AN ASTEROIDAL BRECCIA - THE ANATOMY OF A CLUSTER IDP

We report results of a consortium study of a large interplanetary dust particle known as cluster 1,2008#5. This cluster is composed of fifty -three fragments (>5 mu m in diameter) and several hundred fines (<5 m u m in diameter). Fragments and some fines were characterized using a variety of chemical and mineralogical techniques including: energy dis persive X-ray analyses for bulk chemical compositions for elements car bon through nickel, transmission electron microscopy for mineralogy, n oble gas measurements, synchrotron X-ray fluorescence for trace elemen t abundances, isotopic abundances using an ion probe, trace organic ab undances, and reflectance spectroscopy. Our results show that cluster L2008#5 displays strong chemical and mineralogical heterogeneity on a size scale of the individual fragments (similar to 10 mu m in diameter ). Despite the strong heterogeneity, we believe that nearly all of the analyzed fragments were originally part of the same cluster in space. Several methods were used to estimate the degree of heating that this cluster experienced. Variations in the inferred peak temperatures exp erienced by different fragments suggest that a thermal gradient was ma intained. The cluster as a whole was not strongly heated; it is estima ted to have a low earth-encounter velocity which is consistent with or igin from an object in an asteroidal orbit rather than from a comet, w hich would most likely have a high entry velocity. Our conclusions sho w that cluster 1,2008#5 consists of a chemically and mineralogically d iverse mixture of fragments. We believe that cluster 1,2008#5 represen ts a heterogeneous breccia and that it was most likely derived from an object in an asteroidal orbit. We also present an important cautionar y note for attempts to interpret individual, small-sized 10-15 mu m ID Ps as representative of parent bodies. It is not unique that individua l building blocks of IDPs, such as discrete olivine, pyroxene, sulfide grains, regions of carbonaceous material, and other noncrystalline ma terial, are found in several fragments; however, it is unique that the se building blocks are combined in various proportions in related IDPs from one large cluster particle.