Keck near-infrared detections of Mab and Perdita

Abstract

We report the first near-infrared detection of Uranus's tiny moon Mab, the presumed source of the blue and diffuse μ ring, using the NIRC2 instrument at Keck Observatory. The detection was permitted by an updated shift-and-stack procedure allowing us to integrate on Mab as it moved across the detector in 23 separate exposures taken over ∼2 h, as well as the very low (0.02∘) phase angle at the time of observation. At this phase angle, Mab has an integrated I/F of 24 ± 3 km² at 1.6 μm and ≲37 km² at 2.1 μm. Comparing these values with Mab's visible reflectance as derived by HST reveals that Mab is spectrally blue; its (0.5 μm)/(1.6 μm) color is more consistent with Miranda's value than Puck's value. Mab is therefore more likely a ∼6-km radius body with a Miranda-like surface than a 12-km radius body with a Puck-like surface, in agreement with prior work based on infrared upper limits, but we caution that a Puck-like color is only ruled out at the 2 σ level. We also report the first infrared photometry of Perdita, finding an integrated I/F of 31 ± 3 km² at 1.6 μm.