Where Are the Olivine Asteroids in the Main Belt?
Abstract
As first stated by Chapman [1] and later discussed by Bell et al. [2], one serious problem in our current understanding of the asteroid belt is the explanation for the rarity of olivine-dominated metal-free silicate asteroids in the main belt. The complete differentiation of a chondritic parent body should result in a body with a NiFe core, a thick olivine-dominated mantle, and a thin plagioclase/pyroxene crust [2]. If some fraction of iron meteorites are fragments of the cores of these differentiated asteroids, the disruption of these bodies should produce a number of exposed NiFe cores (historically thought to correspond to type M), a large number of olivine-dominated metal-free asteroids (assumed to correspond to type A) and a number of pyroxene-rich asteroids (assumed to correspond to type V or J). The problem with this scenario is that while M-asteroids are relatively abundant in the main belt (~40 classified asteroids), asteroids classified as A (~5 asteroids), V (~16 asteroids) or J (~6 asteroids) are relatively rare. One possible explanation for this paradox is that A-type asteroids (and V and J-types) are very common at small sizes that have yet to be spectroscopically characterized [2]. Another explanation is that A-types (and V and J-types) are actually very uncommon due to the formation of only a relatively small number of fully differentiated bodies that were subsequently collisionally disrupted. The asteroidal evidence for the existence of olivine-dominated metal-free silicate asteroids in the main belt is not very convincing. A-type asteroids are known to be very rare at diameters larger than ~30 km and also appear rare at smaller diameters as a result of the CCD spectroscopic survey of over 200 small (diameters between 10 and 30 km) asteroids that found no new asteroids with A-type spectra. Also a problem is that all of the newly discovered V and J-type asteroids appear to be fragments of 4 Vesta. Therefore with the exception of Vesta and its assumed fragments, pyroxene-rich objects are also rare in the main belt. Another problem is that the number of M-asteroids in the main belt with NiFe-rich surfaces is very uncertain. One M-asteroid (16 Psyche) has a very high radar albedo that implies a very metal-rich surface. However two M-asteroids [3] have been found to have absorption features in the 3-micrometer wavelength region that indicate hydrated minerals. A final problem is that no asteroid family appears to be the result of the breakup of a fully differentiated body, which should result in a family containing a large M-type asteroid, a large number of A-types and a number of V and J-types. The explanation for this apparent problem may be just that these olivine-dominated metal-free asteroids (and pyroxene-rich objects not related to Vesta) are very small and just have not been spectroscopically observed. This would imply that almost all of the completely differentiated bodies were disrupted relatively early in the age of the solar system, which would account for the A-type asteroids being collisionally broken down into small sizes. However an equally valid argument is that bodies (or fragments of bodies) that completely differentiated (e.g., Vesta) are relatively rare and therefore A-types are very uncommon. In this scenario, partially differentiated bodies would be very common, which implies that S-asteroids are predominately asteroids that only partially differentiated and not the core-mantle boundaries of disrupted differentiated bodies. References: [1] Chapman C. R. (1986) Mem. Soc. Astron. Italiana, 57, 103-114. [2] Bell J. F. et al. (1989) Asteroids II, 921-945. [3] Jones T. D. et al. (1990) Icarus, 88, 172-192.
- Publication:
-
Meteoritics
- Pub Date:
- July 1994
- Bibcode:
- 1994Metic..29..453B
- Keywords:
-
- Asteroid Belts;
- Asteroids;
- Evolution (Development);
- Olivine;
- Silicates;
- Absorption Spectra;
- Iron Meteorites;
- Pyroxenes;
- Spectrum Analysis;
- Astrophysics;
- ASTEROID BELT; OLIVINE