NEAR-INFRARED ABSOLUTE PHOTOMETRIC IMAGING OF THE URANIAN SYSTEM

We report the first multifilter set of absolutely calibrated near-infr ared images of Uranus, its rings, and three major satellites-Titania, Ariel, and Miranda. Along with imagery utilizing the canonical K filte r bandpass (effective wavelength 2.20 mu m), absolutely calibrated ima ges of the uranian system are presented for the first time for three a dditional filter bandpasses: J (1.27 mu m), H (1.62 mu m), and in a na rrow bandpass (0.1 mu m full-width-at-half-maximum) centered at 1.73 m u m thereafter designated H') particularly diagnostic of C-H stretch v ibrational absorptions common in hydrocarbons. Multifilter-derived spe ctra of the southern ring ansa including the bright apoapse of the dom inant epsilon ring show no absorptions due to condensable volatiles, i ncluding water, ammonia, and light thigh H:C) hydrocarbons. Plausible near-infrared spherical geometric and single-scattering particle albed os 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 propertie s of charcoal, carbonaceous chondrite material, and the darkside of Ia petus, and consistent with the hypothesis that a primary component of the ring particles is high stoichiometric ratio C : if organics produc ed by charged-particle and/or photochemical weathering of methane clat hrate and/or hydrocarbon ice material originating from nearby moonlets . Additional components consistent with the ring spectrum include sili cates such as pyroxene, but not olivine. Analytical modeling of the ri ng structure indicates e-ring near-infrared opacities of 0.37 +/- 0.05 and 1.8 +/- 0.3 at apoapsis and periapsis, respectively. Ariel is mor e than 25% brighter than Miranda and 15% brighter than Titania at all near-infrared wavelengths. Comparisons with UV-visible spectra by Kark oschka (1997, Icarus 125, 348-363) show consistency with the hypothesi s that the water-ice surfaces of Titania and Ariel are contaminated wi th low-reflectance, near-IR-reddened substances similar to the weather ed high C:H material postulated for the uranian ring system, The relat ively blue, near-infrared dark surface of Miranda (full-disk near-infr ared albedo of similar to 0.22 at 2.4 degrees phase angle) exhibits th e greatest water ice absorption band-depth and the largest H- and K-fi lter brightness surge measured among the uranian satellites, consisten t 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 weathe red high C:H material. The uranian disk shows marked latitudinal varia bility in tropospheric cloud structure, with a ''polar cap'' of aeroso ls lying within the planet's southern polar region. This is in sharp c ontrast with the stratospheric haze structure, which is nearly globall y uniform. (C) 1998 Academic Press.