and Mars (Warren, 1994). Some meteorites may be from comets (Campins, 1997) or from more distant asteroids, but none have so far been identified as likely candidates, and there are physical reasons that mitigate against that (the high velocity and weak strength of comets result in upper atmospheric disintegration of incoming cometary meteoroids; efficient dynamical mechanisms for delivering asteroidal debris from regions beyond the 5:2 resonance have not been identified). The collisional cascade also generates finer materials as small as interplanetary dust, which is transported by Poynting-Robertson drag and other radiation forces (Burns et al., 1979). The relative contribution of comets and asteroids to particles of various sizes in the interplanetary dust complex is not well known, but both sources contribute a significant fraction (Bradley et al., 1988).

TABLE 4.1 Common Asteroid Types

Type

Reflectance Spectrum

Meteoritic Analog(s)

Undifferentiated C-like types

 

C

Very low albedo, flat longward of 0.4 µm absorption band in UV and sometimes near 3 µm

Carbonaceous chondrites

B

Low albedo; C-like but brighter, more neutral

Carbonaceous chondrites

G

Low albedo; C-like but brighter, strong UV

Carbonaceous chondrites

Undifferentiated metamorphosed types

 

Q

Moderate albedo, strong absorption near 1 µm and 2 µm

Ordinary chondrites

S

Moderate albedo, reddish in visible, weak to moderate absorption near 1 µm and 2 µm

Ordinary chondrites

Differentiated types

 

M

Moderate albedo, slightly reddish linear slope

Irons

V, J

High albedo, like S-types but stronger and additional absorptions

HED basaltic achondrites

A

High albedo, strong absorptions due to olivine

Brachinites

S

Moderate albedo, reddish in visible, weak to moderate absorption near 1 µm 2 µm

Stony-irons, achondrites (?)

E

High albedo, flat or slightly reddish

Aubrites (?)

Others

 

P

Very low albedo, slightly reddish linear slope

None

D

Very low albedo, reddish linear slope

None

NOTE: Other asteroid types have been defined that are not included in this table, e.g., F-types. Some are subdivisions of the types listed. Others are rare, new types, generally seen only among the population of very small asteroids. Some classified asteroids may be atypical of their class (e.g., M-types with 3 µm absorption features) or may have different meteorite analogs, pending availability of better data (e.g., M-types not surveyed by radar for high radar reflectivity could have undifferentiated enstatite chondrites as a meteorite analog).



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