Size and Age Dependence of Koronis Family Colors

Computer Science

Scientific paper

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Scientific paper

The ancient and massive Koronis family now has four identified subfamilies (asteroid families made by the breakup of fragments of the ancient collision), with ages running from 5.7 to 290 My. This presents unique opportunities to explore space weathering processes, along with dynamical processes such as collisions and binary formation and destruction. Analysis of family members with accurate SDSS measurements shows a correlation of average subfamily color with age that for the first time is highly statistically significant. Yet Thomas et al. (2011) report a size dependence of the colors of the ancient family that demands caution when comparing subfamilies with differing size distributions. Reanalyis of the Thomas et al. data show the reported break near asteroid diameter 5 km is not significant. However, analysis of the much more extensive SDSS data set show a significant break past diameter 2.5 km, with smaller objects systematically bluer. The break is not present in the Karin subfamily (the youngest at 5.7 My), but is already fully developed in the Eriphyla subfamily (only 220 My). The reddening trend with age remains even when comparing only asteroids of similar size, confirming the presence of space weathering phenomena. The meaning of the trend with size is not immediately clear. We consider briefly the strengths and weaknesses of several interpretations of the bluer colors for small objects: 1) those objects receive more jolts from random collisions capable of shaking the regolith and exposing fresh material beneath; 2) those objects receive more jolts from the cycle of fission and recombination driven by YORP; and 3) the lower gravity on those objects retains regolith less well.

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