NATURE OF THE M-TYPE ASTEROIDS FROM OPTICAL POLARIMETRY

The curves of polarization of the M-type asteroids are not reproduced by silicate rocks in any surface textures; nor do they correspond to metallic fractures. They fit with powders of 20 to 40-μm diameter fragments of metals. Impacts by meteoroids in metallic bodies are able to produce a superficial coating of small metallic debris because, at the low temperature of asteroids, nickel-iron metals are no longer ductile but brittle. Thus, polarimetric measurements on the M-type asteroids are compatible with metallic bodies probably of iron-nickel composition. Such metallic solar system objects could be the parent bodies for siderites; they may have been formed as the cores of differentiated minor planets, subsequently denuded of their silicaceous mantles by accumulation of impacts. © 1979.