Photometric behavior of 20000 Varuna at very small solar phase angles

Six nights of R-band CCD observations of the classical Kuiper Belt Object (KBO) 20000 Varuna (2000 WR106) were obtained at the Palomar Mountain 60- and 200-in telescopes. The observations were scheduled to take advantage of a particularly favorable apparition which allowed us to sample down to extremely small solar phase angle (α=0.036°). After rotational lightcurve subtraction, we found that the KBO exhibited a strong opposition surge of ∼0.1 mag at phase angles α<0.1°. We modeled our composite solar phase curve of Varuna using both H-G parameterization and Hapke theory and concluded that similar opposition surges may be wide spread among KBOs and that the regolith of Varuna may be significantly more porous than a typical main-belt C-type asteroid. Wide-spread opposition surges lead to higher albedos than derived assuming linear phase behavior: on the whole KBOs may be brighter than previously assumed.