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[23.07] Discovery of leading-trailing asymmetry and CO₂ ice on Ariel

W.M. Grundy (Lowell), L.A. Young, and E.F. Young (SwRI)

As seen from Earth, the Uranian system is oriented more equator-on at present than at any time during the previous 2 decades of infrared spectral observations. This more equatorial aspect, in combination with the excellent capabilities of the new IRTF\slash SpeX infrared spectrometer, offers an opportunity to explore longitudinal compositional variability of Uranian satellites' much more sensitively than was previously possible. We observed Ariel's leading and trailing hemispheres during two half-nights of observations in 2002 July, at sub-solar and sub-Earth latitudes ~20\degr\ IAU ``South'' (IAU coordinates for Ariel define ``South'' as pointing North, along the angular momentum vector). The resulting data have considerably higher spectral resolution (\lambda/\delta\lambda\approx1000) and higher signal precision (SNR up to 50) than previously-published spectra.

Our spectra reveal that H₂O ice absorption bands on Ariel's leading hemisphere are up to 60% deeper than their counterparts on the trailing hemisphere and that the trailing hemisphere has a triplet of CO₂ ice absorption bands near 2~\mum. Deeper leading-hemisphere H₂O ice bands are consistent with leading-trailing dichotomies among icy Jovian and Saturnian satellites. This is their first detection in the Uranian system. The 2~\mum CO₂ ice triplet had previously only been seen on the Neptunian satellite Triton and on Mars. That similar-quality spectra reveal these bands on Ariel's trailing hemisphere but not on its leading hemisphere suggests that the CO₂ could be produced as a result of bombardment by Uranus's corotating plasma, which should predominantly impinge on the satellite's trailing hemisphere. Alternatively, the CO₂ may be primordial or be delivered by impactors, but be preferentially destroyed, buried, or otherwise masked on Ariel's leading hemisphere. Future observations are needed to refine our knowledge of the longitudinal distribution and phase state of CO₂ ice on Ariel.

This work was supported by NSF grant AST-0085614 to Southwest Research Institute and by NASA grant NAG5-10159 to Lowell Observatory.

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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.