Computer Science
Scientific paper
Apr 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998icar..132..266b&link_type=abstract
Icarus, Volume 132, Issue 2, pp. 266-284.
Computer Science
19
Scientific paper
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 epsilon 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 epsilon-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, Icarus 125, 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 deg 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.
Baines Kevin Hays
Golisch William
Kaminski Charles
Lebofsky Larry A.
Momary Thomas W.
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