Colour variability of asteroids in the Sloan Digital Sky Survey Moving Object Catalog

We report a detection of statistically significant colour variations for a sample of 7531 multiply observed asteroids that are listed in the Sloan Digital Sky Survey Moving Object Catalog. Using five-band photometric observations accurate to ∼0.02 mag, we detect colour variations in the range 0.06-0.11 mag (rms). These variations appear to be uncorrelated with the physical characteristics of the asteroids, such as diameter (in the probed 1-10 km range), taxonomic class and family membership. Despite this lack of correlation, which implies a random nature for the cause of colour variability, a suite of tests suggest that the detected variations are not instrumental effects. In particular, the observed colour variations are incompatible with photometric errors, and, for objects observed at least four times, the colour change in the first pair of observations is correlated with the colour change in the second pair. These facts strongly suggest that the observed effect is real, and also indicate that colour variations are larger for some asteroids than for others. The detected colour variations can be explained as being due to inhomogeneous albedo distribution over an asteroid's surface. Although relatively small, these variations suggest that fairly large patches with different colour than their surroundings exist on a significant fraction of asteroids. This conclusion is in agreement with spatially resolved colour images of several large asteroids obtained by the Near Earth Asteroid Rendezvous (NEAR) spacecraft and the Hubble Space Telescope (HST).