REMOTE COMPOSITIONAL ANALYSIS OF ASTEROIDS: METEORITES, MICROSCOPES, AND TELESCOPES

Asteroids represent a huge diversity of geological materials that range from highly differentiated iron-rich fragments of disrupted cores to water-rich, organic-rich, low-temperature primitive materials. The diversity of these objects includes at least 18 spectral types that sample source regions from a wide range of heliocentric distances. The advantage we have in understanding asteroids is partial ground truth in the form of meteorites. Although it is probable that not all asteroid compositional types provide us with meteorite samples, these samples do ground us in the range of at least some (or most?) of the expected mineralogies, reflectance spectra, and physical properties. One application of these data is to use a mix of reflectance spectra of asteroids and meteorite analogues, physical properties studies of meteorites, and bulk density measurements of asteroids to constrain the composition and physical structure of asteroids. The results have given us our emerging view of the asteroid belt. It appears that most asteroids are very collisionally evolved with high levels of porosity being common in almost all objects. Many asteroids are likely to be gravitationally bound rubble piles (examples 253 Mathide, 25143 Itokawa) while others show a coherent, but highly fractured morphology (433 Eros, 243 Ida). Only the largest asteroids (1 Ceres, 4 Vesta) and perhaps the smallest Near Earth Asteroids appear to be low in porosity. These insights are possible because of the synergy between laboratory data and remote sensing.