Near-IR Photometry of the Small Uranian Satellites with Keck at Phase Angles 0–2°

We present photometric measurements of the seven brightest small inner moons of Uranus (Puck, Belinda, Rosalind, Portia, Juliet, Desdemona and Cressida) obtained with the Keck II telescope at 1.6 and 2.2 mu m at phase angles of 0.026 degrees and 0.36 degrees in October-November 2019. We also applied our new calibration technique to the August 2015 data at 2.02 degrees from Paradis et al. (2019). We show that the phase curves in the near-infrared are very similar to those at visible wavelengths, which suggests that the opposition surge at near-infrared wavelengths is also caused by both shadow hiding and coherent backscatter. All moons are either neutral in color or slightly reddish; Puck and Portia show a roughly 20% increase in their 2.2 mu m reflectivity compared to that at 1.6 mu m. This suggests that these moons are not icy bodies like the large classical moons; they are likely covered by a fluffy (similar to 80% void space) layer of carbonaceous material.