Spitzer Observations of the Koronis Asteroid Family: Probing Size-dependent Surface Evolution

The Koronis family is an ideal case study, as its members share a common formation age and composition. This enables investigation of how surface properties vary with asteroid size, independent of shared family age. We present Spitzer Peak-Up Imaging observations of 45 Koronis family asteroids. Using the Near-Earth Asteroid Thermal Model, we derived diameters ( D ), geometric albedos ( p V ), and beaming parameters ( η ), complemented with available rotation periods, light-curve amplitudes, spectral slopes, and taxonomic classifications. We investigated trends with size and family membership using the two-sample Kolmogorov–Smirnov (K-S) test. Smaller Koronis family members ( D ≲ 5 km) show slightly lower η ( η ¯ ∼ 1.0 ) compared to larger targets ( D ≳ 5 km, η ¯ ∼ 1.3 ; D K-S = 0.42, p = 0.08), although the distributions overlap within 1 σ . In contrast, Karin cluster members, representing a younger subfamily, exhibit a much tighter distribution of lower η values ( η ¯ ∼ 0.9 ; D K-S = 0.693, p < 0.0001) relative to the broader Koronis population, which spans η ∼ 0.4–2.5. Albedo ( p V ) differences are modest, with large ( p V = 0.23 ± 0.06) and small ( p V = 0.32 ± 0.18) subpopulations overlapping, and the Karin cluster is consistent with the overall family ( p V = 0.22 ± 0.05; D K-S = 0.22, p = 0.53). Large Koronis family members are predominantly S type, whereas smaller asteroids show a mixture of S, Q, and C types, suggesting size-dependent surface variations. These results demonstrate statistically significant variations in thermal and spectral parameters with size and family membership. The combined trends provide constraints on how regolith development, thermal properties, and space weathering evolve across Koronis family members of different sizes.