The Period of Rotation, Shape, Density, and Homogeneous Surface Color of the Centaur 5145 Pholus

We present optical photometry of the Centaur 5145 Pholus during 05/2003 and 04/2004 using the facility CCD camera on the 1.8-m VATT on Mt. Graham, Arizona. We derive a double-peaked lightcurve and a rotation period of 9.980 hr with amplitude of 0.60 magnitude. We derive four possible rotational pole positions as well as axial ratios of a/b=1.9 and c/b=0.9. If we assume Pholus is a strengthless rubble-pile and its non-spherical shape is due to rotational distortion, our axial ratios and period measurements indicate Pholus has a density of 0.5 g per cubic centimeter, suggestive of an ice-rich, porous interior. By combining B-band and R-band lightcurves, we find B-R=1.94 and any B-R color variation over the surface of Pholus must be smaller than 0.06 magnitudes. By combining our V-R measurements with values in the literature, we find no evidence for any color variegation between the northern and southern hemispheres of Pholus. Our observations add to the evidence that individual Centaurs and KBOs exhibit homogeneous surface colors and hence gray impact craters on radiation reddened crusts are probably not responsible for the colors seen among the Centaur and Kuiper belt object populations.