Albedos of Main-Belt Comets 133P/Elst-Pizarro and 176P/LINEAR

We present the determination of the geometric R-band albedos of two main-belt comet nuclei based on data from the Spitzer Space Telescope and a number of ground-based optical facilities. For 133P/Elst-Pizarro, we find an albedo of p_R=0.05+/-0.02 and an effective radius of r_e=1.9+/-0.3 km (estimated semi-axes of a~2.3 km and b~1.6 km). For 176P/LINEAR, we find an albedo of p_R=0.06+/-0.02 and an effective radius of r_e=2.0+/-0.2 km (estimated semi-axes of a~2.6 km and b~1.5 km). In terms of albedo, 133P and 176P are similar to each other and are typical of other Themis family asteroids, C-class asteroids, and other comet nuclei. We find no indication that 133P and 176P are compositionally unique among other dynamically-similar (but inactive) members of the Themis family, in agreement with previous assertions that the two objects most likely formed in-situ. We also note that low albedo (p_R<0.075) remains a consistent feature of all cometary (i.e., icy) bodies, whether they originate in the inner solar system (the main-belt comets) or in the outer solar system (all other comets).