The Far-UV Albedo of Steins Measured with Rosetta Alice

Astronomy and Astrophysics – Astronomy

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Scientific paper

Alice, the UV imaging spectrometer onboard Rosetta (Stern et al. 2007), has obtained the first far-ultraviolet (FUV) spectrum of an asteroid. During Rosetta's flyby of the main-belt, E-type asteroid (2867) Steins on 5 Sept. 2008, a ten minute integration of the surface of Steins was acquired. The spectrum, which averages over a variety of geometries at closest approach, shows very good signal from 850 Å to 2000 Å representing the first spectrum of an E-type asteroid below the atmospheric cutoff. We find that the FUV albedo of Steins is very low, 5%, compared to its visible albedo, 40% (Keller et al. 2009; Weissman et al. 2008; Jorda et al. 2008), as is expected from the UV behavior of many refractory materials. We also find that the albedo does not show a dramatic color variation over the FUV spectral range; however, there is a pronounced dip near 1600 Å. In addition, Alice obtained the total FUV count rate integrated with 1 second resolution during the encounter to determine the average variation of reflected FUV flux with phase angle. In comparison to the OSIRIS WAC data, Alice data show clear wavelength dependent phase reddening and a larger opposition effect in the FUV than in the visible. FUV spectra of several minerals are being measured in the laboratory to compare with the Steins spectrum in order to shed light on the composition of the E-type asteroid. We thank the Rosetta project team for thoughtful discussion and sharing data. This work is funded by NASA through a contract with JPL and subcontracts to SwRI, UMD, JHU, and JHU-APL.

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