Ground-based near infrared spectroscopy of Jupiterʹs ring and moons

The backscattered reflectivity of Jupiterʹs ring has been previously measured over distinct visible and near infrared wavelength bands by a number of ground-based and spaceborne instruments. We present spectra of Jupiterʹs main ring from 2.21–2.46 μm taken with the NIRSPEC spectrometer at the W.M. Keck observatory. At these wavelengths, scattered light from Jupiter is minimal due to the strong absorption of methane in the planetʹs atmosphere. We find an overall flat spectral slope over this wavelength interval, except for a possible red slope shortward of 2.25 μm. We extended the spectral coverage of the ring to shorter wavelengths by adding a narrow-band image at 1.64 μm, and show results from 2.27-μm images over phase angles of 1.2°–11.0°. Our images at 1.64 and 2.27 μm reveal that the halo contribution is stronger at the shorter wavelength, possibly due to the redder spectrum of the ring parent bodies as compared with the halo dust component. We find no variation in main ring reflectivity over the 1.2°–11.0° phase angle range at 2.27 μm. We use adaptive optics imaging at the longer wavelength L′ band (3.4–4.1 μm) to determine a 2-σ upper limit of 22 m of vertically-integrated I / F . Our observing campaign also produced an L′ image of Callisto, showing a darker leading hemisphere, and a spectrum of Amalthea over the 2.2–2.5 and 2.85–3.03 μm ranges, showing deep 3-μm absorption.