Near-IR photometry of the small Uranian satellites with Keck at phase angles 0–2°

Abstract

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 2.2μm at phase angles of 0.026∘ and 0.36∘ in October–November 2019. We also applied our new calibration technique to the August 2015 data at 2.02∘ 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 2.2μm reflectivity compared to that at 1.6μm. This suggests that these moons are not icy bodies like the large classical moons; they are likely covered by a fluffy (∼80% void space) layer of carbonaceous material.