On The Possibility Of Large KBOs Being Injected Into The Outer Asteroid Belt

In the "Nice model,” an enormous quantity of KBOs are injected into the outer asteroid belt, >2.6 AU (Levison et al., submitted). Most are dynamically lost or collisionally destroyed, yet the remnant of this embedded population may be the source of the D-type asteroids. This raises the issue of whether larger KBOs were also embedded, and what they would look like today (and can we tell?). It is interesting to note that Ceres, the largest asteroid, is not that different from what we imagine dwarf planet KBOs to be like: differentiated, ice-rich (0.72-0.77 anhydrous rock by mass), and possessing unusual surface chemistry. In Levison et al., large KBOs are not considered; the embedded distribution is truncated at 200-km diameter, and the large (bright) end of the size-frequency distribution follows a very steep power-law slope (at least -6.4 differential). This is done, apparently, in order prevent the Trojan population from acquiring large members. If we extend this distribution to larger sizes, using the observed power-law slope of the bright end of today's Kuiper belt (about -4.5; Petit et al., in "The Solar System Beyond Neptune"), the probability of the outer asteroid belt acquiring a "Ceres” is a few percent; if the steep power-law component of Levison et al. is adjusted to the arguably more correct value, this probability rises to 10%. Large embedded asteroids are immune to collisional destruction, but they are subject to dynamical ejection. The probability of Ceres being a refugee from the Kuiper belt would appear to be small, but perhaps it was not as easily ejected as its much smaller D-type brethren. Regardless, its survival in the extreme collisional environment implied above implies bulk ice loss. The probability that one or more of the largest surviving asteroids came from the Kuiper belt should not be discounted.