UV Spectra of the Uranian Satellites, Ariel, Titania, and Oberon

T. L. Roush (SFSU/NASA Ames), K. Noll (STScI), D. P. Cruikshank, and Y.J. Pendleton (NASA Ames)

Using the Faint Object Spectrograph (FOS) on the Hubble Space Telescope (HST) ultraviolet spectra were obtained for the Uranian satellites Ariel, Titania, and Oberon. The FOS data provide clear evidence for the presence of an ultraviolet absorber on these three bodies. FOS grating positions of G270 and G400 provided wavelength coverage of tex2html_wrap_inline25 220-480 nanometers (nm) with a resolving power of tex2html_wrap_inline25 4300-6400. The data were reduced to flux using the standard STScI data pipeline procedures. Prior to ratioing the fluxes to the solar spectrum tex2html_wrap_inline29 , narrow solar lines were used to fine-tune the wavelength positions; the largest shift of the observational data required was 0.16 nm. Spectral geometric albedos were calculated at the solar phase angle of the observations ( tex2html_wrap_inline31 =0.8 tex2html_wrap_inline33 ) and are consistent with recently reported broad-band albedos tex2html_wrap_inline35 within the estimated uncertainties of the two data sets. The geometric albedos of Ariel, Titania, and Oberon all have a broad minimum located at 280 tex2html_wrap_inline37 10 nm. The features are similar in position to those identified with tex2html_wrap_inline39 (280 nm) on the Jovian moons Europa tex2html_wrap_inline41 and Callisto tex2html_wrap_inline43 , although there is no apparent source of tex2html_wrap_inline39 molecules in the Uranian system. Another plausible material producing an absorption near 280 nm is OH, which is a photolysis and radiolysis by-product from tex2html_wrap_inline49 . Any OH formed will be more likely to be stable at the lower surface temperatures of the Uranian system than for the icy Jovian satellites tex2html_wrap_inline53 . None of these minima correspond to features near 260 tex2html_wrap_inline37 5 nm, attributed to tex2html_wrap_inline57 on the surfaces of Ganymede tex2html_wrap_inline59 , Rhea, and Dione tex2html_wrap_inline61 .

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