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The dark and icy surfaces of the large moons of Uranus: Modified by system-wide processes?

June 27, 2019

When: June 27, 2019 3:30PM

Richard Cartwright

NASA Ames / SETI Institute

Ground-based, near-infrared observations have determined that the large and tidally-locked “classical” moons of Uranus have surface compositions dominated by a mixture of H 2 O ice and dark, spectrally neutral material. Laboratory experiments indicate that the dark material is likely carbonaceous in origin, with a spectral signature similar to charcoal. Overprinting this base composition of dirty H 2 O ice, CO 2 ice has been detected, primarily on the trailing hemispheres of the inner moons, Ariel and Umbriel. Spectrally red material has also been detected, primarily on the leading hemispheres of the outer moons, Titania and Oberon. The distribution of CO 2 ice on these moons is broadly consistent with radiolytic generation via irradiation of native H 2 O ice and C-rich material by magnetospherically-embedded charged particles. The distribution of red material is broadly consistent with accumulation of dust from
retrograde irregular satellites, which are spectrally redder than the classical moons.

Although previous work supports these hypotheses, the telescope studies were conducted during southern summer in the Uranian system, when the subsolar point ranged between ~10 to 30ºS (between 2000 and 2006). Over the past seven years, we have been collecting new observations of these moons over their now observable northern latitudes (~15 – 45ºN). The results of our observations are broadly consistent with previous work, supporting surface modification of the Uranian moons by both magnetospherically-embedded charged particles and irregular satellite dust grains. Furthermore, comparison of our new northern latitude data and previously gathered southern latitude data highlights subtle changes in the strengths of detected H 2 O ice bands and hints at the possible presence of NH 3 -bearing species on these moons. In summary, the combined analyses of southern and northern latitude datasets indicate that system- wide processes are likely modifying the surface compositions of the Uranian moons.

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