* Astronomy

Title: Using the youngest asteroid clusters to constrain the Space Weathering and Gardening rate on S-complex asteroids
Authors: Mark Willman, Robert Jedicke, Nicholas Moskovitz, David Nesvorný, David Vokrouhlický, Thais Mothé-Diniz

We have extended our earlier work on space weathering of the youngest S-complex asteroid families to include results from asteroid clusters with ages <10^6 years and to newly identified asteroid pairs with ages <5x10^5 years. We have identified three S-complex asteroid clusters with ages in the range 10^{5-6} years. The average colour of the objects in these clusters agree with the prediction of Willman et al., 2008. SDSS photometry of the members of very young asteroid pairs with ages <10^5 years was used to determine their taxonomy. The average colour of the S-complex pairs is PC_1=0.49±0.03, over 5-sigma redder than predicted by Willman et al., 2008. Therefore, the most likely pair formation mechanism is gentle separation due to YORP spin-up leaving much of the aged and reddened surface undisturbed. In this case our colour measurement allows us to set an upper limit of ~64% on the disturbed surface portion. Using pre-existing colour data and our new results for the youngest S-complex asteroid clusters we have extended our space weather model to explicitly include the effects of regolith gardening and fit separate weathering and gardening characteristic timescales of tau_w=960±160My and tau_g=2000±290My respectively. The first principal component colour for fresh S-complex material is 0.37±0.01 while the maximum amount of local reddening is 0.33±0.06. Our first-ever determination of the gardening time is in stark contrast to our calculated gardening time of tau_g~270My based on main belt impact rates and reasonable assumptions about crater and ejecta blanket sizes. A possible resolution for the discrepancy is through a 'honeycomb' mechanism in which the surface regolith structure absorbs small impactors without producing significant ejecta. This mechanism could also account for the paucity of small craters on (433) Eros.

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