Evidence for differentiation among asteroid families

Introduction There is considerable evidence that planetesimals differentiated in the early solar system. Fragments of the crusts (howardites, eucrites, and diogenites or HEDs) and cores (iron meteorites) of these bodies are prevalent in our meteorite collections (e.g. Grady, 2000). Differentiation also occurred under very oxidizing (angrites) and very reducing conditions (aubrites) (e.g. Jurewicz et al., 1991). Angrites are commonly believed to be from the crusts of these oxidized differentiated bodies (e.g. Warren and Kallemeyn, 1995) while aubrites are thought to be from the mantles of these reduced differentiated bodies (e.g. Watters and Prinz, 1979). Low degrees of partial melting and melt segregation occurred on a number of primitive achondrite (ureilites, acapulcoites-lodranites, winonaites, brachinites) parent bodies (e.g. Goodrich, 1992; McCoy et al., 1997, 2000; Benedix et al., 2005; Gardner-Vandy et al., 2013). While there is ample evidence for differentiated bodies among meteorites, all of their parent bodies experienced rock-metal differentiation. Thermal evolution modeling suggests that ice-rock differentiation may be quite important among asteroids, particularly among the largest members (e.g. Thomas et al., 2005; Castillo-Rogez and McCord, 2010). In ice-rock differentiation, an initial undifferentiated mixture of ice and chondritic material reorganizes itself with sufficient heating into a rocky core and an icy mantle or shell. However, unlike rock-metal differentiation, which can create some of the sturdiest and strongest small bodies and meteorites, it seems likely that ice-rock differentiation would inhibit or even prevent meteorite creation. Heating of planetesimals is commonly thought to be due to the decay of ²⁶Al. Since rocks are composed of mixtures of different minerals that have different melting temperatures, differentiation is primarily due to the partial melting of the precursor chondritic material. The degree of differentiation is due to the amount of partial melting that occurs. It was long hoped that spectroscopically studying asteroid family members would allow the interiors of these differentiated bodies to be studied in situ. Families are groupings of asteroids that have similar proper orbital elements, implying that these objects are fragments from the same parent body. These groupings are defined by a variety of statistical methods (e.g. Zappalà et al., 1990; Bendjoya and Zappalà, 2002; Nesvornyet al., 2015).