The Karin cluster is one of the youngest known families of main-belt asteroids, dating back to a collisional event only 5.8 +/- 0.2 Myr ago. Using the Spitzer Space Telescope we have photometrically sampled the thermal continua (3.5-22 mu m) of 17 Karin cluster asteroids of different sizes. down to the smallest members discovered so far, in order to make the first direct measurements of their sizes and albedos and study the physical properties of their surfaces. Our targets are also amongst the smallest main-belt asteroids observed to date in the mid-infrared. The derived diameters range from 17.3 km for 832 Karin to 1.5 km for 75176, with typical uncertainties of 10%. The mean albedo is p(v) = 0.215 +/- 0.015, compared to 0.20 +/- 0.07 for 832 Karin itself (for H = 11.2 +/- 0.3), consistent with the view that the Karin asteroids are closely related physically as well as dynamically. The albedo distribution (0.12 <= p(v) <= 0.32) is consistent with the range associated with S-type asteroids but the variation from one object to another appears to be significant. Contrary to the case for near-Earth asteroids, our data show no evidence of an albedo dependence on size. However, the mean albedo is lower than expected for young, fresh "S-type" surfaces, suggesting that space weathering can darken main-belt asteroid surfaces on very short timescales. Our data are also suggestive of a connection between surface roughness and albedo, which may reflect rejuvenation of weathered Surfaces by impact gardening. While the available data allow only estimates of lower limits for thermal inertia, we find no evidence for the relatively high values of thermal inertia reported for solve similarly sized near-Earth asteroids. Our results constitute the first observational confirmation of the legitimacy of assumptions made in recent modeling of the formation of the Karin cluster via a single catastrophic collision 5.8 +/- 0.2 Myr ago. (C) 2008 Elsevier Inc. All rights reserved.
