Photometry of the Uranian Satellites with Keck and the Search for Mab

TL;DR

This study provides new near-infrared photometric measurements of six small Uranian satellites, revealing brightness variations and setting upper limits on Mab's reflectivity, but does not detect Mab itself.

Contribution

First near-infrared photometry of six small Uranian satellites with adaptive optics, revealing brightness variations and constraining Mab's surface reflectivity.

Findings

Small satellites are brighter than previously observed.

Hemispheric albedo variations are evident.

Mab was not detected, with an upper reflectivity limit of 0.14.

Abstract

We present photometric properties of six small (radii < 100 km) satellites of Uranus based on 32 H-(1.49-1.78 um) band images taken on August 29, 2015 from the Keck II Telescope on Maunakea, Hawaii with the near-infrared camera NIRC2 coupled to the adaptive optics system. The sub-observer latitude of our observations was 31 deg, i.e., we view much of the satellites' north poles, in contrast to the 1986 Voyager measurements. We derive reflectivities based on mean-stacking measurements of these six minor moons of Uranus. We find that the small satellites are significantly brighter than in previous observations, which we attribute to albedo variations between hemispheres. We also search for Mab, a small satellite with an unknown surface composition, orbiting between Puck and Miranda. Despite the significantly improved signal-to-noise ratio we achieved, we could not detect Mab. We suggest that Mab is more similar to Miranda, an icy body, than to the inner rocky moons. Assuming Mab is spherical with a radius of 6 km, as derived from Hubble Space Telescope (HST) observations if its reflectivity is ~0.46, we derive a 3{\sigma} upper limit to its reflectivity of 0.14 [I/F] at 1.6 um.