Near-Infrared Absolute Photometric Imaging of the Uranian System

Abstract

We report the first multifilter set of absolutely calibrated near-infrared images of Uranus, its rings, and three major satellites—Titania, Ariel, and Miranda. Along with imagery utilizing the canonical K filter bandpass (effective wavelength 2.20 μm), absolutely calibrated images of the uranian system are presented for the first time for three additional filter bandpasses: J (1.27 μm), H (1.62 μm), and in a narrow bandpass (0.1 μm full-width-at-half-maximum) centered at 1.73 μm (hereafter designated H′) particularly diagnostic of C–H stretch vibrational absorptions common in hydrocarbons. Multifilter-derived spectra of the southern ring ansa including the bright apoapse of the dominant ϵ ring show no absorptions due to condensable volatiles, including water, ammonia, and light (high H:C) hydrocarbons. Plausible near-infrared spherical geometric and single-scattering particle albedos consistent with Voyager-derived phase functions range from 0.069 to 0.102 and from 0.030 to 0.037, respectively. These are approximately 50% greater than visible values, consistent with the optical properties of charcoal, carbonaceous chondrite material, and the darkside of Iapetus, and consistent with the hypothesis that a primary component of the ring particles is high stoichiometric ratio C:H organics produced by charged-particle and/or photochemical weathering of methane clathrate and/or hydrocarbon ice material originating from nearby moonlets. Additional components consistent with the ring spectrum include silicates such as pyroxene, but not olivine. Analytical modeling of the ring structure indicates ϵ-ring near-infrared opacities of 0.37 ± 0.05 and 1.8 ± 0.3 at apoapsis and periapsis, respectively. Ariel is more than 25% brighter than Miranda and 15% brighter than Titania at all near-infrared wavelengths. Comparisons with UV–visible spectra by Karkoschka (1997,Icarus125, 348–363) show consistency with the hypothesis that the water-ice surfaces of Titania and Ariel are contaminated with low-reflectance, near-IR-reddened substances similar to the weathered high C:H material postulated for the uranian ring system. The relatively blue, near-infrared dark surface of Miranda (full-disk near-infrared albedo of ∼0.22 at 2.4° phase angle) exhibits the greatest water ice absorption band-depth and the largest H- and K-filter brightness surge measured among the uranian satellites, consistent with (1) a markedly heterogeneous surface punctuated with localized outcroppings of relatively pure water ice and (2), as with Titania and Ariel, a surface composition dominated in the near infrared by weathered high C:H material. The uranian disk shows marked latitudinal variability in tropospheric cloud structure, with a “polar cap” of aerosols lying within the planet's southern polar region. This is in sharp contrast with the stratospheric haze structure, which is nearly globally uniform.